1582 + Interview Questions in WORK IN GENERAL AWARENESS Page 20 InterviewSolution

951.	Asseration : Work-energy theorem can be applied for non-inertial frames also. Reason : Earth is a non-inertial frame.A. If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.B. If both Assertion and Reason are true and the Reason is not the correct explanation of the Assertion.C. If Assertion is true, but the Reason is falseD. If Assertion is false but the Reason is true.
Answer» Correct Answer - B

Discussion

952.	Asseration : A block of mass m starts moving on a rough horizontal surface with a velocity v. It stops due to friction between the block and the surface after moving through a ceratin distance. The surface is now tilted to an angle of `30^@` with the horizontal and same block is made to go up on the surface with the same initial velocity v. The decrease in the mechanical energy in the second situation is small than the first situation. Reason : The coefficient of friction between the block and the surface decreases with the increase in the angle of inclination.A. If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.B. If both Assertion and Reason are true and the Reason is not the correct explanation of the Assertion.C. If Assertion is true, but the Reason is falseD. If Assertion is false but the Reason is true.
Answer» Correct Answer - C Decrease in mechanical energy in first `= E_(i)-E_(f) =1/2mv^(2)-0` `=1/2mv^(2)` Decrease in mechanical energy in second `=E_(i)-E_(f) =1/2mv^(2)-mgh`.

Discussion

953.	A particle is kept fixed on a turntable rotating uniformly. As seen from the ground the particle goes in a circle, its speed is `20 cm//s` & acceleration is `20 cm//s^(2)`. The particle is now shifted to a new position to make the radius half of the original value. The new values of the speed & acceleration will be:A. `10 cm//s,10 cm//s^(2)`B. `10cm//s,80cm//s^(2)`C. `40 cm//s, 10 cm//s^(2)`D. `40 cm//s,40cm//s^(2)`
Answer» Correct Answer - A

Discussion

954.	A body falling on the ground from a height of `10m` rebounds to a height of `2.5m`. Calculate (i) the percentage loss of kinetic energy of the body during its collision with the ground. (ii) ratio of the velocities of the body just before and just after collision.
Answer» Here,`h_(1)=10m, h_(2)=2.5m` Let `upsilon_(1)` be the velocity of the body just before collision with the ground and `upsilon_(2)` be the velocity of the body just after collision. `:. (1)/(2)m upsilon_(1)^(2)=mgh_(1)` and `(1)/(2)m upsilon_(2)^(2)=mgh_(2)` Dividing, we get `(upsilon_(1)^(2))/(upsilon_(2)^(2))=(h_(1))/(h_(2))=(10)/(2.5)=4,` `:. (upsilon_(1))/(upsilon_(2))=2` `%` age loss in KE`=(K_(1)-K_(2))/(K_(1))xx100` `=(mg(h_(1)-h_(2)))/(mgh_(1))xx100=((10-2.5))/(10)xx100=75%`

Discussion

955.	Figure shows a force compression curve of a spring. A body of mass `5kg` moving with the velocity of `8m//s` hits the spring. Calculate the force constant of the spring and also the compression produced in the spring when the body hits it.
Answer» From figure, force constant of spring `k=(F)/(x)=` slope of `F-x` curve`=(AB)/(OB)` `=(32)/(0.4)=80N//m` If `x` is the compression produced on hitting the spring, then as Work done `=`K.E.`:.(1)/(2)kx^(2)=(1)/(2)m upsilon^(2)` `x=sqrt(m(upsilon^(2))/(k))=upsilonsqrt((m)/(k))=8sqrt((5)/(80))=2m`

Discussion

956.	Two masses of 1 gm and 4 gm are moving with equal kinetic energies. The ratio of the magnitudes of their linear momenta isA. `4:1`B. `sqrt2:1`C. `1:2`D. `1:16`
Answer» Correct Answer - C (c ) `P=sqrt(2Km)orP prop sqrt(m)` `:. (P_(1))/(P_(2))sqrt((1)/(4))=(1)/(2)`

Discussion

957.	Two bodies A and B having masses in the ratio of 3 : 1 possess the same kinetic energy. The ratio of their linear momenta is then
Answer» Kinetic energy of body is given by `E_(K)=(1)/(2) mv^(2)` and linear momentum P=mv From Eqs. (i) and (ii), we get `E_(K)=(m^(2)v^(2))/(2m)=(P^(2))/(2m)` When `E_(K_(1)=E_(K_(2)` `implies (P_(1)^(2))/(2m_(1))=(P_(2)^(2))/(2m_(2))" or "(P_(1))/(P_(2))=sqrt((m_(1))/(m_(2)))` or `(P_(1))/(P_(2))=sqrt((3)/(1))` `(P_(2))/(P_(1))=(1)/(sqrt3)`

Discussion

958.	Two bodies of masses `m` and `4m` are moving with equal linear momenta. The ratio of their kinetic energies is `:`
Answer» Here, `m_(1)=m` `m_(2)=4m` If `upsilon_(1), upsilon_(2)` are their respective velocities, then as `p_(1)=p_(2)` `m_(1)upsilon_(1)=m_(2)upsilon_(2)` `(upsilon_(1))/( upsilon_(2))=(m_(2))/( m_(1))=4,` `(E_(1))/(E_(2))=((1)/(2)m_(1)upsilon_(1)^(2))/( (1)/(2)m_(2)upsilon_(2)^(2))=(1)/(4)((4)/(1))^(2)=4`

Discussion

959.	For the force displacement graph shown below. Calculate the work done by the force displacing the body from x=1 cm to x=5 cm.
Answer» Work =Area under the curve and dispalcement axis `=10xx1+20xx1-20xx1+10xx1=20 " erg"`

Discussion

960.	A `100 g ` block moves along `X-` axis. Its acceleration as a function of displacemetn is shown in figure. Calculate work done (in joule) on the block by the force applied as the block moves from `x=0` to `x=8m`.
Answer» As the given graph is linear , figure, `:. a = alphax, ` where` alpha` is slope of graph `alpha=(a)/( x)=(20)/(8)=2.5s^(-2)` Now, `F=ma=m alpha x` `W=int _(0)^(8)F dx = int _(0)^(8)m alpha x dx = m alpha [(x^(2))/( 2)]_(0)^(8)` `=(2.5)/(10)((64-0)/(2))=8J`

Discussion

961.	A force`vecF=(7-2x+3x^(2))` N is applied on a 2 kg mass which displaces it from x = 0 to x = 5 m. Work done in joule is -
Answer» Work done, `W=int_(x_(1))^(x_(2))F dx=int_(0)^(5)(7-2x+3x^(2))dx` Here, the body changes its position from x=0 to x=5 `=[7x-(2x^(2))/(2)+(3x^(3))/(3)]_(0)^(5)=[7(5)-(5)^(2)+(5)^(3)-0]=135 J`

Discussion

962.	A body moves a distance of 5 m along a straight line under the action of a force 10 N. If the work done is `25sqrt3` joule, the angle which the force makes with the direction of motion of the body isA. `0^(@)`B. `30^(@)`C. `45^(@)`D. `60^(@)`
Answer» Correct Answer - B

