This lesson will introduce the idea of work and kinetic energy and investigate the relationship between them.

When a force is exerted on an object and that force contributes to the object's displacement, that force is doing work on an object. When we add up all the work done on an object, that total is called the net work. The net work on an object will cause a change in its kinetic energy which means that the work will cause the object to speed up or slow down. This relationship is called the work-kinetic energy theorem.

As mentioned above, work requires both a force and a displacement. The tricky part is that the force has to contribute to the displacement.

Remember that force and displacement are both vectors which means that they have both magnitude and direction and they can be represented by drawing an arrow. In Module 2, we saw that displacement and force vectors can be broken up into components (usually dx and dy and Fx and Fy). You can say that the force is contributing to the displacement of an object when you can break up the vectors so that they each have a component in the same direction (positive work) or in opposite directions (negative work).


Interactive exercise. Assistance may be required. Take a look at the following alignments and decide if there is positive work, negative work or no work in each case. Hold the mouse pointer still over each image to reveal your answers.

force and displacement parallel Positive work Close force and displacement perpendicular No work Close
force and displacement at an acute angle to each other Positive work Close force and displacement at an obtuse angle to each other Negative work Close