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Given, Velocity (v) = 2 m/s 

Mass of lorry = 4000 kg, Mass of goods on the lorry = 20000 kg 

Therefore, Total mass (m) on the lorry = 4000 kg + 20000 kg = 24000 kg 

Momentum (p) = ? 

We know that, Momentum (p) = Mass (m) × Velocity (v) 

Therefore, p = 24000 kg × 2 m/s 

Or p = 48000 kg m/s 

Thus, the momentum of the lorry = 48000 kg m/s. 

Static friction > Sliding friction > Rolling friction.

This is to let our CG fall within the base of our support.

Given, Velocity of the bullet (v) = 0.1 m/s 

Mass of the bullet (m) = 25 g = 25/1000 kg = 0.025 kg 

Momentum (p) = ? 

We know that, Momentum (p) = Mass (m) × Velocity (v) 

Therefore, p = 0.025 kg × 0.1 m/s 

Or p = 0.0025 kg m/s 

Thus, the momentum of the bullet = 0.0025 kg m/s.

Agent exerting the force is the child himself (muscle of the child). The object on which the force is acting is the child himself. The force is moving the child from a stationary object.

Agent exerting the force is human fingers. The object on which the force is acting is the medicinal tube. The force is changing the shape of the tube and moving the oinment out of the tube.

Agent exerting the force can be a machine (electric power) or human. The object on which the force is acting is the orange. The force is moving the orange and changing the shape of it.

Gravitational force is applicable on all object and it attracts towards earth, while magnetic force is applicable only on magnetic objects.

Correct option is A) Magnetic

A) A force can change the mass of an object.

Difference between mass and weight Mass and weight mean completely different things in physics. The amount of matter that an object contains constitutes its mass. The gravitational force acting on this mass is called the weight of the object. Mass of an object always remains the same, while the weight of the object varies when it is subjected to different gravitational forces. For example, the weight of an astronaut on the earth is six times her/his weight on the moon as the gravitational force of the earth is six times more than the gravitational force of the moon. But the body mass of the astronaut remains the same in both places.

C) I, II and IV only

C) Objects have weight because of frictional force between them and the Earth.

Correct option is A) Increases

Friction is a force that opposes relative motion between two bodies/surfaces in contact with each other. Although friction opposes motion, it also makes motion possible.

 Friction helps us do many things like : 

1. It helps us in walking. In the absence of friction, we may slip or M. 

2. Cars and buses are able to move safely on roads due to the friction between the treaded tyres of these vehicles and the surface of the road. 

3. Friction allows us to hold objects with our hands. 

4. It allows us to write on a paper. 

5. Friction produces heat. In winter, when we rub our cold hands, the heat produced due to friction makes our hands warm.

Pressure P = hdg. So pressure can be depend on height of air column, density of air and acceleration due to gravity. So pressure can be increased by increasing length of air column at a particular place and for a particular gas.

Characteristics of force of friction are : 

1. Friction opposes the direction of motion. 

2. It depends on the nature of the surface that are in contact. Rough surfaces offer greater degree of friction as compared to smooth surfaces. More the roughness of a surface, greater is the friction. 

3. Friction depends upon the weight of the object – greater is the weight of the object, greater is frictional force it faces. 

4. Friction is independent of area of surface in contact, when the weight remains unchanged.

Friction is due to the roughness of surfaces. It can be reduced by the following methods.

– By polishing the rough surface. 

– By applying lubricants like oil and grease to the surfaces. 

– By spreading powder on the rough surface like carromboard. 

– By using soap and detergent solutions, etc. 

Experiment: The rusted pulley makes a rattling noise. Apply some oil in between the wheel and axle. The sound is reduced and now it works smoothly.

Ways by which friction can be increased are : 

1. Tyres of all vehicles have grooved designs or treads to make their surface rough. Roughness of a surface increases friction and prevents vehicle from slipping or skidding on a wet or muddy road. 

2. Spikes in the shoe soles used by players and athletes increase friction and they get firm grip on the ground. The soles of footwear are also provided with grooved designs to increase friction and avoid slipping. 

3. The handles of motorbikes are covered with spiked rubber to provide a good grip and have better control in holding the handles.

Disadvantages of friction: 

1. Friction produces heat which damages the moving parts of a machine. 

2. Friction produces wear and tear on the contacting surfaces. This reduces the life of machine parts, tyres and shoe soles. 