Discussion

963.	A particle moves in a straight line with constant acceleration under a constant force F. Select the correct alternative(s).A. Power development by this force varies linearly with timeB. Power developed by this force varies parabolically with timeC. Power developed by this force varies linearly with displacementD. Power developed by this force varies parabolically with displacement
Answer» Correct Answer - A::D

Discussion

964.	A small ball is rolled with speed `u` from piont A along a smooth circular track as shown in figure. If `x=3R`, then Determine the required speed u so that the ball returns to A, the point of projection after passing through C, the highest point.A. (a) `3/2sqrt(gR)`B. (b) `1/2sqrt(gR)`C. (c) `5/3sqrt(gR)`D. (d) `5/2sqrt(gR)`
Answer» Correct Answer - D Let the velocity at the point is `v_C`. Then for projectile motion from C to A, `Rang e=v_csqrt((2(height))/(g))implies3R=v_csqrt((2xx2R)/(g))` `impliesv_C=3/2sqrt(Rg)` To find velocity at B, apply conservation of energy, i.e., `=1/2mv_B^2=mg2R+1/2mv_C^2=V_B=5/2sqrt(Rg)`

Discussion

965.	From figure, find the work done at the end of displacements: (a) 20cm, (b) 40cm, and (c) 80cm.
Answer» a. Work done at the end of displacement `20cm`, =Area of triangle OAM `=1/2xx(20xx10^-2)xx10=1J` b. Work done at the end of displacement `40cm`, =Area of OABN =Area of OAM+Area of rectangle ABMN `=1+20xx10^-2xx10=3J` c. Work done at the end of displacement `80cm`, =Area of trapezium OACR-Area of rectangle RQD `=1/2(60+30)xx10^-2xx10-1/2xx(20xx10^-2)xx10` `=3.5J` Net area from `50cm` to `60cm` will be zero.

Discussion

966.	A particle of mass 0.01 kg travels along a space curve with velocity given by `4hat(i)+16 hat (k)m//s` After some time its velocity becomes `8hat(i)+20hat(j)m//s` due to the action of a conservative force The work done on the particle during this interval of time is:A. 0.32 JB. 6.9 JC. 9.6 JD. 0.96 J
Answer» Correct Answer - D

Discussion

967.	STATEMENT-1 Work done by a force depends on the frame of reference. STATEMENT-2 Displacement of a body is different in different frames of reference.A. Statement-1 is True, Statement-2 is True, Statement-2 is a correct explanation for Statement-6B. Statement-1 is True, Statement-2 is True, Statement-2 is NOT a correct explanation for Statement-6C. Statement-1 is True, Statement-2 is FalseD. Statement-1 is False, Statement-2 is True
Answer» Correct Answer - A

Discussion

968.	A constatnt force of 10 N act on a block as shown in figure intal velocity of the particle is zero. It is observed from three frames of reference `S_(1),S_(2)` and ground. The frame `S_(1)` start from rest and move with constnat acceleration `10m//s.` The frames `S_(1)and S_(2)` and particle start moving simultaneously at `t=0.` Work done on block in 1 s as observed from frame `S_(1)` isA. 50 JB. `-50J`C. 100 JD. Zero
Answer» Correct Answer - D

Discussion

969.	A constatnt force of 10 N act on a block as shown in figure intal velocity of the particle is zero. It is observed from three frames of reference `S_(1),S_(2)` and ground. The frame `S_(1)` start from rest and move with constnat acceleration `10m//s.` The frames `S_(1)and S_(2)` and particle start moving simultaneously at `t=0.` Net work done on block in 1 s as observed from frame `S_(2)` isA. 50 JB. `-50J`C. 100 JD. Zero
Answer» Correct Answer - B

Discussion

970.	No work is done when:A. a nail is hammered into a wooden box B. a box is pushed along a horizontal floor C. there is no component of force, parallel to the direction of motion D. none of these
Answer» Formula for work done when the force is applied at an angle to the direction of motion is given by, W = FS cos θ When there will be no component of force, parallel to the direction of motion then force F in direction of motion is zero, so work done will also be zero.

970.

No work is done when:A. a nail is hammered into a wooden box B. a box is pushed along a horizontal floor C. there is no component of force, parallel to the direction of motion D. none of these

Answer»

Formula for work done when the force is applied at an angle to the direction of motion is given by,

W = FS cos θ

When there will be no component of force, parallel to the direction of motion then force F in direction of motion is zero, so work done will also be zero.

Discussion

971.	Potential energy of a person is minimum when:A. Person is standing B. Person is sitting on a chair C. Person is sitting on the ground D. Person is lying on the ground
Answer» Potential energy of a body is defined as energy of a body due to its higher position above the earth, Potential energy = M × g × h Where h is the height above ground. If the person will be lying on ground then it will have minimum or zero height above the ground therefore potential energy of the person will also be minimum.

971.

Potential energy of a person is minimum when:A. Person is standing B. Person is sitting on a chair C. Person is sitting on the ground D. Person is lying on the ground

Answer»

Potential energy of a body is defined as energy of a body due to its higher position above the earth,

Potential energy = M × g × h

Where h is the height above ground.

If the person will be lying on ground then it will have minimum or zero height above the ground therefore potential energy of the person will also be minimum.

Discussion

972.	You pay a coolie for carrying your luggage. Does he really do any work?
Answer» Work done by a coolie in carrying load on his head is ZERO. This is because the force of gravity acts in vertical downward direction and displacement of load takes place in horizontal direction hence angle between displacement and force becomes 90°. Since, Work= FS cos θ Work done in this case is zero because cos 90°=0 Note: Even if there is no work done but the energy of the coolie is still consumed in carrying the luggage.

972.

You pay a coolie for carrying your luggage. Does he really do any work?

Answer»

Work done by a coolie in carrying load on his head is ZERO. This is because the force of gravity acts in vertical downward direction and displacement of load takes place in horizontal direction hence angle between displacement and force becomes 90°.

Since, Work= FS cos θ

Work done in this case is zero because cos 90°=0

Note: Even if there is no work done but the energy of the coolie is still consumed in carrying the luggage.

Discussion

973.	Give reason for the following: Why a coolie does not work, when he moves on a level road while carrying a box on his head?
Answer» Work done when force is applied at an angle to motion is given by: W= F×S cos θ Where F = force S = displacement θ = angle between the direction of force and direction of motion. Work done by a coolie in carrying a load on his head is ZERO with respect to gravity. This is because the coolie applies force against the force of gravity which acts in a vertically downward direction and displacement of load takes place in a horizontal direction hence the angle between displacement and force becomes 90°. cos 90°= 0 which means work done = 0 So, a coolie does not work, when he moves on a level road while carrying a box on his head.

973.

Give reason for the following: Why a coolie does not work, when he moves on a level road while carrying a box on his head?

Answer»

Work done when force is applied at an angle to motion is given by:

W= F×S cos θ

Where F = force

S = displacement

θ = angle between the direction of force and direction of motion.

