Question

which statement best describes why a machine is useful?
a machine does a greater amount of work than the amount of work that is done on it.
a machine does a smaller amount of work than the amount of work that is done on it.
a machine can decrease both the input force and the input distance needed to do a given amount of work.
a machine can decrease either the input force or the input distance needed to do a given amount of work.

Explanation:

Machines work on the principle of mechanical advantage. They can't create more work than input (due to conservation of energy). They can change the way work is done by reducing either the input force or input distance for a given work - output.

Answer:

A machine can decrease either the input force or the input distance needed to do a given amount of work.