Question:

A river flows north at 4 m/s and a boy swims straight across it at 2 m/s. If the river is 8 m wide, how far will the boy be from his starting point after crossing the river?

Answer:

B. 16m

First, solve for the time it takes you to reach the other side. Let's let north be the y-direction and the direction across the river be the x-direction.

Δx=vxt

8m=2m/s×t

thus,

t=4s

Now, use the time you are in the water to find how far the river has carried you north.

Δy=vyt

Δy=4m/s×4s

4x4=16

Δy=16m

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