(a) A Boat is paddling upstream at 1.25 m/s relative to the water in a river which is flowing at 0.50 m/s relative to the banks of the river.What is the resultant velocity of Rosa's boat...

(a) A Boat is paddling upstream at 1.25 m/s relative to the water in a river which is flowing at 0.50 m/s relative to the banks of the river.

What is the resultant velocity of Rosa's boat (relative to the banks)?

(b)Boat is paddling downstream at 1.25 m/s relative to the water in a river which is flowing at 0.50 m/s relative to the banks of the river. What is the resultant velocity of Rosa's boat (relative to the banks)?

Asked on by diarie

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pacorz | High School Teacher | (Level 3) Educator

Posted on

This is a fairly straightforward vector problem. Imagine for a moment that Rosa stops paddling, and think about what that would look like to you if you were standing on the bank. Rosa, now floating with the current, would be going past you at 0.5 m/s, wouldn't she?

Now for a different picture. If the river had zero current and Rosa paddled past you, she would be going 1.25 m/s.

The motions of Rosa and the river do not depend on each other. Because they are both going in the same direction, you can just add them up. Rosa's actually going downstream at 0.5 + 1.25 = 1.75 m/s.

Just for the record, if Rosa was paddling upstream (against the current)  we would subtract the two speeds because they would partially cancel each other out.

The link below explains how to do these problems with vector diagrams, which may help you to picture them more clearly.

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