a) What is the difference between gravitational potential energy and elastic potential energy? Give one example of a body having gravitational potential energy and other 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.

Hint: a) In this question we are asked the difference between the gravitational potential energy and elastic potential energy. Therefore, in this question we will be discussing the cause of potential energy and their effects. While doing so, we will point out the major difference.
b) We have been given the work done in Joules and the mass of the object. We are asked to calculate the height through which it was lifted. Therefore, we will be using the equation of work done by gravitational force.
Formula used:
W = mgh
Where,
W is the work done,
m is the mass of the object
g is gravitational acceleration
h is the height

Complete answer:
a) We know that gravitational potential energy is obtained when a body is lifted vertically from a surface to some height. If an object is lifted from ground to a certain height the gravitational potential energy will depend directly on the height and the mass of the object. However, the elastic potential energy is the energy due to change in shape or size of the object. The elastic potential energy is the energy obtained by the work done on an object to restore the deformed object to its original shape.
Origin of the gravitational potential energy is the gravitational force acting on an object. While, the electrostatic force in the molecules is responsible for elastic potential energy.

The water stored in a dam is an example of gravitational potential energy. The stretched rubber band is an example of elastic potential energy.

b) We have been given the work done and mass of the object
Therefore, by using the equation for gravitational potential energy
W = mgh
After substituting the given values
We get,
\[784=20\times 9.8\times h\]
On solving
\[h=\dfrac{784}{20\times 9.8}\]
Therefore,
\[h=4m\]
The object will be lifted by 4 meters if the work done on the 20 kg object is 784 J.

Note:
The gravitational potential energy depends on the acceleration due to gravity (g). Acceleration due to gravity is a parameter which depends on the mass of the planet and distance of the object from the centre of the planet. Gravitational potential energy of the object if its position is changed horizontally will be zero.