Work Done

by a Constant ForceW = F s cos Units?

[W] = N-m = J = joule named for James Prescott Joule

In the simplest case, when the force

Fand the displacementsareparallel, this definition of the work becomesW = F s This is also an convenient way to recall the

units; work is measured inN-mwhich is calledjoules ( J ).But, in general, work is the component of the the force

parallelto the displacement multiplied by the displacement,W = F s cos

W = F s cos

You can think of work as the product of the component of the force

parallelto the displacement multiplied by the displacement.That means that a force

perpendicularto the displacement doesno work. As a block slides along a plane, thenormal forcedoesno workon it.

If the force is in the

opposite directionof the displacement ( so the angle is 180^{o}) the work done by that force isnegative( of course, the work is negative for 90^{o}< < 270^{o}). For theopposite directionas shown in the sketch here,W = - F s We will see that positive work on an object increases its speed (and Kinetic Energy). Likewise, negative work on an object

decreases its speed (and Kinetic Energy).

ToCScalar ProductReturn to ToC, Ch7, Work and Energy(c) Doug Davis, 2001; all rights reserved