A 13.6 kg shelf is held 1.50 m above the ground for 25.0 s while it is screwed in to the wall.
a) What is the potential energy of the shelf with respect to the ground?

b) How much work is done on the shelf in this time?

When the displacement of a mass on a spring is 12a the half of the amplitude, what fraction of the mechanical energy is kinetic energy? at what displacement, as a fraction of a, is the mechanical energy half kinetic and half potential?