Motion at constant speed

It is relatively easy to convince oneself of the correctness of Galilei's statement that it is impossible to decide whether a body is at rest or moving at constant speed. All that is required is a moving platform capable of smooth movement and elimination of outside visual reference points:

• On a smooth road the movement of a car is only felt through the occasional bump, and its speed has to be judged by looking at the speedometer.
• A train on a modern welded track can run so smoothly that it feels like standing still even at high speed.
• Elevators in high rise buildings seem to stand still when going up or down as soon as the initial acceleration is complete.
• People in aircraft can judge the speed at which they travel only by looking at the earth below.

By similar argument it is also easy to demonstrate that circular motion is not "moving at constant speed" in Galilei's sense: A person on a merry-go-round can never get the impression to be at rest but has to hold on in order not to be thrown off. Only uniform movement in a straight line is equivalent to rest.

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