Question

A boy can swim with a speed of 26m/s in still water. He wants to swim across a 150m river from a point A to point B which is directly opposite the other side of the river. The river flows with a speed 10m/s?

1. If he always swim in the direction parallel to AB, find how far he lands downstream of B.
   2.in what direction relative to the bank must the boy swim so as to cross directly from A to B

Answer 1

Please see below

Explanation:

Note about one thing,why in the `1` st case the pathway of the boy will be along `AC`, because,the boy goes from one side of the river to the opposite side because of its own velocity . At the same time he is being laterally pushed by velocity of river water,again as the two velocity vectors are perpendicular,one can't cancel each other and they give a resultant,so the boy is reaching the opposite bank of the river but with a lateral shift, So,we call `AC = AB +BC`

In the `2` nd case, the boy needs to swim in shown direction aiming to reach left of `B`,so that,here again,due to river flow,he ultimately reaches point `B` due to lateral shift.