# by a Constant Force

W = F s cos

Units?

[W] = N-m = J = joule

named for James Prescott Joule

In the simplest case, when the force F and the displacement s are parallel, this definition of the work becomes

W = F s

This is also an convenient way to recall the units; work is measured in N-m which is called joules ( J ).

But, in general, work is the component of the the force parallel to 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 parallel to the displacement multiplied by the displacement.

That means that a force perpendicular to the displacement does no work. As a block slides along a plane, the normal force does no work on it.

If the force is in the opposite direction of the displacement ( so the angle is 180o ) the work done by that force is negative ( of course, the work is negative for 90o < < 270o ). For the opposite direction as 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).