Question

A river flows due north with a velocity of 3 km per hour. A man rows a boat across the river with a velocity of 4 kilometre per hour relative to the water due east. What is his velocity relative to the earth?

Answer 1

it will be 4km/hr only because 4 +cos90=4km/hr

Answer 2

Hey buddy,

● Answer-
Boat velocity vb = 7 km/hr

● Explaination-
Let vb be velocity of boat and vr be velocity of river.

Let vbc be velocity of boat against the river stream.
vbc = vb - vr

Substituting given values-
4 = vb - 3
vb = 4 + 3
vb = 7 km/hr

Therefore, velocity of boat wrt earth is 7 km/hr.

Hope this helps...