(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A girl can row a boat at 3 m/s in still water. She wishes to cross a 165m wide river in which the current flows at 1.5 m/s. Find the direction in which the boat must be steered in order to cross the river:

a) by the shortest possible route

b) in the shortest possible time

2. Relevant equations

3. The attempt at a solution

The answer from my book says that for

a) In order to cross the river by the shortest possible path, the actual path must be at right angles to the banks, which is θ=cos^{-1}1.5/3 = 60º

b) in order to cross in shortest possible time, the boat must be steered at right angles to the bank, i.e θ=90º.

But i don't really understand. Can anyone elaborate more for me...?

Why in order to cross the river by the shortest possible path, the actual path must be at right angles to the banks?

and why in order to cross in shortest possible time, the boat must be steered at right angles to the bank?

Please guide me.

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# Vector - Need explanation

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