The velocity of an object measured by an observer is the sum of velocities it is moving at relative to the observer. For example if an object is moving at a velocity v1 to the right relative to the observer and also has a velocity v2 to the left, the...

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The velocity of an object measured by an observer is the sum of velocities it is moving at relative to the observer. For example if an object is moving at a velocity v1 to the right relative to the observer and also has a velocity v2 to the left, the net velocity measured by the observer is (v1 - v2) to the right.

Here, the passenger is traveling in a train an moves at 17 m/s relative to the Earth. A baseball is thrown by the passenger in a direction opposite to that in which the train is traveling at 15 m/s.

The net velocity of the baseball when it leaves the passenger's hand is the sum of its velocity relative to the Earth. That is 17 + (-15), (one of the velocities has a negative sign as it is opposite in direction to the other).

This gives the velocity of the baseball relative to the Earth when it leaves the thrower's hand as 2 m/s.