Machine Effectiveness The percentage of effort put into a machine by the user (input work) that is converted into work done by the machine is known as efficiency (output work). Because some of the input work is utilized to overcome friction, the output work is always less than the input work. As a result, efficiency is never 100 percent. For example, if 10 pounds of force are required to lift a one-pound weight one foot high, then the efficiency of lifting such a weight is 10/1, or 100%. The amount by which efficiency can exceed 100 percent depends on how much friction there is in the system. If the resistance to movement is very low, more than 100 percent efficiency is possible. For example, if five pounds of force are needed to lift a two-pound weight one foot high, then the efficiency would be 200 percent. In practice, however, it is difficult to achieve efficiencies higher than 100 percent due to limitations of human strength. Also, in many systems friction does not allow for high efficiencies. That is why most machines are inefficient compared with natural processes like humans. For example, a person can walk up steep hills without getting too tired because their body adapts to the stress caused by gravity, but most machines must be given time to rest before being used again because they consume energy even when they are idle!