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Silicon and Gallium. Solar cell produces d.c. current. State whether a solar cell produces a.c. or d.c. current. 

It is not advisable to use wood as fuel even though it can be replenished because burning of wood produces smoke that causes pollution which results in environmental imbalance.

The stability off a body is increased if: 

(i) The centre of gravity is as low as possible. It should be above the base and near the geometric centre of the body. This is the case when mass of the body is large at the bottom. 

(ii) The area of base of the body is large so that the vertical line through the CG of the body passes through its base. If the body is slightly tilted, its CG rises higher and it has a tendency to regain its original position to lower the CG.

Solar cell. Its two uses are: 

(i) All artificial satellites and space probes mainly depend upon electricity generated by solar cells. 

(ii) Solar cells are used for lighting, operating water pumps, for running radio and television sets in remote areas and traffic signals etc.

 Advantages of Solar Cells: 

(i) Solar cells have no moving parts, easy to construct, require little maintenance. 

(ii) Solar cells derive their energy from solar radiations which is a renewable source of energy and also inexhaustible. 

(iii) They are most suitable for the remote inaccessible and isolated places where laying a power transmission line may be expensive and not commercially viable. 

(iv) They are environment friendly as they do not cause any pollution. 

(v) Use of solar cells enables us to save usage of fossil fuels as solar cells require no fuel. 

Disadvantages of Solar Cells: 

(i) The manufacturing cost of solar cells is sufficiently expensive. 

(ii) The efficiency of energy conversion of solar energy to electricity is low. 

(iii) The electricity produced in solar cells is direct current (DC) which cannot be directly used for many household appliances. 

The forces which act on bodies when they are in physical contact, are called contact forces. For e.g. frictional force and force exerted on two bodies during collision.

Advantages: 

(i) Wind is free and will not run not (renewable sources). 

(ii) Wind energy does not create greenhouse gases. 

Disadvantages: 

(i) The wind farms can be established only at places near the coastal areas where wind blows around the year steadily with a speed not less than 15Kmh^-1. 

(ii) The establishment of wind farm is expensive. 

The forces experienced by bodies even without being physically touched, are called the non-contact forces. For e.g. Gravitational force and Electrostatic force.

Work does not depend on time, while the power depends on time. Power is the work done in unit time.

(d) all of these

The blowing wind has kinetic energy is called wind energy. 

Principle: It works on the principle of conversion of kinetic energy of blowing wind into electrical energy. When the blowing wind strikes across the blades of a windmill it exerts a force which rotates its blade. The turning of the blades would simultaneously cause the shaft to turn, which is connected to a generator. This makes the generator produce electricity.

Principle of conservation of energy: It states that energy can neither be created nor be destroyed but can be transferred from one form to another form.

Contact force: (a) frictional force (b) normal reaction force (c) force of tension in a string.

Non-contact force: (d) gravitational force (e) electric force (f) magnetic force.

(c) Electrostatic force

Newton’s Third Law of Motion states that there is always reaction for every action in opposite direction and of equal magnitude.

Explanation: Whenever a force is applied over a body, that body also applies same force of equal magnitude and in opposite direction.

Absolute units of moment of force in 

1. CGS system is dyne-cm. 

2. S.I system is newton-metre(N-m)

Two equal and opposite parallel forces not acting along the same line, form a couple. A couple is always needed to produce the rotation. For example, turning a key in a lock and turning a steering wheel.

NEWTON’S SECOND LAW OF MOTION IN EVERYDAY LIFE:

(a) A fielder pulls his hand backward; while catching a cricket ball coming with a great speed, to reduce the momentum of the ball with a little delay. According to Newton’s Second Law of Motion; rate of change of momentum is directly proportional to the force applied in the direction.

(b) While catching a cricket ball the momentum of ball is reduced to zero when it is stopped after coming in the hands of fielder. If the ball is stopped suddenly, its momentum will be reduced to zero instantly. The rate of change in momentum is very quick and as a result, the player’s hand may get injured. Therefore, by pulling the hand backward a fielder gives more time to the change of momentum to become zero. This prevents the hands of fielder from getting hurt.

