Prime movers convert natural forms of energy, such as those found in streams, wind, and potentially in fuels, into mechanical energy. Similarly, electric machines convert mechanical to electrical energy in such devices as generators and alternators, or convert electrical to mechanical energy in electric motors and loudspeakers. Mechanical machines, the focus of this article, modify mechanical energy to convert limited input forces and motions into those required to perform specific work. The manner in which a machine modifies and transmits motion is called a mechanism. Work Work is defined as force multiplied by the distance over which the force acts.
Work is measured in foot-pounds.
Since very few humans can lift 300 pounds directly, a device must be employed to modify the required effort into something manageable. If the ramp were 10 feet long and friction forces were negligibly small, then 75 pounds of force would be required to roll the refrigerator up the ramp. A device that decreases the necessary applied force while increasing the distance over which the smaller force acts is called a force multiplier. Machines can also multiply speed and distance. A broom is an example of a speed and distance multiplier because it converts input force and distance at the handle into a lower force and larger distance at the sweeping end.