Question

(a) What is the difference between gravitational potential energy and elastic potential energy? Give one example of a body having gravitational potential energy and another having elastic potential energy.
(b) If 784 J of work was done in lifting a 20 kg mass, calculate the height through which it was lifted. (g = 9.8 m/s²)

Answer 1

(a) We can differentiate between gravitational and elastic potential energy as -

Gravitational potential energy	Elastic potential energy
A body possesses gravitational potential energy if it is placed any height above the ground. (ground is considered as the refence level)	A body possesses elastic potential energy if its shape is changed by squeezing, pushing or pulling.
Gravitational force is responsible for this energy.	Elastic property of the body is responsible for this energy.

Example:
(i) Gravitational potential energy - A book kept on a table possesses gravitational potential energy because of its position above the ground.
(ii) Elastic potential energy - The stretched rubber string of a catapult possesses elastic potential energy because of the change in shape of the string.

(b) Mass of the box (m) = 20 kg
Acceleration due to gravity (g) = 9.8 m/s²
Work done against gravity (W) = 784 J
Let the height above the ground be (h)
So, W = m × g × h
So, height can be calculated as,
$$\begin{gathered} h=\frac{W}{m\times g} \\ \mathrm{So}, \\ h=\frac{784}{20\times 9.8}m \\ =\frac{784}{196}m \\ =\boxed{4\mathrm{m}} \end{gathered}$$
So the body is lifted to a height of 4 m.