Posted by **Minte** on Thursday, December 27, 2012 at 10:48pm.

A boat takes 4 min. to cross a river 90m wide directly when there is no current & 5min. When there is current. find the velocity of the current.

- physics -
**drwls**, Thursday, December 27, 2012 at 11:36pm
With no river flow, the boat crosses the river at 90/4 = 22.5 meters/min.

With river flow, it's speed relative to water is the same as above, but it crosses at a speed of 90/5 = 18 m/min, relative to and perpendicular to the shore.

Consider the vector right triangle with sides of river speed, speed relative to shore, and speed relative to water.

If v is the river speed,

v^2 + 18^2 = 22.5^2

Solve for v.

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