The reason is because the force due to the acceleration from gravity is constant. It's the same as the typical "dropping a bowling ball and feather (with no air resistance) at the same time". Gravity acts on all object with the same acceleration regardless of physical properties.
Aluminum foil (I think, it may not be right)
Force = -kx
80N=0.15m * -k
K=-80/0.15=533.333. Spring constant
Energy=1/2kx^2
1/2*(-80/0.15)*80^2=Energy
