work

Work is the physicist's precise name for the transaction between a force and the body it acts on. If a force F acts on a body and the body moves a distance d while the force is applied, the work done is the product of force and displacement — but only the component of the force aligned with the motion counts. A force perpendicular to the displacement does no work at all, which is why gravity does no work on a satellite in a circular orbit and why holding a heavy suitcase stationary costs biochemical effort but no mechanical work.

For constant forces the formula is W = F·d·cos θ. For variable forces, W = ∫F(x)·dx, the integral taken along the path. The SI unit is the joule, one newton-metre. The work–kinetic-energy theorem says that the net work done on a body equals the change in its kinetic energy — a direct consequence of Newton's second law.

Work is the channel through which energy flows into and out of a mechanical system. Work done on a body increases its kinetic energy or its stored potential energy; work done by a body transfers energy from the body to whatever it is pushing against. All of macroscopic mechanical life is the bookkeeping of work flowing between bodies.