Question 1

Question 44:
Explain by an example that a body may possess energy even when it is not in motion.

Accepted Answer

Answer:
A body can have energy even if it is not in motion. The statement can be justified by:
Water stored in the reservoir at the roof of a building possesses gravitational potential energy due to its position above the ground.

Question 2

Question 45:
(a) On what factors does the gravitational potential energy of a body depend?
(b) Give one example each of a body possessing : (i) kinetic energy, and (ii) potential energy.

Accepted Answer

Answer:
(a) Gravitational potential energy of a body depends on its position above the ground.
     Example - A book kept on a table possesses gravitational potential energy because of its position above the ground.

(b) (i) Kinetic energy - A car moving with some speed on a road possesses kinetic energy.
     (ii) Potential energy- A stretched rubber band possesses potential energy due to the change in its configuration.

Question 3

Question 46:
Give two examples where a body possesses both, kinetic energy as well as potential energy.

Accepted Answer

Answer:
Two examples in which the body possesses kinetic energy as well as potential energy are-
(i) A man climbing a hill
(ii) A bird flying in the sky
In both of the above examples, we can observe that the body has speed and its position is above the ground. Therefore, these bodies have kinetic and potential energy.

Question 4

Question 50:
What type of energy is possessed :
(a) by the stretched rubber strings of a catapult?
(b) by the piece of stone which is thrown away on releasing the stretched rubber strings of catapult?

Figure

Accepted Answer

Answer:
(a) The stretched rubber string of a catapult possesses potential energy, as the work done to change the shape of the rubber string is stored in it.

(b) The stone possesses kinetic energy because on releasing the rubber string of the catapult,  potential energy of the rubber string is transformed into the kinetic energy of the stone thrown.

Question 5

Question 56:
(a) What do you understand by the kinetic energy of a body?
(b) A body is thrown vertically upwards. Its velocity goes on decreasing. What happens to its kinetic energy as its velocity becomes zero?
(c) A horse and a dog are running with the same speed. If the weight of the horse is ten times that of the dog, What is the ration of their kinetic energies?

Accepted Answer

Answer:
(a) The energy possessed by a body by virtue of its motion is known as kinetic energy. It is directly proportional to the mass and square of the velocity of the moving body.

(b) When a body is thrown vertically upwards against the force of gravity, its kinetic energy keeps on decreasing as its velocity decreases. At the maximum height, kinetic energy becomes zero, as the velocity of the body becomes zero at this moment. The kinetic energy of the body is completely transformed in to potential energy at the maximum height.

(c) Kinetic energy of a moving body is directly proportional to its mass. Hence, if the mass of the horse is 10 times the mass of the dog, then,
(Kinetic energy)Dog​(Kinetic energy)Horse​​=110​

Question 6

Question 57:
(a) Explain by an example what is meant by potential energy. Write down the expression for gravitational potential energy of a body of mass m placed at a height h above the surface of the earth.
(b) What is the difference between potential energy and kinetic energy?
(c) A ball of mass 0.5 kg slows down from a speed of 5 m/s so that of 3 m/s. Calculate the change in kinetic energy of the ball. State your answer giving proper units.

Accepted Answer

Answer:
(a) An example to explain the fundamental meaning of potential energy is as follows:
A brick lying on the ground has no energy, so it cannot do any work. If we lift the brick to the roof of a house, the work done in lifting the brick against gravity gets stored in it in the form of potential energy. Thus, the energy in the brick is due to its higher position with respect to the ground, which is known as potential energy.

We can calculate the potential energy of a body of mass (m) lifted by a height (h) as,
Potential energy = (Weight of body) × (Vertical distance)
So,
P.E = m × g × h
Where,
(P.E) - Potential energy
(m) - Mass of body
(g) - Acceleration due to gravity
(h) - Height by which the body is lifted

(b) The difference between potential and kinetic energy are:

Potential Energy    
Kinetic Energy

It is the energy of a body by virtue of its change in shape, size or configuration.
It is the energy of a body by virtue of its motion.

It is independent on the speed of the body.
It is directly proportional to the square of the speed of the moving body.

(c) Mass of the ball, (m) = 0.5 kg
Initial velocity, (v1) = 5 m/s
Final velocity, (v2) = 3 m/s
So, initial kinetic energy can be calculated as,
K.E=21​mv2 
Therefore, initial kinetic energy,
(K.E)1​=21​×0.5×52 J           =6.25 J
Similarly, final kinetic energy,
(K.E)2​=21​×0.5×32 J           =2.25 J
So, Work done = Change in kinetic energy
Therefore work done,
Work done = (KE)2 – (KE)1
                  = (2.25 – 6.25) J
                   = – 4 J
A negative sign shows that the force applied is opposite to the direction of motion of the body.

Question 7

Question 59:
A car is accelerated on a levelled road and acquires a velocity 4 times of its initial velocity. During this process, the potential energy of the car :

(a) does not change
(b) becomes twice that of initial potential energy
(c) becomes 4 times that of initial potential energy
(d) becomes 16 times that of initial potential energy

Accepted Answer

Answer:
Potential energy of an object depends only on its position above the ground and mass.. So, the potential energy of a car traveling on a levelled road does not change.
Therefore, the answer is,
(a) does not change

Question 8

Question 60:
A car is accelerated on a levelled road and attains a speed of 4 times its initial speed. In this process, the kinetic energy of the car :

(a) does not change
(b) becomes 4 times that of initial kinetic energy
(c) becomes 8 times that of initial kinetic energy
(d) becomes 16 times that of initial kinetic energy

Accepted Answer

Answer:
Kinetic energy of a body is directly proportional to the square of the velocity. So, the kinetic energy will become 16 times its initial value if the velocity is increased 4 times.
Therefore, the answer is,
(d) becomes 16 times that of initial kinetic energy.

Chapter 4 : Work and Energy