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Question: A police boat is chasing a boat with criminals along a straight river by moving against the stream. The speed of the river stream is 3 miles per hour, the speed of the boat with criminals relative to the river is 30 miles per hour, and the police boat is 4 miles per hour faster than the boat with criminals.

Currently the criminals are ahead of the police, and horizontally throw a stone at the police boat at a speed 16 miles per hour relative to their boat (i.e. relative to the boat of criminals).

What is the horizontal velocity of the stone relative to the police boat and to the river bank? You need to state what the origin and the positive direction of motion are.

What should the origin be?

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There's not "right/wrong" answer for where to put the origin (or which direction you choose to be the positive direction), but your answer will be relative to those choices and that's why they are asking you to declare them.

I'd select the origin to be at the front of the criminals' boat and the positive direction to be pointing back toward the police boat.

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  • $\begingroup$ Alright I will try that, and see what answers I get. So the positive direction you're suggesting, is against the direction of both boats, right? $\endgroup$ – Michael Nov 26 '13 at 21:41

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