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?