Potential energy is the energy^* an object has because of its position. Potential energy can be gravitational or elastic. Holding a book in the air gives it gravitational potential energy.

• If you release the book, gravity will do work^* on it.
• It will fall, gaining kinetic energy as it gains speed.
• When it strikes something, it will do work on the object by moving it.
• Gravitational potential energy is directly proportional to the mass^* of the object and its altitude^*.

Drawing back an arrow in a bow gives it elastic potential energy.

• If you release the arrow, the bow string will do work on it.
• It will move through the air as a projectile, losing speed as it climbs higher in the air and gaining speed as it falls back toward the ground.
• When it strikes something, it will do work on the object by moving it.
• Elastic potential energy is directly proportional to both the force^* needed to deform^* the elastic material and the distance it is deformed.

When potential and kinetic energy change from one type of energy to the other, the total amount of energy stays the same. This relationship is called conservation of energy.