In order to understand the work-energy theorem, let’s take a few moments to review the term "work." Watch the following video about work and take notes.

Source: eureka 8 work, hongjia zhou, YouTube

Now that you’ve refreshed your understanding of work, let’s look more closely at work in connection to forces and kinetic energy.

When a force is exerted on an object and that force contributes to the object's displacement, that force is doing work on the object. When the work done on an object is added up, 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, force and displacement are both vectors. This means that they have both magnitude and direction, and they can be represented with an arrow. Displacement and force vectors can be broken up into components (usually dx and dy and Fx and Fy). Force contributes to the displacement of an object when the vectors are broken up; each has a component in the same direction (positive work) or opposite directions (negative work).

Look at the following alignments and decide if there is positive work, negative work, or no work in each case. Place the cursor over each image to reveal the answers.