Physics

The key is recognizing that total mechanical energy in a spring system is conserved and splits between kinetic and potential forms. At any position $x$, the potential energy is $U = \tfrac{1}{2}kx^2$, which depends on how far you are from equilibrium—not on how fast you're moving. When the block is only one-third of the way to maximum displacement, it's still relatively close to equilibrium where the spring force is weak, so potential energy is small and most of the total energy remains kinetic.

The key is recognizing that total mechanical energy in a spring system is conserved and splits between kinetic and potential forms. At any position $x$, the potential energy is $U = \tfrac{1}{2}kx^2$, which depends on how far you are from equilibrium—not on how fast you're moving. When the block is only one-third of the way to maximum displacement, it's still relatively close to equilibrium where the spring force is weak, so potential energy is small and most of the total energy remains kinetic.