Question

If he is crossing a river that is flowing at 2.4 km/h, then the direction in which he must row to reach a point directly opposite his starting point is :

• A. 120⁰ with river flow.
• B. 90⁰ with rive flow.
• C. 135⁰ with river flow.
• D. 150⁰ with river flow.

Answer 1

Answer: (A)

120⁰ with river flow.

Answer 2

$\overrightarrow{V}$_{m,g. =} $\overrightarrow{V}$_m,r. + $\overrightarrow{V}$_r,g

thus $\overrightarrow{V}$_m,g is resultant of $\overrightarrow{V}$_m,r and $\overrightarrow{V}$_r,g. To each exactly at the opposite point, net horizontal velocity must be zero. That is possible if

V_m,r sin θ = V_r,g ⇒ sin θ = 1/2 = 30⁰

i.e. 120⁰ with the river flow.