Energy is a number that we associate with a system of one or more objects. If a force acts on one of the objects, making it move, then the number changes. After countless experiments, scientists and engineers realised that if the scheme by which we assign these energy numbers is planned carefully, then the numbers can be used to predict the outcome of experiments. However, learning how to use the numbering scheme is not easy. Let us focus on one form of energy - kinetic energy.
1) A bullet fired from a gun can pierce a target on account of the kinetic energy of the bullet.