Question

If a boat can have a speed of 4 km/hr in still water, for what values of speed of river flow, it can be managed to row boat right across the river, without any drift?

• A. ≥ 4 km/hr
• B. Greater than zero but less than 4 km/hr
• C. Only 4 km /hr
• D. None of these

Answer 1

Answer: (B)

Greater than zero but less than 4 km/hr

Answer 2

Drift (∆x) = (v_{b, x}) ∆t

= (v_br cos θ + v_r)∆t

for ∆x = 0,

v_r = -v_br cos θ

⇒ (v_r)_max = v_br

For, v_r > v_br we can not have zero drift. Hence (2) is correct.