Fr. Victor Badillo, SJ: Newton’s apple and celestial mechanics

NEWTON’S APPLE

by Fr. Victor Badillo, SJ of the Pedro Calungsod Blog:

The young man had read or wrote physics books under his favorite apple tree many times before.  During the days he read, flowers blossomed attracting bees.  They came to carry off apple nectar and deposit pollen from other trees. Bees knew that there was no free nectar. In time the flowers wilted to give way to fruits. One was directly above Newton’s head.  It had never fallen before.  Gravity tugged at it to no avail, not because Newton’s body was blocking the force, but because it was held in place by its stem.

Eventually the tree said, “The little fellow is now equipped with the
necessities of life, but he cannot propagate our species up here.” So it stopped nourishing the stem and gravity had its way. Cut was the umbilical cord that kept the apple from falling.

It bounced off Newton’s head and fell to the ground, some distance from the tree.  This was good, for it had a better chance of life away from the mother tree.  Better still, if picked up and eaten far away, the species would be propagated over the globe.

Another person would have looked up and reasoned, “The fruit fell.  That leaf when it dries will fall.  That branch when it rots will fall.  Anything not held up will fall.”  For Newton it was, “The fruit fell.  Therefore the moon is falling.”  He had discovered the law of universal gravitation. Physical laws valid on earth are valid out of the earth.

When he got home, he told his mother, “The moon is falling.”  To which she replied, “So is the sky”. Clearly Newton had his work cut up.  His intuition had to result in something measurable.   It had to predict results.

Earlier, Kepler reduced to a neat mathematical expression mountains of observations of planets.  He showed that the paths of planets were elliptical, correcting Galileo and others who thought they were circular. He found too the relation of the distance of a planet to its period (the time it took to orbit the sun).  Knowing the period of a planet (which is easy to determine) he did not have to spend years and equipment to determine its distance.  He just had to use paper and pencil.   This was a big step. What Kepler did was curve fitting, or summarizing data neatly, namely mathematics.  But why are planetary paths elliptical and their distances from the sun calculable from their periods?

Newton gave the answer with his celestial mechanics.  It was because gravity makes planets fall to the sun.  He provided the physics to the mathematics of Kepler.

Celestial mechanics provide the means to determine the existence of extra solar planets, the mass of a galaxy (containing billions of stars), even the mass of the whole universe.  Newton provided the method  to weigh the universe.

Humbly Newton acknowledged, “I have seen further than other men, because I stood on the shoulders of giants”.  On another occasion he said, “To myself I seem to he only like a boy, playing on the seashore, and now and then finding a smoother pebble, or a prettier shell, while the great ocean of truth lay undiscovered before me.”

It was not the shiny red color of the skin, or the delicious taste of the apple that was decisive.  All that was needed was an object that could fall, an object that had mass.  It could have been a stone, or even a McIntosh.

Adam’s apple made him lose heaven.  Newton’s apple made him conquer the heavens.

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About ateneophysicsnews
Physics News and Features from Ateneo de Manila University

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