14.1  Work and Power (continued)

What Is Work?

Recall that an object begins moving only when an unbalanced force acts on it. In science, work is the product of force and distance. Work is done when a force acts on an object in the direction the object moves. For example, work is done by the weight lifter when he exerts an upward force to raise the barbell over his head.

Work Requires Motion Though it may seem surprising, the weight lifter in Figure 1 does no work on the barbell as he holds it over his head. Why is that? Because although force is applied to the barbell, the force does not cause the barbell to move.   For a force to do work on an object, some of the force must act in the same direction as the object moves. If there is no movement, no work is done.

Work Depends on Direction The amount of work done on an object, if any, depends on the direction of the force and the direction of the movement. For example, though you would never do this, imagine pulling a suitcase as shown in Figure 2A. Note that all of the force acts in the same direction as the suitcase moves—all of the force does work on the suitcase.

Figure 2  The work done on an object depends on the size of the force acting in the direction of motion and on the distance the object moves. A When force and motion are in the same direction, the work done is maximized.

A force does not have to act entirely in the direction of movement to do work. Look at the forces and direction of motion shown as the suitcase is pulled in Figure 2B. The force acts upward and to the right along the handle, whereas the suitcase moves only to the right along the ground. Figure 2B shows that there is a horizontal portion of the force acting in the direction of motion. Only the horizontal part of the applied force—the part in the direction of movement—does work.

Figure 2B  Only the horizontal part of the force does work to move the suitcase to the right.

  Any part of a force that does not act in the direction of motion does no work on an object. As shown in Figure 2C, the vertical lifting force does not act in the direction of motion. Thus, this vertical force does no work on the suitcase.

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Figure 2C  Because the lifting force is not in the direction the suitcase moves, the force does no work on the suitcase.