A boat man can row with a speed of ` 10 km h^(-1)` in still water. If the river flows steadily at ` 5 km //h`, in which direction should the boatman row in order to reach a point on the other bank directly opposite to the point from where he stared ? The width of the river is ` 2 km`.

Options

A. in a direction inclined at `120^@` to the direction of river flow.
B. in a direction inclined at `90^@` to the direction of river flow.
C. ` 60^@` in the north-west direction.
D. should row directly along the river flow.

Answer

A

Explanation

Refer figure . The boat man should go along OC in order to cross the river straight (i.e , along OB)
`sin theta=(CB)/(OC)=v_r/v_b=5/10=1/2= sin 30^(@) , " or "theta= 30^@`
`:.` Boat man should go along in a direction inclined at
`= 90^(@) +30^(@) = 120^(@)` to the direction inclined at the direction of river flow.