Question

SAT物理两道题目 thanks!!

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?
答案22
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?
答案 11.2m/s at 63.4 W of N

Answer 1

1

自由落体：

V=√(2gh)=√(2*9.8*25)m/s≈22m/s

2

“合速度”垂直“河岸”。

V合=d/t=(5000)/(15*60)≈5.56m/s

船在“静水中”速度：

V船=√(V水^2+V合^2)

=√(10^2+5.56^2)

≈11.4m/s

“船头”与“垂直河岸”夹角为θ：（北偏西）

sinθ=V水/V船=10/11.4≈0.877

θ≈61.3°