Question

Example 12. A river 800 m wide flows at the rate of
5 kmh-1. A swimmer who can swim at 10 kmh-1 in still
water, wishes to cross the river straight. (i) Along what
direction must he strike ? (ii) What should be his resultant
velocity? (iii) How much time he would take ?
​

Answer 1

Answer:

Let the angle between the swimmer and the perpendicular to the flow of river is θ with the perpendicular to the flow of river and time to cross the river is t

(i)

The direction is given as,

sinθ=

10

5

θ=30

∘

(ii)

The resultant velocity is given as,

V=V

sg

cos30

∘

=10cos30

∘

=8.66km/h

(iii)

Displacement in vertical direction is given as,

(V

sg

cosθ)t=800×10

−3

(10cos30

∘

)t=800×10

−3

t=0.092s

t=5.5min