Work done by a coolie in carrying a load on his head is ZERO with respect to gravity. This is because the coolie applies force against the force of gravity which acts in a vertically downward direction and displacement of load takes place in a horizontal direction hence the angle between displacement and force becomes 90°.

cos 90°= 0 which means work done = 0

So, a coolie does not work, when he moves on a level road while carrying a box on his head.

Discussion

974.	When a coil spring is compressed, the work is done in the spring. The elastic potential energy:A. increases B. decreases C. disappears D. remains unchanged
Answer» Work done by a coil spring is given by, W = \(\frac{1}{2}\)KX² Where k is the spring constant/stiffness X is the extension or compression produced This work done is stored in spring as elastic potential energy (PE) when the spring is compressed and hence elastic potential energy decreases.

974.

When a coil spring is compressed, the work is done in the spring. The elastic potential energy:A. increases B. decreases C. disappears D. remains unchanged

Answer»

Work done by a coil spring is given by,

W = \(\frac{1}{2}\)KX²

Where k is the spring constant/stiffness

X is the extension or compression produced

This work done is stored in spring as elastic potential energy (PE) when the spring is compressed and hence elastic potential energy decreases.

Discussion

975.	Class 9 Science MCQ Questions of Work, Power and Energy with Answers?
Answer» The Class 9 Science MCQ Questions of Work, Power and Energy are recognized because of the most useful and widely applicable form of objective test elements. By preparing these MCQ Questions once after completing every topic, we are ready to attempt competitive exams easily and effectively. Here you will find MCQ Questions for class 9 Science and answers on Work, Energy and Power which can able to understand the mechanism. They assist you in your preparations for various examinations. Practice MCQ Questions for class 9 Chapter-Wise 1. When a body falls freely towards the earth, then its total energy (a) increases (b) decreases (c) remains constant (d) first increases and then decreases 2. An iron sphere of mass 10 kg has the same diameter as an aluminum sphere of mass is 3.5 kg. Both spheres are dropped simultaneously from a tower. When they are lo m above the ground, they have the same. (a) acceleration (b) momenta (c) potential energy (d) kinetic energy 3. The work done on an object does not depend upon the (a) displacement (b) force applied (c) angle between force and displacement (d) initial velocity of the object 4. The number of joules contained in 1 kWh is (a) \(36\times10^5\;J\) (b) \(3.6\times10^7\;J\) (c) \(36\times10^8\;J\) (d) \(3.7\times10^7\;J\) 5. If speed of a car becomes 2 times, its kinetic energy becomes (a) 4 times (b) 8 times (c) 16 times (d) 12 times 6. One-joule work is said to be done when (a) a force of 1 N displaces a body by 1 cm (b) a force of 1 N displaces a body by 1 m (c) a force of 1 dyne displaces a body by 1 m (d) a force of 1 dyne displaces a body by 1 cm 7. Mechanical energy of a body includes (a) kinetic energy only (b) potential energy only (c) kinetic energy and potential energy (d) none of these 8. ……….. is defined as the rate of doing work. (a) Power (b) Energy (c) Current (d) Electric current 9. Work done by centripetal force is (a) \(>0\) (b) \(\geq0\) (c) \(<0\) (d) 0 10. If work is done at a faster rate, then (a) power is more (b) No power is required to do work (c) power is less (d) Infinite power is required 11. Moon revolves around the earth due to gravitational force (F) of earth on moon. The work done by the gravitational force is (r = radius of the circular orbit of moon) (a) \(F.2\pi r\) (b) \(F.\pi r\) (c) zero (d) negative work 12. The energy possessed by an oscillating pendulum of a clock is (a) kinetic energy (b) potential energy (c) restoring energy (d) mechanical energy 13. On tripling the speed of motion of a body, the change in K.E. is (a) 9 times (b) 8 times (c) 4 times (d) 2 times 14. A body at rest can have: (a) Energy (b) Power (c) Momentum (d) Velocity 15. A flying airplane possesses (a)Only potential energy (b) Only kinetic energy (c) Both potential and kinetic energy (d) None of these 16. The kinetic energy of a body is increased most by doubling its: (a) Speed (b) Density (c) Weight (d) Mass 17. In winters, rubbing of hands together for some time, causes a sensation of warmth mainly because of (a) heat caused by the force of friction (b) heat caused by the momentum (c) heat caused by the motion (d) heat flows from the blood to skin 18. The commercial unit of energy consumption in households, industries and commercial establishments is (a) Joule (b) Watt (c) kW (d) KW h (kilowatt-hour) 19. In Force vs Displacement graph, What does the area under the curve represents : (a) Velocity (b) Acceleration (c) Force (d) Work done 20. The person will have maximum Potential Energy, when? (a) he is sleeping on the ground (b) he is sitting on the ground (c) he is sleeping on the bed (d) he is standing on the roof Answer: 1. Answer : (c) remains constant Explanation: Since, the total energy of the system is always conserved, so when a body falls freely towards the earth, then its total energy remains constant i.e.the sum of the potential energy and kinetic energy of the body would be the same at all points. 2. Answer : (a) acceleration Explanation: When both spheres are dropped simultaneously from a tower, then they have same acceleration because during free-fall acceleration of the body becomes equals to acceleration due to gravity. The acceleration due to gravity depends on mass of earth and the radius of earth. Hence, The acceleration will be same. 3. Answer : (d) initial velocity of the object Explanation: Here, F= force applied on the object, d= displacement, and 0 is the angle between force and displacement. So, the work done on an object does not depend upon the initial velocity of the object. 4. Answer : (a) \(36\times10^5\;J\) Explanation: One kilowatt-hour is equivalent to the energy of 1,000 joules used for 3,600 seconds or 3.6 million Joules. In equation form: 1 kWh = 3.6 million J. 5. Answer : (a) 4 times Explanation: The change in the kinetic energy of the object as the speed changes is proportional to the square of the factor by which the speed changes. For example, if the speed of the object becomes double, its kinetic energy changes to four times the initial kinetic energy. 6. Answer : (b) a force of 1 N displaces a body by 1 m Explanation: 1 Joule is the amount of work done when a force of 1 Newton displaces a body through a distance of 1m in the direction of the force applied. 7. Answer : (c) kinetic energy and potential energy Explanation: Mechanical Energy of a body includes both kinetic energy and potential energy. 8. Answer : (a) Power Explanation: power is the rate of doing work. It is the amount of energy consumed per unit of time. 9. Answer : (d) 0 Explanation: Work done by a centripetal force is always zero as centripetal force always act perpendicular to the direction of motion. 10. Answer : (a) power is more Explanation: \(Power=\frac{Work}{time}\) If work is done faster, power is higher. If work is done slower, power is smaller. 11. Answer : (c) zero Explanation: The moon is revolving around the earth. In this process, the work done by the gravitational force is zero because the displacement of the moon is at the right angle to the gravitational force. 12. Answer : (d) mechanical energy Explanation: The energy possessed by an oscillating pendulum of a clock is the sum of potential energy and kinetic energy and this sum is known as mechanical energy. 13. Answer : (a) 9 times Explanation: The Kinetic Energy is increased by a factor of 9 if the velocity is tripled. 14.Answer : (a) Energy Explanation: A body at rest can have energy, i.e. potential energy, or momentum. Energy is the ability to do work. The Law of conservation of energy: Energy can neither be created nor be destroyed, only Energy can be converted from one form to another. 15. Answer : (c) Both potential and kinetic energy Explanation: An airplane flying has both kinetic and potential energy because it is in motion due to which it has kinetic energy and it is flying at some height from the earth's surface due to which it has potential energy also. 16. Answer : (a) Speed Explanation: If the speed of an object is doubled by increasing its velocity, its kinetic energy becomes four times its initial value. Thus kinetic energy increases by four times when its momentum is doubled. Hence kinetic energy of a body is increased most by doubling its speed. 17. Answer : (a) heat caused by the force of friction Explanation: That warmth is caused by a force called friction. When objects like your hands come in contact and move against each other, they produce friction. Friction happens when you overcome the resistance of one object rubbing against the other. Rub them together and there's friction. 18. Answer : (d) KW h (kilowatt-hour) Explanation: The energy used in households, industries, and commercial establishments are usually expressed in kilowatt hour 1 kWh. 19. Answer : (d) Work done Explanation: The area under the force-displacement curve represents the work done. It is the distance (s) of the object displaced by the application of the force (f). 20. Answer : (d) he is standing on the roof Explanation: Potential energy is the multiplication of mass, acceleration due to gravity and height of the object. So, the maximum potential energy at height. Hence, The person will have maximum potential energy when he is standing on roof. Click here to Know more MCQ Questions for class 9 Work, Power and Energy

