A swimmer can swim in still water at of `10ms^(-1)`. While crossing a river his average speed is `6ms^(-1)`. If he crosses the river in the shortest possible time, what is the speed of flow of water?

Options

A. `16ms^(-1)`
B. `4ms^(-1)`
C. `60ms^(-1)`
D. `8ms^(-1)`

Answer

A

Explanation

The resultant velocity of swimmer must be perpendicular to speed of water to cross the river in a shortest time
`:.v_(s)^(2)=v^(2)+v_(w)^(2)`
`v_(w)^(2)=v_(s)^(2)-v^(2)=100-36=64" ":.V_(w)=8m//s`