An average man can reach a maximum height of about 60 centimeter when jumping straight up from a crouched position. During the jump itself, the person's body from the knees up typically rises a distance of around 50 centimeter. To keep the calculations simple and yet get a reasonable result, assume that the entire body rises this much during the jump.
In terms of this jumper's weight W, find what force does the ground exert on him or her during the jump?