1. In CGS system unit of moment of couple-dyne-cm 

2. In S.I. system unit of moment of couple-N-m.

Examples of couple in everyday life : 

1. Steering wheel of a car. 

2. Turning of a screw driver. 

3. Opening and closing of water tap. 

4. Winding the watch. 

Mathematical expression : Moment of couple = force × Arm of couple. 

Moment of force about a given axis = Force x perpendicular distance of force from the axis of rotation.

Moment of force depends on the distance of line of action of the force from the axis of rotation. Decreasing the perpendicular distance from the axis reduces the moment of a given force.

If the turning effect on the body is anticlockwise, moment of force is called anticlockwise moment and it is taken as positive while if the turning effect on the body is clockwise, moment of force is called clockwise moment and is taken negative.

From Boyle’s law P x \(\frac{1}{v}\) (Pressure is inversely proportional to volume) Initial pressure 

P₁ = Atmospheric pressure + Pressure of liquid column 

∴ p₁ = hpg + Hpg 

h = depth of the pond 

H = 10 m of water

V₁ = \(\frac{4}{3}\)πr³

P₂ = Hpg

V₂ = \(\frac{4}{3}\) π(3r)³ = \(\frac{4}{3}\) x 27 r³ x π

Hence by substituting these values,

P₁ V₁ = P₂ V₂

1. Down ward force (mg) = weight of body is more that upthrust (upward force or resistance due to air). 

2. Force of friction due to ground and air on car is greater than the moving force acting on car. 

Effects produced by unbalanced force : 

1. It produces motion in the body. 

2. It can stop a body in motion or it slows down it. 

3. It changes the direction of motion of body. 

W = R and F = F'

If two or more forces act on a body such that their resultant is not zero, the resultant is an unbalanced force. 

[Note: Unbalanced force acting on a body = mass of the body × acceleration of the body.]

Consider a rigid body acted upon by two forces, equal in magnitude, opposite in . direction and having the same line of action. These forces are called balanced forces as their net effect on the body is zero. 

Example: A glass slab kept on a table is acted upon by two balanced forces: (i) the weight of the slab acting downward and (ii) the upward force on the slab due to the table. Their net effect on the slab being zero, the slab remains at rest.

If a body is acted upon by two forces, equal in magnitude, opposite in direction and having the same line of action, the forces are called balanced forces. Here, the net force acting on the body is zero.

Unbalanced forces : “The set of forces acting on a body simultaneously, such that their resultant force is not zero and the body does not continue in the state of rest or of uniform motion in a straight line, the forces are called unbalanced forces.”

 Non-contact force decreases with the increase in separation.

Balanced force : “When a number of forces act on a body simultaneously such that their resultant is zero and hence, the body continues in its state of rest or of uniform motion in a straight line, then the set of forces acting on the body are said to constitute balanced forces.” 

Centripetal force is required for circular motion. It is always directed towards the centre of circle.

Uniform Circular motion : 

1. Speed of body is uniform. 

2. It is accelerated motion. 

3. Velocity of body is variable. 

4. External force is needed. 

Uniform Linear motion : 

1. Speed is uniform. 

2. Acceleration is zero. 

3. Velocity is constant. 

4. No external force is needed.

Characteristics of non-contact forces : 

1. Non – contact forces can pass through any kind of medium. They can pass through vacuum. 

2. Gravitational force is always attractive in nature. 

3. Magnitude of non-contact forces on two bodies is inversely proportional to the square of distance between their centres. 

Yes, a body moving in circular path has accelerated motion with constant speed.

Motion of moon around the earth is example of moon (with uniform speed and accelerated motion). Gravitational force of the earth provides necessary centripetal force. 

When the object moves in a circular path with uniform speed, it means that its magnitude of velocity does not change, only its direction changes continuously. Hence, it is considered as uniformly accelerated motion.

Force F = 50 dynes 

⊥ distance = 75cm 

∴ Moment of force = F × ⊥ distance 

= 50 × 75 = 3750 dynecm

Yes, uniform circular motion has an accelerated motion with a constant speed.

(d) both (b) and (c)

(a) speed  is constant.

When a particle moves with a constant speed in a circular path, its motion is said to be the uniform circular motion. For example: Revolution of earth around sun is an example of uniform circular motion.