Peoples Physics Concepts

8.4. Work http://www.ck12.org

8.4 Work

• Define work and calculate the amount of work done.

Students will learn that work is simply the transfer of energy into or out of a system. Students will learn how to

calculate the work done and how to incorporate it into energy conservation.

Key Equations

W=Fx∆x=F dcos; Work is equal to the distance an object moves multiplied by the component of the force in

the direction that object is moving.

W=work (in Joules; work is just energy being transferred)

Guidance

When an object moves in the direction of an applied force, we say that the force doesworkon the object. Note that
the force may be slowing the object down, speeding it up, maintaining its velocity — any number of things. In all
cases, the net work done is given by this formula:

W=~F·d~=~F·∆~x Work is the dot product of force and displacement.

In other words, if an object has traveled a distancedunder force~F, the work done on it will equal todmultiplied

by the component of~Falong the object’s path. Consider the following example of a block moving horizontally

with a force applied at some angle:

Here the net work done on the object by the force will beF dcosθ.

Example 1

A 1kg ball has been attached to a 2m string and is at rest on a frictionless surface. If you exert a constant force of

10N on the string and pull the ball over the course of 5m and then begin spinning the ball in a circle, after 3

revolutions, what is the total amount of work you have done on the ball?