The correct answer to the question is : C) A pendulum swings back and forth.
EXPLANATION:
Before answering this question, first we have to understand the law of conservation of energy.
As per law of conservation of energy, energy can neither be created not be destroyed. It can only change from one form to another form, and the total energy of the universe is always constant.
As per the question, we have a pendulum which is moving back and forth.
Let us consider a pendulum which is taken to some height from its equilibrium or mean point. The energy possessed by the pendulum at this height is gravitational potential energy. When the pendulum is released, the potential energy is converted into kinetic energy. At mean point, whole of its potential energy is converted into kinetic energy.
Due to inertia, the pendulum reaches at the other extreme point. During its movement from mean point to extreme point, the kinetic energy is converted into potential energy. At extreme point, whole of its kinetic energy must have converted into potential energy. The same process will be repeated.
Hence, it obeys law of conservation of energy.
Hence, swinging of pendulum back and forth is the best example to demonstrate the law of conservation of energy.
