the lever
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
bicycle
The computer.
The most useful object in the world is a screw. not kidding
labor saving
A machine saves you force, changes distance, and changes direction. Those are the three ways in science that why machines are useful.
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A pulley
The gamma knife is a stationary machine that is most useful for small tumors, blood vessels, or similar targets
Efficiency
90%