When the apple plunked Newton on the head, what does legend tell that he realized?
Sir Issac Newton's theory of "universal gravitation" is believed to have been inspired by the falling of an apple from a tree. Although the event may not occur exactly as depicted in the legend, accounts of the story indicate that portions of the legend hold valid. William Stukely, Newton's closer friend, published Memoires of Sir Issac Newton's Life in 1752. This biography contains a passage that mirrors the legend:
After dinner, the weather became warm, and we went to the garden, and drank tea under the shade of some appletrees, only he, and myself. Amidst other discourse, he told me, he was just in the same situation, as when formerly, the notion of gravitation came to his mind: "why should that apple always descend perpendicularly to the ground" thought he to himself... "Why should it not go sideways or upwards? but constantly to the Earth's center? Assuredly the reason is that the earth draws it.