## Essential University Physics: Volume 1 (3rd Edition)

Potential energy can be negative, for where the ground is can be considered an arbitrary definition. For instance, if I were to say that the ground was the floor while a different person said the ground was the top of a table, neither of us would be wrong. Therefore, anything below whatever is defined to be ground level has negative potential energy. Kinetic energy, however, equals $mv^2$. $v^2$ and can never be negative, for it is squared, and mass can never be negative, so kinetic energy will never be negative.