Work

The teens in the picture on the left are having fun playing basketball. The teen in the picture on the right is working hard studying for an exam. It’s obvious who is doing work—or is it? Would it surprise you to learn that the teens who are working are the ones who are having fun playing basketball, while the teen who is studying isn’t doing any work at all? The reason why has to do with how work is defined in physics.

Defining Work

Work is defined differently in physics than in everyday language. In physics, work means the use of force to move an object. The teens who are playing basketball in the picture above are using force to move their bodies and the basketball, so they are doing work. The teen who is studying isn’t moving anything, so she isn’t doing work. Not all force that is used to move an object does work. For work to be done, the force must be applied in the same direction that the object moves. If a force is applied in a different direction than the object moves, no work is done. The Figure below illustrates this point.

Q: If the box the man is carrying is very heavy, does he do any work as he walks across the room with it?

A: Regardless of the weight of the box, the man does no work on it as he holds it while walking across the room. However, he does more work when he first lifts a heavier box to chest height.

Work, Force, and Distance

Work is directly related to both the force applied to an object and the distance the object moves. It can be represented by the equation:

Work = Force × Distance

The unit for work is the Joule abbreviated with the letter J

This equation shows that the greater the force that is used to move an object or the farther the object is moved, the more work that is done. You can see a short video introduction to work as the product of force and distance below:

To see the effects of force and distance on work, compare the weight lifters in the Figure below. The two weight lifters on the left are lifting the same amount of weight, but the one on the bottom is lifting the weight a greater distance. Therefore, this weight lifter is doing more work. The two weight lifters on the bottom right are both lifting the weight the same distance, but the weight lifter on the left is lifting a heavier weight, so she is doing more work.

Summary

  • In physics, work is defined as the use of force to move an object. For work to be done, the force must be applied in the same direction that the object moves.
  • Work is directly related to both the force applied to an object and the distance the object moves. It can be represented by the equation: Work = Force × Distance.

Vocabulary

  • work: Use of force to move an object; calculated as force multiplied by distance.

Practice

At the following URL, review the meaning of work by reading the first two paragraphs of the article. Then do the quick quiz. Be sure to check your answers and read the explanations.

http://www.physicsclassroom.com/Class/energy/u5l1a.cfm

Review

  1. How is work defined in physics?
  2. Write the equation that relates work to force and distance.
  3. Assume that a friend hands you a heavy book to hold as he turns the combination lock on his locker. Which of you does more work?