Work done

The work done by a force is when the force moves an object through a distance, a transfer of energy. $w = Fs$ where $w$ is work done, $F$ is the force acting on the object and $s$ is the distance the object was displaced.

Example

Given the following system moves 3.5m, and the work done on the object is 28J, find the force of friction.

$$\begin{align} w &= Fs\\ 28 &= (60\cos(40) - F) \times 3.5\\ 8 &= 60\cos(40) - F\\ 8 - 60\cos(40) &= -F\\ F &= -37.963\\ &= 38.0 \end{align}$$