Setting the Hooke's law force equal to the weight defines and equilibrium
position where net force is zero. When the man steps onto the spring, he
begins to move down, and in fact passes the equilibrium position at a speed
which is the maximum speed in this problem. He "overshoots" and comes
INSTANTANEOUSLY to rest at an equal distance below the equilibrium position.
At this lowest point, the net force is NOT zero but is rather pushing upward
to give a net upward acceleration.
Jim Goff
Pima Community College
Tucson, AZ
JGOFF@PIMACC.PIMA.EDU