1
问题描述：
1
The 1000 newton weight of a pile driver falls freely from rest; it drops 25 meters,strikes a steel beam,and drives it 3 centimeters into the ground.The speed of the weight just before hitting the beam is ,in meters per second,approximately?
2
The operator of a boat wishes to cross a 5 kilometer-wide river that is flowing the the east at 10 meters persecond.He wishes to reach the exact point on the opposite shore 15 minutes after starting.At what speed and in what direction should the boat travel?
11.2m/s at 63.4 W of N
答
BARRON上的题吧 1能量守恒,1/2mvv=mgh ,即1/2*100kg*v*v=100kg*g*25m ,v为根号下500,所以是22 2意思是要河的正对面,即水平方向没有位移.合速度是垂直于岸的.合速度为 5000m/900s=5.5,船速度=根号下5.5*5.5+10*10 约...