Watching an Apple Falling from a Tree

By Tom Logsdon

In 1665, Isaac Newton left Cambridge University and returned to his hometown of Woolsthorpe to escape the worst ravages of the Black Plague. Safely back among familiar surroundings, he made landmark discoveries that have provided us with precisely the keys we needed to conquer space.

Although the young Newton had reportedly been a mediocre student in the early grades, his powerful intelligence asserted itself even before he reached his teenage years. When he was still a tow-headed youngster, for instance, he managed to construct a charming little windmill backed up by one mouse-power so it could go on turning when the wind refused to blow. Later, he made a paper kite rigged to carry a small lantern high above the British countryside. The people of Woolsthorpe had never before seen flickering lights floating across the nighttime sky, so the young Isaac may have been responsible for some of the earliest sightings of UFOs.

At the age of 23, while relaxing on his mother's farm, Isaac Newton, by his own account, saw an apple falling from a tree. That simple incident caused him to wonder why apples always tumble down. That apple tumbled down toward the ground while the pale August moon continued to sail contentedly overhead. Soon he theorized that the force of gravity tugged on apple and moon falls off systematically with increasing altitude in the same way a light beam dissipates as we move farther away from its source. Double the distance and its intensity falls of by a factor of 4.

Thus, by Newton's reckoning, the force of gravity pulling on the moon should be about 1/3000th as strong as the gravity we experience at the surface of the earth. In 1 minute, he soon calculated a falling apple would be pulled downward about 10 miles, but the moon would fall toward the earth only about 16 ft. During that same 1-minute interval, the moon's orbital velocity also carried it sideways 38 miles. Consequently, its horizontal and vertical motion combine to bring it back onto the same gently curving circular path over and over again.