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Compare the motion of the window observed by a passenger on a train with its motion observed by a person watching the train go by.

Two trains are standing beside one another in a station and one starts to move. A passenger in one train thinks it is his train that is moving. Certainly he seems to be moving past the windows and doors of the carriages of the other train. Suddenly he turns to look through the opposite window and, to his surprise, finds that his window is still opposite the door of the refreshment room on the station platform. His train is still standing in the station so it must have been the train standing at the next platform that has departed in the opposite direction.

Many railway passengers must have experienced this rather strange sensation at one time or another. Although at first the passenger thinks the train is moving, he can only really be certain that one train is moving in relation to the other. But a porter standing on the platform knows which is moving because he is stationary and can tell when either objects are moving.

For velocity to have any meaning at all it must be measured in relation to some reference point. This reference may be stationary or may itself be moving. For instance, an express train appears to be moving much faster to some standing beside the track than to a passenger in a slow moving train traveling in the same direction alongside the express.

The stationary observer and the moving passenger get different results when they measure the velocity of the express train because of the difference in their own velocities. If no reference is mentioned, it is normal to assume that all velocities are measured with reference to the stationary ground. Velocities measured from a moving observation point are called relative velocities, but to avoid confusion in problems concerning moving objects, the velocity of the observer or the position of the observation point should always be stated.


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