14.2 Work and Machines

Two good friends, each one wearing her most glamorous dress, are on their way to a school dance when disaster strikes. Their car gets a flat tire! To fix it, they'll have to remove the flat tire and put on the spare tire. But won't loosening the lug nuts and lifting the car off of the ground require more force than they can exert just using their hands? How can they change the tire?

As shown in Figure 5, machines come to the rescue. Fortunately, all cars come with the tools designed to help you change a tire. With the help of a lug wrench and jack, which are actually simple machines, they'll have no problem changing the tire. Read on to learn how a car jack and a lug wrench alter forces to make changing a tire a relatively easy task.

Machines Do Work

A machine is a device that changes a force. When using the jack, you apply a force to the jack handle. The jack changes this force and applies a much stronger force to lift the car. Because the jack increases the force you exert, it is a machine. Machines make work easier to do. They change the size of a force needed, the direction of a force, or the distance over which a force acts. Both the lug wrench and the jack allow a single person to apply enough force to accomplish tasks they would normally not be able to.

Figure 5 All cars come equipped with simple machines designed to make changing a tire a fairly easy task.

Inferring Does the jack used to lift the car increase or decrease the force applied to it?

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