person jumps off building 4.0 meters high. when he strikes the ground, he bends his knees so that his torso decelerates over an approximate distance...

person jumps off building 4.0 meters high. when he strikes the ground, he bends his knees so that his torso decelerates over an approximate distance of .70 meters. The mass of his torso excluding his legs is 50kg. What is average force exerted on his torso by his legs during deceleartion.