Question

A man can swim in still water with a velocity of 2.5km/h. He wants to cross the river flowing at a speed of 1.5km/h. He always swims at an angle of 120° with downstream. The width of river is 800m . Then the time in which he will cross the river is

Answer 1

Answer:

0.37 hr  = 22.2 Mins

Explanation:

A man can swim in still water with a velocity of 2.5km/h. He wants to cross the river flowing at a speed of 1.5km/h. He always swims at an angle of 120° with downstream. The width of river is 800m . Then the time in which he will cross the river is

River Speed = 1.5 km/hr

Man speed in still water = 2.5 km/hr  and angle = 120°

Speed across Downstream = 2.5Cos120° = 2.5(-1/2) = -1.25 km

Speed toward other bank = 2.5Sin120° = 2.5(√3/2) = 1.25√3

Net Speed = (1.5 - 1.25) i  + 1.25√3 j

= 0.25i + 1.25√3 j

only  1.25√3 speed will help him to cross river width

=>  1.25√3  * time = 0.8     800 m = 0.8 km

=> Time = 0.64/√3

=> Time = 0.37 hr  = 22.2 Mins