A 2.8-kg block is hanging stationary from the end of a vertical spring that is attached to the ceiling. The elastic potential energy of the spring/mass system is 1.2 J. What is the elastic potential energy of the system when the 2.8-kg block is replaced by a 5.4-kg block?

Answer 1

The elastic spring energy is related to distance stretched squared. But distance stretched is proportional to force.

Here force doubles, so distance stretched doubles, so PE in the spring doubles squared.