Question

Two boats A and B having same speed relative to river are moving in a river. Boat A moves normal to the river current as observed by an observer moving with velocity of river current. Boat B moves normal to the river as observed by the observer on the ground.

A : To a ground observer Boat B moves faster than A

B : To a ground observer boat A moves faster than B

C : to the given moving observer boat B moves faster than A

D : To the given moving observer the speeds of boat A and B are same

Answer 1

Answer:

option D: To the given moving observer the speeds of boat A and B are same

Explanation:

Let the river be flowing with a speed of x m/s  

And, the speed of the boat w.r.t to water be y m/s.

Since we are given that the boat A is moving normal to the river, as observed by the observer moving with the velocity of the river and boat B is normal to the river, observed from the ground.

So,  

The speed of boat A w.r.t the ground = $\sqrt{x^2 + y^2}$

Also,  

The speed of boat B w.r.t the ground = $\sqrt{x^2 + y^2}$

Thus we can clearly see that, the speeds of boat A and B w.r.t the given moving observer are same.

Answer 2

Ans = D

to given moving observer the speeds of boat A and B are same