975.

Class 9 Science MCQ Questions of Work, Power and Energy with Answers?

Answer»

The Class 9 Science MCQ Questions of Work, Power and Energy are recognized because of the most useful and widely applicable form of objective test elements. By preparing these MCQ Questions once after completing every topic, we are ready to attempt competitive exams easily and effectively.

Here you will find MCQ Questions for class 9 Science and answers on Work, Energy and Power which can able to understand the mechanism. They assist you in your preparations for various examinations.

Practice MCQ Questions for class 9 Chapter-Wise

1. When a body falls freely towards the earth, then its total energy

(a) increases
(b) decreases
(c) remains constant
(d) first increases and then decreases

2. An iron sphere of mass 10 kg has the same diameter as an aluminum sphere of mass is 3.5 kg. Both spheres are dropped simultaneously from a tower. When they are lo m above the ground, they have the same.

(a) acceleration
(b) momenta
(c) potential energy
(d) kinetic energy

3. The work done on an object does not depend upon the

(a) displacement
(b) force applied
(c) angle between force and displacement
(d) initial velocity of the object

4. The number of joules contained in 1 kWh is

(a) \(36\times10^5\;J\)
(b) \(3.6\times10^7\;J\)
(c) \(36\times10^8\;J\)
(d) \(3.7\times10^7\;J\)

5. If speed of a car becomes 2 times, its kinetic energy becomes

(a) 4 times
(b) 8 times
(c) 16 times
(d) 12 times

6. One-joule work is said to be done when

(a) a force of 1 N displaces a body by 1 cm
(b) a force of 1 N displaces a body by 1 m
(c) a force of 1 dyne displaces a body by 1 m
(d) a force of 1 dyne displaces a body by 1 cm

7. Mechanical energy of a body includes

(a) kinetic energy only
(b) potential energy only
(c) kinetic energy and potential energy
(d) none of these

8. ……….. is defined as the rate of doing work.

(a) Power
(b) Energy
(c) Current
(d) Electric current

9. Work done by centripetal force is

(a) \(>0\)
(b) \(\geq0\)
(c) \(<0\)
(d) 0

10. If work is done at a faster rate, then

(a) power is more
(b) No power is required to do work
(c) power is less
(d) Infinite power is required

11. Moon revolves around the earth due to gravitational force (F) of earth on moon. The work done by the gravitational force is (r = radius of the circular orbit of moon)

(a) \(F.2\pi r\)
(b) \(F.\pi r\)
(c) zero
(d) negative work

12. The energy possessed by an oscillating pendulum of a clock is

(a) kinetic energy
(b) potential energy
(c) restoring energy
(d) mechanical energy

13. On tripling the speed of motion of a body, the change in K.E. is

(a) 9 times
(b) 8 times
(c) 4 times
(d) 2 times

14. A body at rest can have:

(a) Energy
(b) Power
(c) Momentum
(d) Velocity

15. A flying airplane possesses

(a)Only potential energy
(b) Only kinetic energy
(c) Both potential and kinetic energy
(d) None of these

16. The kinetic energy of a body is increased most by doubling its:

(a) Speed
(b) Density
(c) Weight
(d) Mass

17. In winters, rubbing of hands together for some time, causes a sensation of warmth mainly because of

(a) heat caused by the force of friction
(b) heat caused by the momentum
(c) heat caused by the motion
(d) heat flows from the blood to skin

18. The commercial unit of energy consumption in households, industries and commercial establishments is

(a) Joule
(b) Watt
(c) kW
(d) KW h (kilowatt-hour)

19. In Force vs Displacement graph, What does the area under the curve represents :

(a) Velocity
(b) Acceleration
(c) Force
(d) Work done

20. The person will have maximum Potential Energy, when?

(a) he is sleeping on the ground
(b) he is sitting on the ground
(c) he is sleeping on the bed
(d) he is standing on the roof

Answer:

1. Answer : (c) remains constant

Explanation: Since, the total energy of the system is always conserved, so when a body falls freely towards the earth, then its total energy remains constant i.e.the sum of the potential energy and kinetic energy of the body would be the same at all points.

2. Answer : (a) acceleration

Explanation: When both spheres are dropped simultaneously from a tower, then they have same acceleration because during free-fall acceleration of the body becomes equals to acceleration due to gravity. The acceleration due to gravity depends on mass of earth and the radius of earth. Hence, The acceleration will be same.

3. Answer : (d) initial velocity of the object

Explanation: Here, F= force applied on the object, d= displacement, and 0 is the angle between force and displacement. So, the work done on an object does not depend upon the initial velocity of the object.

4. Answer : (a) \(36\times10^5\;J\)

Explanation: One kilowatt-hour is equivalent to the energy of 1,000 joules used for 3,600 seconds or 3.6 million Joules. In equation form: 1 kWh = 3.6 million J.

5. Answer : (a) 4 times

Explanation: The change in the kinetic energy of the object as the speed changes is proportional to the square of the factor by which the speed changes. For example, if the speed of the object becomes double, its kinetic energy changes to four times the initial kinetic energy.

6. Answer : (b) a force of 1 N displaces a body by 1 m

Explanation: 1 Joule is the amount of work done when a force of 1 Newton displaces a body through a distance of 1m in the direction of the force applied.

7. Answer : (c) kinetic energy and potential energy

Explanation: Mechanical Energy of a body includes both kinetic energy and potential energy.

8. Answer : (a) Power

Explanation: power is the rate of doing work. It is the amount of energy consumed per unit of time.

9. Answer : (d) 0

Explanation: Work done by a centripetal force is always zero as centripetal force always act perpendicular to the direction of motion.

