All machines are constructed from these simple machines: lever, pulley, inclined plane, wedge, screw, and wheel and axle (Figure 14).

A machine is a device that is used to transfer energy from one object to another and allow work to be done easily. Consider the bottle opener shown in Figure 16. When we use the opener, we lift the handle, thereby doing work on the opener. The opener lifts the cap, doing work on it. The work we do is called the input work, W_in. The work the machine does is called the output work, W_out.

Work, as you recall, is the transfer of energy by mechanical means. We put work into a machine, in this case, the bottle opener. That is, we transfer energy to the opener. The opener, in turn, does work on the cap, transferring energy to it. The opener is not a source of energy, so the cap cannot receive more energy than you put into the opener. Thus, the output work can never be greater than the input work. The machine simply helps in the transfer of energy from us to the bottle cap.