A machine saves you force, changes distance, and changes direction. Those are the three ways in science that why machines are useful.
What the most useful machine is, is a matter of opinion. Some would argue that the wheel is the most useful machine, and others would argue that the lever is the most useful.
the lever
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the lever
bicycle
Efficiency
90%
The computer.
Machine guns are useful for killing people. They are also useful for aiming bigger weapons; since machine guns fire tracer ammo, a machine gunner can fire on a target so the forward observer can train guns onto it.
It depends on the efficiency of the machine. No machine is perfect and there are always losses and waste. Usually the losses are a greater percentage than the useful energy.
The efficiency of a machine is typically calculated as the ratio of useful work output divided by total work input, then multiplied by 100 to get a percentage. For example, if a machine outputs 80 units of useful work for every 100 units put in, the efficiency would be 80%.
The efficiency ratio for a machine usually refers to the ratio of the useful energy available from a machine and the energy put into it.