As an archer pulls back on the bowstring to set up her shot, she is doing work on that bowstring by applying a force that causes the displacement. But, why does she do that? To give the bow the ability to shoot the arrow forward toward the target. The work that the archer does gives something to the bowstring so that it is able to do work on the arrow. In physics, we define this as energy. Energy is the ability to do work. A substance with a lot of energy can do a lot of work and a substance that has no energy can do no work.

The work that the archer does on the bowstring gives the bowstring elastic potential energy. It is one of three types of mechanical energy which is the energy in an object due to its position or motion. The elastic potential energy in the stretched bowstring is transferred to another type of mechanical energy called kinetic energy. By doing work on the arrow, the arrow speeds up and is given energy due to its motion. As the arrow shots out and falls to the ground, the Earth does work on it to change both its kinetic energy (by speeding it up in the vertical direction) and its gravitational potential energy (the energy due to its height). In this lesson, we will focus on gravitational potential energy and kinetic energy as well as the conservation of that mechanical energy within a system.