Why is the work output of a machine never equal to the work input?

What if? if the temperature near the cliff suddenly falls to 0°c, reducing the speed of sound to 331 m/s, what would the initial speed of the rock have to be (in m/s) for the soccer player to hear the sound of the splash 2.90 s after kicking the rock?

How does the entropy of steam compare to the entropy of ice?

The built in flash in a compact camera is usally capable of giving correct exsposure for distance up to how many meters?