A man can swim with a speed of 4.0 km/h in still water. How long does he take to cross a river 1.0 km wide if the river flows steadily at 3.0 km/h and he makes his strokes normal to the river current? How far down the river does he go when he reaches the other bank? | Learn NCERT solution | Education portal Class 11 Physics | Study online Unit-4 Motion In A Plane



Q.13:- A man can swim with a speed of 4.0 km/h in still water. How long does he take to cross a river 1.0 km wide if the river flows steadily at 3.0 km/h and he makes his strokes normal to the river current? How far down the river does he go when he reaches the other bank?

 

Answer:-



Speed of the man, vm = 4 km/h
Width of the river = 1 km
Time taken to cross the river = Width of the river / Speed of the river
= 1/4 h = 1 × 60 / 4 = 15 min
Speed of the river, vr = 3 km/h
Distance covered with flow of the river = vr × t
= 3 × 1 / 4 = 3 / 4 km
= 3 × 1000 / 4
= 750 m