10. Answer : (a) power is more

Explanation: \(Power=\frac{Work}{time}\)

If work is done faster, power is higher. If work is done slower, power is smaller.

11. Answer : (c) zero

Explanation: The moon is revolving around the earth. In this process, the work done by the gravitational force is zero because the displacement of the moon is at the right angle to the gravitational force.

12. Answer : (d) mechanical energy

Explanation: The energy possessed by an oscillating pendulum of a clock is the sum of potential energy and kinetic energy and this sum is known as mechanical energy.

13. Answer : (a) 9 times

Explanation: The Kinetic Energy is increased by a factor of 9 if the velocity is tripled.

14.Answer : (a) Energy

Explanation: A body at rest can have energy, i.e. potential energy, or momentum. Energy is the ability to do work. The Law of conservation of energy: Energy can neither be created nor be destroyed, only Energy can be converted from one form to another.

15. Answer : (c) Both potential and kinetic energy

Explanation: An airplane flying has both kinetic and potential energy because it is in motion due to which it has kinetic energy and it is flying at some height from the earth's surface due to which it has potential energy also.

16. Answer : (a) Speed

Explanation: If the speed of an object is doubled by increasing its velocity, its kinetic energy becomes four times its initial value. Thus kinetic energy increases by four times when its momentum is doubled. Hence kinetic energy of a body is increased most by doubling its speed.

17. Answer : (a) heat caused by the force of friction

Explanation: That warmth is caused by a force called friction. When objects like your hands come in contact and move against each other, they produce friction. Friction happens when you overcome the resistance of one object rubbing against the other. Rub them together and there's friction.

18. Answer : (d) KW h (kilowatt-hour)

Explanation: The energy used in households, industries, and commercial establishments are usually expressed in kilowatt hour 1 kWh.

19. Answer : (d) Work done

Explanation: The area under the force-displacement curve represents the work done. It is the distance (s) of the object displaced by the application of the force (f).

20. Answer : (d) he is standing on the roof

Explanation: Potential energy is the multiplication of mass, acceleration due to gravity and height of the object. So, the maximum potential energy at height. Hence, The person will have maximum potential energy when he is standing on roof.

Click here to Know more MCQ Questions for class 9 Work, Power and Energy

Discussion

976.	An elastic spring of spring constant k is compressed by an amount x . show that its potential energy is \(\frac{kx^2}{2}\).
Answer» We know that as we stretch or compress a spring, a restoring force acts on it which increases linearly with the distance from the equilibrium (unstretched position) restoring force F is directly proportional to distance stretched/compressed from unstretched position. Let this distance be x then F = -kx k is proportionality constant called 'spring constant'. (negative sign shows that restoring force is opposite to displacement) Now we want to know how much potential energy is stored in the compressed spring. Potential energy of any configuration = work done to get that configuration So we basically have to calculate the work done by us in compressing the spring by a distance x. W = Force. Distance The applied force is equal to the restoring force in magnitude. Let us compress the spring by a small distance dx, then small work done in doing so dW = kx. dx Total work done will be equal to integrating all the small works dW from x = 0 to x = x ∫dW = ∫kx. dx [ from x = 0 to x = x ] W = k ∫ x.dx = \(\frac{k [x^2]}{2}\) W = \(\frac{kx^2}{2}\) Therefore Potential energy stored in the spring = W = \(\frac{kx^2}{2}\)

976.

An elastic spring of spring constant k is compressed by an amount x . show that its potential energy is \(\frac{kx^2}{2}\).

Answer»

We know that as we stretch or compress a spring, a restoring force acts on it which increases linearly with the distance from the equilibrium (unstretched position) restoring force F is directly proportional to distance stretched/compressed from unstretched position.

Let this distance be x

then

F = -kx

k is proportionality constant called 'spring constant'.

(negative sign shows that restoring force is opposite to displacement)

Now we want to know how much potential energy is stored in the compressed spring.

Potential energy of any configuration = work done to get that configuration

So we basically have to calculate the work done by us in compressing the spring by a distance x.

W = Force. Distance

The applied force is equal to the restoring force in magnitude. Let us compress the spring by a small distance dx, then small work done in doing so

dW = kx. dx

Total work done will be equal to integrating all the small works dW from x = 0 to x = x

∫dW = ∫kx. dx [ from x = 0 to x = x ]

W = k ∫ x.dx = \(\frac{k [x^2]}{2}\)

W = \(\frac{kx^2}{2}\)

Therefore Potential energy stored in the spring = W = \(\frac{kx^2}{2}\)

Discussion

977.	The cgs unit of work is:A. joule B. erg C. dyne D. watt
Answer» CGS unit means units of work in form of centimeter, gram and seconds. 1 erg = 1 g cm²s^-2 One erg is defined as the amount of work done by a force of one dyne which produces the displacement of one centimeter. Also 1 erg = 10^-7 J

977.

The cgs unit of work is:A. joule B. erg C. dyne D. watt

Answer»

CGS unit means units of work in form of centimeter, gram and seconds.

1 erg = 1 g cm²s^-2

One erg is defined as the amount of work done by a force of one dyne which produces the displacement of one centimeter.

Also 1 erg = 10^-7 J

Discussion

978.	Give reason for the following: Can a body have momentum without having energy? Explain.
Answer» Momentum = mass × velocity A body can only possess velocity if it has kinetic energy and if does not have that energy than velocity = 0, using this value in the above-mentioned formula Momentum = mass × 0 Momentum = 0 So, a body cannot have momentum without having energy. Sometimes even if a body has energy, its momentum can be zero. For example, a brick lying on the roof of a building possesses potential energy but no velocity so momentum will be zero.

978.

Give reason for the following: Can a body have momentum without having energy? Explain.

Answer»

Momentum = mass × velocity

A body can only possess velocity if it has kinetic energy and if does not have that energy than velocity = 0, using this value in the above-mentioned formula

Momentum = mass × 0

Momentum = 0

So, a body cannot have momentum without having energy.

Sometimes even if a body has energy, its momentum can be zero.

For example, a brick lying on the roof of a building possesses potential energy but no velocity so momentum will be zero.

Discussion

979.	Write true or false for the following statements: A force does not work, if it produces no motion.
Answer» False Explanation: work is said to be done when a force produces motion. Work done (W) in moving an object is equal to the product of force (F) and the displacement(s) of body in the direction of force. Work done= F×s If there is no motion then there will be no displacement, i.e. s=0, so work done will be zero.

979.

Write true or false for the following statements: A force does not work, if it produces no motion.

Answer»

False

Explanation: work is said to be done when a force produces motion. Work done (W) in moving an object is equal to the product of force (F) and the displacement(s) of body in the direction of force.

Work done= F×s

If there is no motion then there will be no displacement, i.e. s=0, so work done will be zero.

Discussion

980.	The work done by a force on a body will be positive if the:A. body does not move B. body moves perpendicular to the direction of motion C. body moves along the direction of the applied force D. body moves opposite to the direction of the applied force
Answer» Work done by a force will be positive when the direction of applied force and the direction of motion is same. When force is applied at an angle to the direction of motion of object then the work in the direction of motion is given by: W = Fs cos θ When F and s are in same direction then cos θ will give positive value and work done will be positive.

