[填空题]
A person jumps from the roof of w+s .:)snznsbxt 4d2ga house 3.9-m high. When he str cq-.zxp*nztv) )j7w dikes the ground below, he bends his knees so that his torso decelerates over an approximan-w )xtz)7*.z dpvcqj te 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.
