About Newton's apple

Question

I read:

Suddenly – boink! -an apple hits him on the head. “Aha!” he shouts, or perhaps, “Eureka!” In a flash he understands that the very same force that brought the apple crashing toward the ground also keeps the moon falling toward the Earth and the Earth falling toward the sun: gravity

Now this story is not litterally true. It is written though that Newton sat in the garden where he spent his his childhood and saw an apple falling. He pondered why apples don't fall sideways but straight down instead.

How did this lead him to his laws? He imagined the apple being the moon that fell (or the moon being the apple). And because of the Earth's curvature it would fall forever, so orbiting. But what is the connection with his universal law of gravitation? Didn't he think before that the same force that makes the apple fall can be applied to the moon, and in fact is the force responsible for the moon's motion. How were the orbits of the planets accounted for before his law? By the planets moving through invisible tubes?

Answer 1

The first main point to make in answer is : beware Newton myths -- there are far too many of them, and people have made money and (flaky) reputations fabricating catchpenny fake history about Newton.

The 'apple' story has been plenty embroidered over time. But there is a source, a notebook handwritten by Newton's friend William Stukeley. It was written possibly a long time after Newton in the 1720s in his old age appears to have told a reminiscent story to Stukeley. It is impossible to say whether the story in the notebook was itself embroidered, but the notebook can be read in scanned form at the Royal Society website at

https://ttp.royalsociety.org//ttp/ttp.html?id=1807da00-909a-4abf-b9c1-0279a08e4bf2&type=book

and the story about the apple as recorded by Stukeley reads (in part) as follows:

“After dinner, the weather being warm, we {i.e. Newton and Stukeley} went into the garden and drank thea, under the shade of some apple trees … he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. It was occasion’d by the fall of an apple, as he sat in contemplative mood. Why should that apple always descend perpendicularly to the ground, thought he to himself …”

It can be seen that the story is connected with Newton's ideas of gravitation only, not with his laws of motion. It seems that Newton thought of some influence of the earth extending upwards as far as the apple hanging off the tree, and he seems to have wondered how much farther up that influence might also extend, maybe as far as the moon? That seems to be as far as the initial germ of an idea went.

The origin of the laws of motion is a quite separate matter, certainly nothing to do with the apple story. Newton was aware of many previous contributions to possible laws of motion (and later acknowledged those he approved of, in the Principia). Various notes in the Newton papers show how he wrote and amended numerous sets of draft versions of laws of motion, worrying about this aspect or that, until it seems he was more or less satisfied. Tracing through them all involves consulting bulky sources and if you look for a good place to start, it is probably 'Mathematical Papers of Isaac Newton, Volume VI, 1684-1691' edited by D T Whiteside (Cambridge University press, 1974).