980.

The work done by a force on a body will be positive if the:A. body does not move B. body moves perpendicular to the direction of motion C. body moves along the direction of the applied force D. body moves opposite to the direction of the applied force

Answer»

Work done by a force will be positive when the direction of applied force and the direction of motion is same.

When force is applied at an angle to the direction of motion of object then the work in the direction of motion is given by: W = Fs cos θ

When F and s are in same direction then cos θ will give positive value and work done will be positive.

Discussion

981.	When a force retards the motion of a body, the work done is :A. positive B. zero C. negative D. uncertain
Answer» A force can retard the motion of an object only when it will acts in opposite direction of motion of body, this means that angle between the direction of motion of body and direction of force is 180°. When force is applied at an angle to the direction of motion of object then the work in the direction of motion is given by: W = F s cosθ Since cos 180°= -1, So, Work done = -F×s which is negative.

981.

When a force retards the motion of a body, the work done is :A. positive B. zero C. negative D. uncertain

Answer»

A force can retard the motion of an object only when it will acts in opposite direction of motion of body, this means that angle between the direction of motion of body and direction of force is 180°.

When force is applied at an angle to the direction of motion of object then the work in the direction of motion is given by: W = F s cosθ

Since cos 180°= -1,

So, Work done = -F×s

which is negative.

Discussion

982.	Give reason for the following: Can there be displacement of an object in the absence of any force acting on it?
Answer» Yes, there can be displacement on an object even if there is no force acting on it. Force = mass × acceleration Acceleration = \(\frac{Change\,in\,velocity}{Time}\) Velocity = \(\frac{Displacement}{Time}\) If a body possesses constant velocity then it will have some displacement but there is no change in velocity so the acceleration of the body will be zero. If there is no acceleration then there will be no force acting on the body.

982.

Give reason for the following: Can there be displacement of an object in the absence of any force acting on it?

Answer»

Yes, there can be displacement on an object even if there is no force acting on it.

Force = mass × acceleration

Acceleration = \(\frac{Change\,in\,velocity}{Time}\)

Velocity = \(\frac{Displacement}{Time}\)

If a body possesses constant velocity then it will have some displacement but there is no change in velocity so the acceleration of the body will be zero. If there is no acceleration then there will be no force acting on the body.

Discussion

983.	A weightlifter lifts 240 kg from the ground to a height of 2.5 in 3 seconds. The average power is:A. 1960 W B. 19.6 KW C. 1.96 MW D. 196 W
Answer» Given, Mass = 240 kg Height = 2.5 m Time = 3 s Let g = 9.8 ms^-2 We know, Average Power = \(\frac{Energy\,consumed}{Time\,taken}\) Energy consumed = m × g × h = 240 kg×9.8 ms^-2×2.5 m = 5880 J Substituting the require values Average Power = \(\frac{5880\,J}{3\,s}\) = 1960 W

983.

A weightlifter lifts 240 kg from the ground to a height of 2.5 in 3 seconds. The average power is:A. 1960 W B. 19.6 KW C. 1.96 MW D. 196 W

Answer»

Given,

Mass = 240 kg

Height = 2.5 m

Time = 3 s

Let g = 9.8 ms^-2

We know,

Average Power = \(\frac{Energy\,consumed}{Time\,taken}\)

Energy consumed = m × g × h

= 240 kg×9.8 ms^-2×2.5 m

= 5880 J

Substituting the require values

Average Power = \(\frac{5880\,J}{3\,s}\)

= 1960 W

Discussion

984.	Write true or false for the following statements: In order to get minimum work, the angle between force and displacement should be 90.
Answer» True Explanation: When force is applied at an angle to the direction of motion of object then the work in the direction of motion is given by: W = Fs cos θ If angle θ between force and displacement is 90 then cos θ will be equal to zero. So, work done will also be zero by the given formula which means minimum.

984.

Write true or false for the following statements: In order to get minimum work, the angle between force and displacement should be 90.

Answer»

True

Explanation: When force is applied at an angle to the direction of motion of object then the work in the direction of motion is given by:

W = Fs cos θ

If angle θ between force and displacement is 90 then cos θ will be equal to zero.

So, work done will also be zero by the given formula which means minimum.

Discussion

985.	Write true or false for the following statements: Work and energy have different units.
Answer» False Explanation: Work is said to be done when a force produces motion. SI Unit of work is Joule and the capability to do that work is called energy. SI unit of energy is also Joule. Energy required to do 1 joule of work is called 1 joule energy.

985.

Write true or false for the following statements: Work and energy have different units.

Answer»

False

Explanation: Work is said to be done when a force produces motion. SI Unit of work is Joule and the capability to do that work is called energy. SI unit of energy is also Joule.

Energy required to do 1 joule of work is called 1 joule energy.

Discussion

986.	Write true or false for the following statements: The rate of doing work is called power.
Answer» True Explanation: Yes, the rate at which work is done is called power. Power = \(\frac{Work\,done}{Time\,taken}\) Power is also defined as the rate at which energy is being consumed. SI unit of power is Watt. Power = \(\frac{Energy \,consumed}{Time\,taken}\)

986.

Write true or false for the following statements: The rate of doing work is called power.

Answer»

True

Explanation: Yes, the rate at which work is done is called power.

Power = \(\frac{Work\,done}{Time\,taken}\)

Power is also defined as the rate at which energy is being consumed. SI unit of power is Watt.

Power = \(\frac{Energy \,consumed}{Time\,taken}\)

Discussion

987.	Write true or false for the following statements: Work done by the centripetal force is zero.
Answer» True Explanation: Whenever a body is moving in a circular path, centripetal force is directed towards the center of circle and it acts at right angle to the direction of motion. When force is applied at an angle to the direction of motion of object then the work in the direction of motion is given by: W = F × s cos θ If angle θ is 90º then cos θ will be equal to zero. So, work done will also be zero

987.

Write true or false for the following statements: Work done by the centripetal force is zero.

Answer»

True

Explanation: Whenever a body is moving in a circular path, centripetal force is directed towards the center of circle and it acts at right angle to the direction of motion.

When force is applied at an angle to the direction of motion of object then the work in the direction of motion is given by: W = F × s cos θ

If angle θ is 90º then cos θ will be equal to zero.

So, work done will also be zero

Discussion

988.	Write true or false for the following statements: The unit of work is watt.
Answer» False Explanation: SI unit of work is Joule which is denoted by J and SI unit of power is watt which is denoted by W. 1 joule = 1 newton × 1 meter 1 J = 1 Nm 1 joule of work is said to be done when a force of 1 Newton moves an object through a distance of 1 meter in direction of force.

988.

Write true or false for the following statements: The unit of work is watt.

Answer»

False

Explanation: SI unit of work is Joule which is denoted by J and SI unit of power is watt which is denoted by W.

1 joule = 1 newton × 1 meter

1 J = 1 Nm

1 joule of work is said to be done when a force of 1 Newton moves an object through a distance of 1 meter in direction of force.

