Energy can be thought of as the capacity to do work. Potential energy is energy that a motionless object has by virtue of its position. A rock poised at a great height has gravitational potential energy because it can do work while falling, if harnessed to a suitable apparatus. Galileo's cannonball had considerable potential energy before he dropped it (see cartoon).

Two separated positive and negative charges have electrostatic potential energy, for they can do electrical work as they come together again. A bar magnet turned crosswise to a magnetic field has magnetic potential energy. Work is required to turn the magnet out of alignment with the field, and this work can be regained when the magnet aligns itself with the field once more.

Kinetic energy is the energy that an object has because of its motion. Work is required initially to get the object going, and this work can be obtained again when the projectile collides with a target and comes to rest. If the moving object has mass m, and is moving with velocity v, its kinetic energy of motion is

E = �mv²