I need help on how to approach this problem and which equations I can use. It was a problem that we were supposed to think about, but even that's difficult. Here it is: If there is a 70 lb. girl roller blading at a constant speed of 5 mph and she suddenly hits a rock and trips downward, what would be the impact force on her knee if it is 1 ft above the ground and the distance traveled after impact is 0.25 in? We had a similar problem without the girl traveling at an initial speed which was simple enough using conservation of energy, potential and kinetic energy, impact velocity, and work. But I'm not sure how the rotational momentum from her fall downward comes into play. Can anyone point me in the right direction? PLEASE!!!