## Stories of Science:

The apple that fell on Newton's head

*By ItsNotMagicItsScience*

*Image Source: **http://en.wikipedia.org/wiki/File:Sir_Isaac_Newton_by_Sir_Godfrey_Kneller,_Bt.jpg*

In our last article on uncovering the truth behind some of science's famous stories, we looked at Archimedes's discovery of buoyancy after he took a soak in the bathtub. This time, we'll skip forward some 1,500 years from Archimedes' time to another great scientist, Sir Isaac Newton – whom many regard as the Father of Physics – and the apple that got him thinking about gravity.

**The story**

The scientist and physicist Sir Isaac Newton made one of the greatest discoveries – gravitational force – while sitting down under an apple tree when an apple fell on his head. Besides giving him a headache, it made Newton think about why the apple fell down straight to earth, and later realised that it didn't exactly “fall”, but rather it was drawn to the Earth's centre. Expanding on this thought to the moon, sun, and beyond, he realised that everything in the universe draws every other object – a gravitational pull – which was in proportion to its mass.

Newton went on to formulate and publish his work in the ground-breaking *Philosophiae Naturalis Principia Mathematica* ("the Principia"), which was published in July 1687.

In that book, Newton wrote out his three Laws of Motion, which govern and explain why objects move the way they do, as well as his law of universal gravitation, which explains the behaviour of planets in the solar system, and the universe in general.

**What probably happened**

The story is close to what was commonly told – but instead of the apple falling on Newton's head, it was more likely that Newton was sitting on a front porch sipping some tea while observing an apple falling down. The story of Newton's apple can be traced back to a 1752 manuscript by William Stukeley, who recorded in Memoirs of Sir Isaac Newton's Life a conversation with Newton in Kensington on April 15, 1726.

“It (the notion of gravity) was occasioned 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. Why should it not go sideways or upwards, but constantly to the Earth's centre?”

“Therefore does this apple fall perpendicularly or towards the centre? If matter thus draws matter; it must be proportion of its quantity. Therefore the apple draws the Earth, as well as the Earth draws the apple."

The apple tree that supposedly inspired Newton still exists, amazingly. The centuries-old tree is now owned by The National Trust, which owns Newton's childhood home at Woolsthorpe Manor near Lincolnshire in England.

**What Newton's discovery meant**

Newton's law of universal gravitation explains a great deal of why the universe exists the way it is. Planets, solar systems, galaxies are all formed and behave in accordance with this Newtonian law, which states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

The formula he came up with looks like this:

where:

- F is the force between the masses,
- G is the gravitational constant,
- m1 is the first mass,
- m2 is the second mass, and
- r is the distance between the masses.

Suffice it to say, without Newton's discovery, humanity's modern understanding of physics and astronomy would have been impossible. As far as the story of the apple falling on Newton's head goes, however, it's more myth than fact.

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