Question

(i) 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.
(ii) What is the difference between potential energy and kinetic energy?
​(iii) 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 kinetic energy of the ball. State your answer giving proper units.

Answer 1

(i) The energy possessed by a body by virtue of its position with respect to the earth is known as the potential energy of the body.

Example: A body placed at a height with respect to the ground have the potential energy.

The potential energy of a body of mass 'm' placed at a height 'h' above the surface of the earth = mgh (Where 'g' is the acceleration due to gravity)

(ii) The energy possessed by a body by virtue of its position with respect to the ground is known as the potential energy of the body. Whereas, the energy possessed by a body by virtue of its motion is known as the kinetic energy.

(iii) Mass of the ball = 0.5 kg

Initial speed of the ball, u = 5 m/s

Initial kinetic energy = $\frac{1}{2}m{u}^2=\frac{1}{2}\times 0.5\times (5{)}^2=\frac{{\displaystyle 25}}{4}\mathrm{J}$

Final velocity of the ball, v = 3 m/s

Final kinetic energy of the ball = $\frac{1}{2}m{v}^2=\frac{1}{2}\times 0.5\times (3{)}^2=\frac{9}{4}\mathrm{J}$

Change in kinetic energy = Initial kinetic energy $-$ Final kinetic energy = $\frac{25}{4}\mathrm{J}-\frac{9}{4}\mathrm{J}=4\mathrm{J}$