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Q14 (CISF/2019) Science & Technology › Basic Science (Physics, Chemistry, Biology)

A man rows at 6 km/hr in still water and with the same effort he rows 4-5 km/hr against the stream of a river. What is the speed if he rows (with the same effort) along the stream of the river?

Explanation

To find the speed along the stream (downstream speed), we use the relationship between the speed in still water, the speed against the stream (upstream), and the speed along the stream (downstream).

  • Speed in still water (u) = 6 km/hr
  • Speed against the stream (upstream speed, u − v) = 5 km/hr (Note: The "4-" in "4-5" is a common typographical artifact from question numbering; the intended upstream speed is 5 km/hr).

The speed of the stream (v) is calculated as:
v = Speed in still water − Upstream speed
v = 6 − 5 = 1 km/hr

The speed along the stream (downstream speed, u + v) is:
Downstream speed = Speed in still water + Speed of stream
Downstream speed = 6 + 1 = 7 km/hr.

Alternatively, using the midpoint formula:
Speed in still water = (Downstream + Upstream) / 2
6 = (Downstream + 5) / 2 ⇒ 12 = Downstream + 5 ⇒ Downstream = 7 km/hr.

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