Motion in a Plane - Result Question 53

56. A boat which has a speed of $5 km / hr$ in still water crosses a river of width $1 km$ along the shortest possible path in 15 minutes. The velocity of the river water in $km / hr$ is

[2000, 1998]

(a) 3

(b) 4

(c) $\sqrt{21}$

(d) 1

Show Answer

Answer:

Correct Answer: 56. (a)

Solution:

  1. (a)

Speed along the shortest path

$=\frac{1}{15 / 60}=4 km / hr$

Speed of river $v=\sqrt{5^{2}-4^{2}}=3 km / hr$