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Chapter 6: Problem 3

Must you do work to whirl a ball around on the end of a string? Explain.

Short Answer

Expert verified
From a physics perspective, no, you do not do work when whirling a ball around on a string. Although it requires physical effort, the force applied is perpendicular to the motion of the ball, which, in physics, means that no work is being done.

Step by step solution

01

Understand the situation

The first thing to understand is what forces are at play when a ball is being whirled in a circle at the end of a string. There are two forces: the gravitational force pulling the ball downwards, and the tension in the string pulling the ball in a circular path.
02

Identify the force that does work

In physics, work is defined as the product of the displacement of an object and the component of the force applied on the object in the direction of the displacement. When the ball is whirled around in a circle, it does not move up or down (against gravity); it only moves along the circular path. Thus, the gravitational force does not do any work on the ball because it does not cause any displacement. On the other hand, the tension in the string does do work because it is causing the ball to move in the circular path. Note that the tension force is always perpendicular to the path of the ball.
03

Explain the conclusion

Even though the person whirling the ball is exerting a force on the ball through the string, from a physics perspective, no work is being done on the ball. This is because work in physics is only done when a force causes an object to move in the direction of the force, and in this case, the ball is not moving in the direction of the applied force. Therefore, while it might require physical effort from a person to whirl the ball around, from a physics perspective, no work is being done on the ball itself.

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