Answer:
work lost due to friction
Explanation:
The work output of any real machine is usually less than the work needed to operate the machine due to the fact that some input work is lost due to friction.
Frictional forces are those forces that oppose the motion of 2 surfaces sliding against each other and this is one of the reasons machines do not work at 100% efficiency. When the opposition happens, some of the input work is converted to heat.
