Up And Down And Up And Down

Classical Mechanics Level 1

A block is attached to a spring so that it oscillates up and down between A and C.

The total potential energy of the system is given by the sum of its gravitational and elastic potential energy. At what point(s) of the block's motion is the total potential energy of the system greatest?

Details and Assumptions

In case A, the mass starts from rest and the spring is fully compressed.
In case B, the spring is in motion.
In case C, the mass is at rest.