A pole-vaulter just clears the bar at 4.07 m and falls back to the ground. The change in the vaulter's potential energy during the fall is -3900 J. What is his weight?
PEmax = mg*hmax = mg*4.07 J.
Change = 0-mg*4.07 = -3900
mg = 958.2 N. = Wt. of the vaulter.