Question

What is gravitational potential energy? Explain with the help of an example.

Answer 1

Hint : The potential energy is related to the work done on an object when it is displaced against a conservative force. It is always defined with respect to a reference point.

Complete step by step answer:
We know that the potential energy of an object is the energy stored within it due to its configuration in a conservative force field. We know that the conservative forces are the forces by which the work done is independent of the path. The examples of these conservative forces include spring force, electrostatic force, magnetic force etc. The gravitational force is also one of these conservative forces.
Now, the gravitational potential energy, as other kinds of potential energies, is defined with respect to a reference point. The reference point is the point in the force field, where the value of potential energy is assumed to be zero.
So the gravitational potential energy at a point is defined as the work done by the external force to move an object from the reference point to that point, against the force of gravity. That work done is stored as the gravitational potential energy of the object. We know that the force of gravity always acts vertically downwards. So the gravitational potential energy increases when the object is moved upwards, and decreases in the opposite case.
For example, when the bob of a pendulum is at the mean position, then its entire mechanical energy is in the form of the kinetic energy. As it travels from the mean position to either of its two extreme positions, then this kinetic energy begins to be converted to its potential energy. When it reaches the extreme position, the mechanical energy is entirely potential.
Let a point A be selected as the reference point. Also, let B be a point situated at a height of $ H $ above the ground. Then the potential energy of a particle of mass $ M $ at the point B with respect to the point A is $ {U_{BA}} = MgH $ , where $ g $ is the acceleration due to gravity.

Note:
We should not ignore the concept of the reference point. The potential energy has no meaning without the reference point. This is because the absolute value of the potential energy at a point is not defined.