Force is among the most fundamental concepts in Newtonian physics. Energy became an important unifying concept in nineteenth-century physics. Energy and force take on somewhat different roles in relativity and quantum mechanics.

Force in Classical Physics

In classical physics, force is a vector quantity. Isaac Newton's second law of motion (F = ma) relates the net force (F) on a body to its mass (m) and acceleration (a) in an inertial reference frame. Newton's third law says that the force exerted by body A on body B is equal and opposite to the force that B exerts on A. To apply Newton's laws of motion in a non-inertial frame, correction factors with the dimensions of force ("pseudoforces") must be introduced, such as the Coriolis and centrifugal forces.

The constituents of a system of bodies (such as a macroscopic object) exert "internal forces" upon one another, whereas "external...