Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

(a) The kinetic energy when a person lands on the ground will be equal to the potential energy at the height $h$. We can find the height $h$ when the potential energy is 200 J. $PE = 200~J$ $mgh = 200~J$ $h = \frac{200~J}{mg}$ $h = \frac{200~J}{(60~kg)(9.80~m/s^2)}$ $h = 0.34~m$ A person could jump from a maximum height of 0.34 meters without breaking the leg bones. (b) Of course people can jump from a height that is higher than 0.34 meters without breaking the leg bones. This is possible because people bend the knees during the impact in order to absorb the energy over a greater distance. If the impact is spread over a larger distance, then the average force exerted on the bones is decreased. (Remember that Work = Force X Distance)