What is meant by the statement "motion is relative"?


Motion is relative to an observer or to an object. For example, a woman driving a car is not in motion relative to the car, but she is in motion relative to an observer standing on the side of the road.

A similar example is a cat sleeping on a rug. The cat appears to be at rest to an observer on Earth, but relative to an object at a fixed point in space, the cat is moving as fast as the Earth turns. As these examples show, discussions of motion must include a point of reference, especially when calculating measurements such as speed. For example, if a train is traveling 90 miles per hour and a man walks from the back of a train car to the front, how fast is the man moving? The answer depends on the point of reference. To another passenger in the train, the man is moving at a rate much less than 90 miles per hour. To an observer outside of the train, however, the man is moving at a rate slightly more than 90 miles per hour.

Most people consider motion relative to a fixed point on the Earth. Physicists consider that the universe has no fixed point, so all motion is relative to what is defined as a fixed point. In fact, this basic idea is part of Einstein's theory of relativity.

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