[填空题]
A person jumps from the roof of a house 3l5qwi.q l+.yz ) y)nfz.d3-slh67jva+ liyxs +9-m high. When he strikes the ground below, he bends hhj7ss l 6i+ vax+d3ly-is knees so that his torso decelerates over an approximate distance of 0.70 m. If the mass of his torso (excluding legs) is 42 kg, find
(a) his velocity just before his feet strike the ground, and
(b) the average force exerted on his torso by his legs during deceleration.