 # Lakhmir Singh Solutions Class 9 Physics Solutions for Exercise in Chapter 4 - Work And Energy

a) Explain by an example of what is meant by potential energy. Write down the expression for the 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 to that of 3 m/s. Calculate the change in the kinetic energy of the ball. State your answer giving proper units.

a) The energy stored in a body due to either its position or change in shape is known as potential energy. Water stored in a reservoir is an example of potential energy.

PE = mgh

Where,

PE is the potential energy

m is the mass of the body

g is the acceleration due to gravity

h is the height above the surface of the earth

b) The following are the differences between kinetic energy and potential energy:

i) Kinetic energy is defined as the energy in the body due to its motion while potential energy is defined as the energy due to its position.

ii) The kinetic energy of a body when it is at rest is zero whereas the potential energy of a body when it is at rest is not zero.

c) Mass of ball, m = 0.5 kg

Speed, v1 = 5 m/s

Speed, v2 = 3 m/s

KE1 = 1/2mv_1^2

KE2 =1/2mv_2^2

Change in KE = KE1 – KE2

= 4J

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