Question

A man can row a boat at 1.8 m/s in still water. He heads straight downstream for 2.7 km in a river where the current is 0.9 m/s and then returns to the starting point. Calculate the time for the round trip

Answer 1

Speed of the boat in still water, b = 1.8 m/s

speed of the current, c = 0.9 m/s

Therefore,

speed of the boat in downstream = 1.8 + 0.9 = 2.7 m/s

speed of the boat in upstream = 1.8-0.9 = 0.9 m/s

We know that time = distance / speed

For downstream time taken to cover distance of 2.7 km

t₁ = 2700/2.7 = 1000 s

For upstream time taken to cover distance of 2.7 km

t₂ = 2700 / 0.9 = 3000 s

Hence total time for a round trip ,

T = t₁ + t₂ = 1000+3000 = 4000sec

Answer 2

Given:

Speed still water = 1.8 m / s

Speed of the current = 0.9 m / s

Distance = 2.7 km

To find:

The time for the round trip.

Solution:

During downstream, the speed increases with respect to the speed of the stream,

Speed ( downstream ) = Speed in still water + Speed of the current

1.8 + 0.9

Speed ( downstream ) = 2.7 m/s

Speed ( upstream ) = Speed in still water - Speed of the current

1.8-0.9

Speed ( upstream ) = 0.9 m/s

By formula,

Time = distance / speed

For downstream,

Time ( downstream ) = 2700/2.7

Time ( downstream ) = 1000 s

For upstream,

Time ( upstream ) = 2700 / 0.9 = 3000 s

Time ( upstream ) = 3000 s

Total time = Time ( downstream ) + Time ( upstream )

1000+3000

Total time = 4000 s

Hence, it takes approximately 1 hour and 2 minutes for the round trip.