# Gravity requires energy, right? Where does this energy come from? What am I misunderstanding?

It's easy to understand that as an object is lifted, it gains gravitational potential energy; as it falls, that potential energy is converted into kinetic energy. But what about when it's at rest? If a book sits on a table at rest, gravity exerts a force on the book. But the exertion of force requires energy, right? And what about the normal force? It exerts force as well; where does the energy to exert THAT force come from? Am I misunderstanding what energy is?

EDIT: Question answered - humans have a flawed understanding of force, and force actually isn't conserved in nature like energy is.

• @Somniad To make it more explicit, for "conservative forces" such as gravity the work done is $\int \mathbf{F}\cdot d\mathbf{s}$, which implies to do work the object must not only move, but move in a direction that's not perpendicular to the force. (Forces are parallel to acceleration, not motion. Magnetism causes charges to move perpendicular to the force, so it does no work on them!) – J.G. Jul 30 '17 at 9:20