Discussion

989.	Does a stationary body possess kinetic energy?
Answer» Kinetic energy of a body is due to its motion or velocity, if the body is stationary then velocity = 0 Kinetic energy =\(\frac{1}{2}\) mv² Substituting value of velocity v in the equation we get kinetic energy = 0 So, a stationary body does not possess kinetic energy.

989.

Does a stationary body possess kinetic energy?

Answer»

Kinetic energy of a body is due to its motion or velocity, if the body is stationary then velocity = 0

Kinetic energy =\(\frac{1}{2}\) mv²

Substituting value of velocity v in the equation we get kinetic energy = 0

So, a stationary body does not possess kinetic energy.

Discussion

990.	Write true or false for the following statements: If we know the speed and mass of an object, we can find out its kinetic energy.
Answer» True Explanation: Kinetic energy KE of a body of mass m moving with velocity or speed v is given by the formula: KE = \(\frac{1}{2}\)mv² The formula suggests that kinetic energy depends on mass and speed of the object. So, if we know the speed v and mass m of an object, we can find out its kinetic energy.

990.

Write true or false for the following statements: If we know the speed and mass of an object, we can find out its kinetic energy.

Answer»

True

Explanation: Kinetic energy KE of a body of mass m moving with velocity or speed v is given by the formula:

KE = \(\frac{1}{2}\)mv²

The formula suggests that kinetic energy depends on mass and speed of the object. So, if we know the speed v and mass m of an object, we can find out its kinetic energy.

Discussion

991.	Write true or false for the following statements: When velocity is halved, its kinetic energy becomes 1/4th.
Answer» True Explanation: Kinetic energy KE of a body of mass m moving with velocity or speed v is given by the formula: KE = \(\frac{1}{2}\) mv² so kinetic energy is directly proportional to the mass of object and to the square of speed of the body. KE₁ = \(\frac{1}{2}\) mv₁² If the velocity of body is halved then kinetic energy KE₂ = \(\frac{1}{2}\) mv₂²where, v₂ = \(\frac{1}{2}\) v₁ Substituting the value of v₂in the formula, KE₂ =\(\frac{1}{2}\)m (\(\frac{1}{2}\) v₁)² = \(\frac{1}{4}\) ( \(\frac{1}{2}\)mv₁² ) KE₂= \(\frac{1}{4}\) KE₁ So kinetic energy will become \(\frac{1}{4}\) if the speed is halved.

991.

Write true or false for the following statements: When velocity is halved, its kinetic energy becomes 1/4th.

Answer»

True

Explanation: Kinetic energy KE of a body of mass m moving with velocity or speed v is given by the formula:

KE = \(\frac{1}{2}\) mv²

so kinetic energy is directly proportional to the mass of object and to the square of speed of the body.

KE₁ = \(\frac{1}{2}\) mv₁²

If the velocity of body is halved then kinetic energy

KE₂ = \(\frac{1}{2}\) mv₂²where, v₂ = \(\frac{1}{2}\) v₁

Substituting the value of v₂in the formula,

KE₂ =\(\frac{1}{2}\)m (\(\frac{1}{2}\) v₁)²

= \(\frac{1}{4}\) ( \(\frac{1}{2}\)mv₁² )

KE₂= \(\frac{1}{4}\) KE₁

So kinetic energy will become \(\frac{1}{4}\) if the speed is halved.

Discussion

992.	Write true or false for the following statements: When an arrow is released from a bow, potential energy changes into kinetic energy.
Answer» True Explanation: When is arrow is pulled from the bow it attains potential energy but when this bow is released it starts moving with speed which means that the stored potential energy in arrow is progressively being converted into kinetic energy.

992.

Write true or false for the following statements: When an arrow is released from a bow, potential energy changes into kinetic energy.

Answer»

True

Explanation: When is arrow is pulled from the bow it attains potential energy but when this bow is released it starts moving with speed which means that the stored potential energy in arrow is progressively being converted into kinetic energy.

Discussion

993.	Calculate the kinetic energy of a body of mass 50 Kg moving with a velocity of 50 cm.
Answer» Given, Mass m = 50 kg Velocity = 50 cms^-1 1 cm = 0.01 m So, velocity = 50 ×0.01 ms^-1 v = 0.5 ms^-1 We know, Kinetic energy = \(\frac{1}{2}\)mv² Substituting value of m and v in the equation: Kinetic energy = \(\frac{1}{2}\) × 50 × 0.5² = \(\frac{1}{2}\) × 12.5 kgm²s^-2 = 6 J

993.

Calculate the kinetic energy of a body of mass 50 Kg moving with a velocity of 50 cm.

Answer»

Given,

Mass m = 50 kg

Velocity = 50 cms^-1

1 cm = 0.01 m

So, velocity = 50 ×0.01 ms^-1

v = 0.5 ms^-1

We know,

Kinetic energy = \(\frac{1}{2}\)mv²

Substituting value of m and v in the equation:

Kinetic energy = \(\frac{1}{2}\) × 50 × 0.5²

= \(\frac{1}{2}\) × 12.5 kgm²s^-2

= 6 J

Discussion

994.	A girl of mass 40 Kg climbs a rope 6 m long at constant speed in 15 seconds. What power does she expend during the climb?
Answer» Given, Mass m = 40 kg Height h = 6m Time = 15 s Power =???? Let g = 10 ms^-2 Potential energy = m × g × h = 40×10×6 J = 2400 J power = \(\frac{Energy\, consumed}{Time\,taken}\) Power = \(\frac{2400\, J}{15\,s}\) Power = 160 J/s So power spent by the girl is 600 watts

994.

A girl of mass 40 Kg climbs a rope 6 m long at constant speed in 15 seconds. What power does she expend during the climb?

Answer»

Given,

Mass m = 40 kg

Height h = 6m

Time = 15 s

Power =????

Let g = 10 ms^-2

Potential energy = m × g × h

= 40×10×6 J

= 2400 J

power = \(\frac{Energy\, consumed}{Time\,taken}\)

Power = \(\frac{2400\, J}{15\,s}\)

Power = 160 J/s

So power spent by the girl is 600 watts

Discussion

995.	A man is drawing water from a well with a bucket which leaks uniformly. The bucket when full weights 20 kg and when it arrives the top only half the water remains. The depth of the water is 20 m What is the work done ?
Answer» Correct Answer - C

Discussion

996.	Which of the following statements is correct?A. Heat is absorbed in exothermic reaction.B. Heat is released in endothermic reaction.C. Energy released in burning 1 litre of gasoline is 300 MJ.D. Chemical energy is associated with the forces that give rise to the stability of substances
Answer» Correct Answer - D In exothermic reaction heat is released In endothermic reaction heat is absorbed. Energy released in burning 1 litre of gasoline is 30 MJ `(=3xx 10^7 J )`.

Discussion

997.	One man takes 1 minute to raise a box to a height of 1 metre and another man takes 1/2 minute to do so. The energy of the two isA. differentB. sameC. energy of the first is moreD. energy of the second is more
Answer» Correct Answer - B Energy required = mgh. In both cases, h is the same. Hence, energy given by both is same. [It is worth noting here that powers of two men will be different a power is the energy expense per unit time and times are different here.]

