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

Explanation

Refer to Fig. 2 (c ) . 84. (b) , the velocity of river, ` v_r = 5 k//m along ` (OA). The velocity of boatman, ` v_b = 10 km//h`. The boatman can reach the other bank directly opposite to the point from where be has strated, if he goes along (OB) so that the resultant velocity may act along (OC).
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Let ` alpha` be the angle which the boatman mades with upstream. Then. ltbRgt ` theta + alpha= 90^@ ` or theta = ( 90^@- alpha)`
In ` Delta OBC, sin theta = sin ( 90^@- alpha)`
` = cos alpha= (BC)/(OB)= 5/(10) = 1/2 cos 60^@`
or ` alpha = 60^@` upsteram.