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Bernard Cohen ties a tennis ball to the end of a string. Holding the string, he whirls the ball faster and faster in a circle. Then he releases the string. The ball flies off at a tangent. As you watch this, he invites your common sense to speculate on what might happen to a bird, sitting on a branch of a tree firmly rooted in earth, if that earth happened to be whirling through space at seventy thousand miles an hour. If the bird releases its grip on the branch, will it be hurled tangentially out into space by this tremendous speed? Obviously not, or the earth would have a very low bird population. For the ancients, the explanation was that the earth did not move, but stood fixed in space. The new physics of Galileo and Newton refused to accept this easy way out. Cohen tells of the start of this new physics and how it now only transcends common sense, but is the foundation of all understanding of motion today: the motion of machines, of heavenly bodies, of our watch springs, of the earth.
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