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Mechanical Efficiency

Mechanical efficiency is the measure of the effectiveness of the performance of a mechanical system. Efficiency of a machine measures the useful work done by the machine in comparison to the amount of work needed to operate the machine. Useful work is the work that the machine is designed to do. For example, a bicycle is designed to move a certain distance. The useful output of the bicycle would be the bicycle's motion.The non-useful output would be the heat given off by the bicycle. But why are these other non-useful forms of energies created?
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Work Done By Friction

Whenever a machine does work, part of the energy is given off as heat. For example, when a bicycle moves forward a certain distance, the wheels on the bicycle also move. Friction is created when the two wheels rotate on their shafts. Since the friction is applied over the distance that the bicycle moved, work is done by the force of friction. The work done by friction then transforms into heat when the two wheels turn. Therefore, the bicycle must now input extra work to compensate for friction. This is also why the output force never exceeds the input force.
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Efficiency is improved when friction is reduced. This is because the machine uses less input to produce the same amount of output work, since there is less friction working against the input force. An ideal machine would have no friction, thus all input work work would be transferred into output work. However, an ideal machine is a type of perpetual-motion machine, therefore it would never exist since there will always be friction.

Below is a video explaining mechanical efficiency.
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