posted by Elaine Jetton on .
A rowboat crosses a river with a velocity of 3.42 mi/h at an angle 62.5° north of west relative to the water. The river is 0.805 mi wide and carries an eastward current of 1.25 mi/h. How far upstream is the boat when it reaches the opposite shore?
north speed = 3.42 sin 62.5 = 3.03
time = .805/3.03 = .265 hours
west speed = 3.42 cos 62.5 - 1.25
= .329 mi/hr
distance west = .329*.265 = .0872 miles
By the way to cross a river fast, forget where you end up downstream and row or swim directly across in direction, not trying to get the right component upstream to end up across from where you started. This is important if swimming and tired. Do not fight the current.