Definition

In science, work happens when a push or pull makes something move; if you push and nothing moves, no work is done, no matter how tired you get. Work is done when a force causes displacement in the direction of the force, $W = Fd$, with $F$ in newtons and $d$ in meters, measured in joules ($1\ \text{J} = 1\ \text{N}\cdot\text{m}$); work transfers energy, so doing work on an object increases its energy by that amount, and a force perpendicular to the motion does no work. In general, work is the scalar product $W = \vec{F}\cdot\vec{d} = Fd\cos\theta$, where $\theta$ is the angle between force and displacement, and for a variable force $W = \int \vec{F}\cdot d\vec{r}$, the area under a force-versus-position graph. The work-energy theorem states $W_{\text{net}} = \Delta K$. Work can be negative when a force opposes motion, as with friction, and the work done by a conservative force equals $-\Delta U$.