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

What is the mass of the same rock on the earth

The average density of the planet uranus is 1.27 103 kg/m3. the ratio of the mass of neptune to that of uranus is 1.19. the ratio of the radius of neptune to that of uranus is 0.969. find the average density of neptune.

The voltage applied across a given parallel-plate capacitor is doubled. how is the energy stored in the capacitor affected?