3. A lot of energy is wasted due to friction to overcome it before moving., 

Ways to reduce friction : 

1. Providing ball bearings or wheels between the moving parts of machine or vehicles reduce friction and allow smooth movement as rolling friction is less than sliding friction.

2. Oiling or lubricating (with graphite or grease) the moving parts of a machine reduces friction. Fine powder like talcum powder also works as a lubricant to reduce friction. 

3. Polishing the rough surface reduces friction offered by it. 

4. Streamlining (giving special shape to experience minimum drag) the bodies of aeroplane, cars, boats and ships help reduce drag (fluid friction) while travelling through air or water.

Force: Force is a physical cause that changes or may tend to change the state of rest or the state of motion of an object. The S.I. unit of force is Newton.

(a) False

(b) True

(c) True

(d) False

Force of tension in the thread provides the centripetal force.

A planet moves around the Sun in a circular path and the gravitational force of Sun acts on planet and provides necessary centripetal force.

Force acting on a body which is in circular motion is called centripetal force. It acts towards the centre of circular path.

 Ideal machine has 100% efficiency i.e., work done on the machine = work done by the machine.

(i) Total anticlockwise moment about O

= 150 gf × 40 cm = 6000 gf cm

(ii) Total clockwise moment about O,

= 250gf × 20 cm = 5000 gf cm

(iii) The difference of anticlockwise and clockwise moment = 6000 – 5000 = 1000gf cm

(iv) From the principle of moments,

Anticlockwise moment = Clockwise moment 

To balance it, 100gf weight should be kept on right hand side so as to produce a clockwise moment about the O. Let its distance from the point O be d cm. Then,

150gf × 40 cm = 250gf × 20 cm + 100gf × d

6000gf cm = 5000gf cm + 100gf × d

1000gf cm = 100 gf × d

So, d= 1000gf cm/100gf= 10 cm on the right side of O.

Let the 50 gf weight produce anticlockwise moment about the middle point of metre rule .i.e, at 50cm.

Let a weight of 100gf produce a clockwise moment about the middle point. Let its distance from the middle be d cm. Then, according to principle of moments,

Anticlockwise moment = Clockwise moment

50gf × 50 cm = 100gf × d

So, d = 50 x 50/100 = 25cm from the other end

Anticlockwise moment = 40gf × 40 cm

Clockwise moment = 80 gf × d cm

From the principle of moments,

Anticlockwise moment = Clockwise moment

40 gf × 40 cm = 80 gf × d

So, d = 40gf x 40/80 = 20cm to the righte of point O.

The SI unit of pressure is N/m² , also called the pascal.

The lactometer is used to determine the purity of a sample of milk.

The hydrometer is used to determine the density of a liquid.

The figure shows that THE LIQUID PRESSURE INCREASES WITH THE HEIGHT OF THE LIQUID COLUMN ABOVE IT.

Yes force of gravitation exist between two astronauts in space.

Force of gravity. No, without the parachute his speed will be higher.

FACTORS are : (hdg)

(i) h — Depth of the point below free surface.

(ii) d — Density of liquid.

(iii) g — Acceleration due to gravity, (constant)

While standing, the area touching the floor is less, the pressure exerted will be more. While walking, area touching the floor is more and pressure exerted will be less.

Towards east.

Oiling or lubricating.

True Gases exert pressure in all directions.

When the box is sliding, the contact points on its surface do not get enough time to lock in to the contact points on the floor. So friction is less when compared with box at rest so we required to apply lesser force to maintain motion.

Frictional force (friction)

Because smooth surface has very less friction.

Can use pencils as rollers under the wooden block.

1. The maximum force exerted by a surface on a body so long as it remains stationary is called the force of static friction. 

2. The minimum force required to keep the body moving over a surface such that it moves equal distances in equal intervals of time is called the force of sliding friction.. 

3. The minimum force required to roll a body on a surface is called the force of rolling friction.

The force of friction is offered by all the three states of matter – solids, liquids and gases.

The can gets gradually crushed. 

[Note: The steam inside the can condenses to form water as the can cools. Therefore, the pressure inside the can becomes much less than the external pressure of the air. Hence, the can gets crushed.]

If you pour some oil onto a floor, the friction would

2. decrease