Discussion

998.	A particle of mass m is lying on smooth horizontal table. A constant force F tangential to the surface is applied on it. Find . (a) average power over a time interval from `t=0` to `t=t,` (b) instantaneous power as function of time t.
Answer» Correct Answer - B (a) `a=F/m="constant"` `v=at=F/m t rArr P_(av)=W/t=(1/2mv^2)/t=((1/2)(m)((Ft)/m)^(2))/(t)` `=(F^(2)t)/2_(m)` (b)`P^(i)=Fvcos 0^@=Fv=(F) ((F ^(2)t)/(m))`

Discussion

999.	A `100g` iron ball having velocity `10m//s` collies with a wall at an angle `30^(@)` and rebounds with the same angle. If the period of contact between the ball and wall is `0.1` second, then the force experinced by the wall isA. 100 NB. 10 NC. 0.1 ND. 1.0 N
Answer» Correct Answer - B

Discussion

1000.	A person trying to lose weight by burning fat filts a mass of `10 kg` upto a being of `1m 1000` time . Assume that the potential energy lost each time be lower the mass is dissipated . How much far will be use up considering the work done only when the weight is lifted up ? Far supplies `3.8 xx 10^(7) J` of energy per kg wich is canverted to mechanical energy with `x 20 %` efficiency rate Take ` = 9.8 ms^(-2)`A. `9.89 xx 10^(-3) kg`B. `12.89 xx 10^(-3) kg`C. `2.45 xx 10^(-3) kg`D. `6.45 xx 10^(-3) kg`
Answer» Correct Answer - B (b)`n = (W)/(input) = (mgh xx 1000 )/(input) = (10 xx 9.8 xx 1 xx 1000)/(input)` ` input = (98000)/(0.2) = 49 xx 10^(4) J` `Fat used = (49 xx 10^(4))/(3.8 xx 10^(7)) = 12.89 xx 10 ^(-3) kg` .

Discussion

Explore topic-wise InterviewSolutions in .

A body falling on the ground from a height of `10m` rebounds to a height of `2.5m`. Calculate (i) the percentage loss of kinetic energy of the body during its collision with the ground. (ii) ratio of the velocities of the body just before and just after collision.

Figure shows a force compression curve of a spring. A body of mass `5kg` moving with the velocity of `8m//s` hits the spring. Calculate the force constant of the spring and also the compression produced in the spring when the body hits it.

Two masses of 1 gm and 4 gm are moving with equal kinetic energies. The ratio of the magnitudes of their linear momenta isA. `4:1`B. `sqrt2:1`C. `1:2`D. `1:16`

Two bodies A and B having masses in the ratio of 3 : 1 possess the same kinetic energy. The ratio of their linear momenta is then

Two bodies of masses `m` and `4m` are moving with equal linear momenta. The ratio of their kinetic energies is `:`

For the force displacement graph shown below. Calculate the work done by the force displacing the body from x=1 cm to x=5 cm.

A `100 g ` block moves along `X-` axis. Its acceleration as a function of displacemetn is shown in figure. Calculate work done (in joule) on the block by the force applied as the block moves from `x=0` to `x=8m`.

A force`vecF=(7-2x+3x^(2))` N is applied on a 2 kg mass which displaces it from x = 0 to x = 5 m. Work done in joule is -

A body moves a distance of 5 m along a straight line under the action of a force 10 N. If the work done is `25sqrt3` joule, the angle which the force makes with the direction of motion of the body isA. `0^(@)`B. `30^(@)`C. `45^(@)`D. `60^(@)`

From figure, find the work done at the end of displacements: (a) 20cm, (b) 40cm, and (c) 80cm.

No work is done when:A. a nail is hammered into a wooden box B. a box is pushed along a horizontal floor C. there is no component of force, parallel to the direction of motion D. none of these

Potential energy of a person is minimum when:A. Person is standing B. Person is sitting on a chair C. Person is sitting on the ground D. Person is lying on the ground

You pay a coolie for carrying your luggage. Does he really do any work?

Give reason for the following: Why a coolie does not work, when he moves on a level road while carrying a box on his head?

When a coil spring is compressed, the work is done in the spring. The elastic potential energy:A. increases B. decreases C. disappears D. remains unchanged

Class 9 Science MCQ Questions of Work, Power and Energy with Answers?

An elastic spring of spring constant k is compressed by an amount x . show that its potential energy is \(\frac{kx^2}{2}\).

The cgs unit of work is:A. joule B. erg C. dyne D. watt

Give reason for the following: Can a body have momentum without having energy? Explain.

Write true or false for the following statements: A force does not work, if it produces no motion.

The work done by a force on a body will be positive if the:A. body does not move B. body moves perpendicular to the direction of motion C. body moves along the direction of the applied force D. body moves opposite to the direction of the applied force

When a force retards the motion of a body, the work done is :A. positive B. zero C. negative D. uncertain

Give reason for the following: Can there be displacement of an object in the absence of any force acting on it?

A weightlifter lifts 240 kg from the ground to a height of 2.5 in 3 seconds. The average power is:A. 1960 W B. 19.6 KW C. 1.96 MW D. 196 W

Write true or false for the following statements: In order to get minimum work, the angle between force and displacement should be 90.

Write true or false for the following statements: Work and energy have different units.

Write true or false for the following statements: The rate of doing work is called power.

Write true or false for the following statements: Work done by the centripetal force is zero.

Write true or false for the following statements: The unit of work is watt.

Does a stationary body possess kinetic energy?

Write true or false for the following statements: If we know the speed and mass of an object, we can find out its kinetic energy.

Write true or false for the following statements: When velocity is halved, its kinetic energy becomes 1/4th.

Write true or false for the following statements: When an arrow is released from a bow, potential energy changes into kinetic energy.

Calculate the kinetic energy of a body of mass 50 Kg moving with a velocity of 50 cm.

A girl of mass 40 Kg climbs a rope 6 m long at constant speed in 15 seconds. What power does she expend during the climb?

A man is drawing water from a well with a bucket which leaks uniformly. The bucket when full weights 20 kg and when it arrives the top only half the water remains. The depth of the water is 20 m What is the work done ?

Which of the following statements is correct?A. Heat is absorbed in exothermic reaction.B. Heat is released in endothermic reaction.C. Energy released in burning 1 litre of gasoline is 300 MJ.D. Chemical energy is associated with the forces that give rise to the stability of substances

One man takes 1 minute to raise a box to a height of 1 metre and another man takes 1/2 minute to do so. The energy of the two isA. differentB. sameC. energy of the first is moreD. energy of the second is more

A particle of mass m is lying on smooth horizontal table. A constant force F tangential to the surface is applied on it. Find . (a) average power over a time interval from `t=0` to `t=t,` (b) instantaneous power as function of time t.

A `100g` iron ball having velocity `10m//s` collies with a wall at an angle `30^(@)` and rebounds with the same angle. If the period of contact between the ball and wall is `0.1` second, then the force experinced by the wall isA. 100 NB. 10 NC. 0.1 ND. 1.0 N