A swimmer wishes to cross a `500 m` river flowing at `5 km h^-1`. His speed with respect to water is `3 km h^-1`. The shortest possible time to cross the river is.

Answer

A, B

Explanation

a. The vertical component 3 sin theta take him to opposite side.
Distance = 0.5 km
Velocity= 3 sin theta km/h
`Time = Distance/ Velocity = 0.5/(3 sin theta) hr`
`= 0.5/(3 sin theta) xx 60 1/(sin theta) min`
(1 hr = 60 min)`

b. Here vertical component of velocity, i.e. 3 km/hr takes him to opposite.
`:. t= d/v'= 500/(3xx1000)/60`
`= 10 minutes `