A 60 meter wide river has a current of 9 m/s East. A boat with a constant speed of 15m/s in still water is able to go straight across the river from the North shore to the South shore without drifting downstream by aiming West of North. Aiming in any direction, what is the minimum time needed for the boat to cross the river?

Question

anonymous · Answer

You need to use pythagorous to find the hypotenuse of your velocity vectors. 9m/s can be you opposite side of a right triangle and 15 m/s can be you adjacent side. Solve for the hypotenuse and that is the speed needed provided you boat is angled west of north.

anonymous · Answer

You wouldn't want to aim west of north though. If you wanted to get across as fast as possible you would want as great of southern velocity as possible, so you would aim straight south. The river will carry the boat downstream some, but will not affect the boat's southern velocity. So the minimum time to cross the river will be 
60 m/ (15 m/s)=4 s