A tool or machine is composed of one or more of the following mechanical elements: a lever, wheel and axle, pulley, inclined plane, wedge and screw. The force exerted on the mechanical principle is called acting force or power, and that given out is called weight or resisting force. We must bear in mind that none of these simple machines or mechanical elements can generate energy, but that they enable energy to be distributed and utilized to the best advantage. As an illustration, the ability to work hard and without rest varies according to the manner in which a workman applies his force, and the number of muscles he brings into action. In the operation of turning a crank, a man's strength changes in every part of the circle which the handle describes. It is greatest when he pulls the handle upward from the height of his knees, and weakest at the top and bottom of the circle, where the handle is pushed or drawn horizontally.