Question

Question 4.13 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?

Chapter Motion In A Plane Page 86

Answer 1

Actual speed of man = 4 km/h
Speed of river = 3 km/h
And the width of river = 1 km
Time taken by man to cross the river = width of river/speed of man
= 1 km/4 km/h
= 1/4 hour .

He makes his stroke normal to the river current . so, distance covered along the river = speed of river × time taken by man to reach the other Bank
= 3 × 1/4 = 0.75 km
= 750 m

Answer 2

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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

=15 min

Speed of the river, vr = 3 km/h

Distance covered with flow of the river

Distance covered with flow of the river = vr × t

= vr × t = 3× 1/4 = 3/4 km

= 3/4 × 1000 = 750 m

I hope, this will help you

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