The Story of Gravity: The weakling, the plague and the apple.

“Albert, is the gravity that makes stars the same as the gravity on Earth?”

It’s the very same force that keeps your feet on the ground and makes apples fall out of trees.

“Was that story true about the man who invented gravity being hit on the head with an apple really true?”

Well the man who was supposed to be under the tree was Isaac Newton and he certainly existed. Many people think he was the most brilliant scientist who ever existed.

“I thought you were the most brilliant scientist that ever existed Albert.”

Like Newton said, if I have seen farther it is by standing on the shoulders of giants. Newton was one my giants.

“So tell me about Isaac Newton then.”

Well he was born on Christmas Day in the year 1642 in the small village in Lincolnshire in England and he almost died the same day.

"What happened?"

He was tiny and feeble when he was born. He was so small his mother, Hannah Newton, said he'd fit in a two pint pot. Luckily for science young Isaac proved them wrong. He proved them so wrong he lived for 84 years which was almost a miracle back then. He also survived an outbreak of a deadly plague in 1665 that closed down Cambridge University. That was when he was sent back to deepest Lincolnshire for some uninterrupted thinking time. This was when he was supposed to sitting under the apple tree.

“Surely people had always known apples fell of trees?”

Of course they did. The difference was that Newton asked the question ‘Why?’
He was also smart enough to answer the question by developing his theory of gravity.

“So what was his theory?”

Newton said that everything big and small has a gravitational pull. The bigger the thing the stronger the gravity. He also worked out that the pull of gravity from an object gets less and less as you get further away.

“So even an apple creates gravity? I thought it was just the earth pulling the apple down.”

Exactly. The Earth pulls the apple and the apple pulls the Earth so they move together. It’s just that the Earth is so huge it has a much greater gravitational pull than the apple does. In mathematical terms gravity increases with the sum of the masses of two objects and reduces with the square of their distance apart.

“Whoa, hold on try that again in English.”

OK, for a start if you double in weight (or mass as we scientists call it) your gravity doubles.

“Got that.”

Right, the next thing is that if there are two objects then each pulls the other.

“Sounds weird, but OK.”

And lastly if you move away from a big object gravity reduces so that at twice the distance it is reduced by four times, two times two.

“I know what two times two is Albert.”

OK, so what happens to gravity at fives times the distance?

“Oh, er…”

It’s just five times five.

“Twenty five. I get it, so at three times the distance gravity is reduced by 9, three times three.”

Exactly, so now you know Newton’s theory of gravity.

“Can you really explain how a whole planet moves with this law of gravity?"

You certainly can, But there is one thing that gravity doesn’t do.

“What’s that?”

Gravity doesn’t make people fall in love.

“Thanks Albert, I’ll try and remember that.”

Now Newton didn’t stop there he worked out the rules for how things move in the universe, his laws of motion. How many rules or laws do you think that would take?

“Oh, millions I suppose. It’s a complicated place the universe.”

Newton only needed three extra laws but.these three laws explain how objects, from footballs to planets, move and what happens when you kick a football. Now pay attention.
“I’m listening.”

Newton's first law of motion is that objects keep doing whatever they are doing unless a force is applied to them. This works if the object is still or moving. Leave a football on the ground and its stays there. Kick it and it moves.

"But it doesn't keep moving for ever."

No because two other forces act on it. Gravity pulls it down to earth and friction between the ball and the air and ground stops it. In space there is no friction so a football could keep going forever as long as it didn't hit anything.

Newton's second law is a bit more mathematical but explains the link between how much an object accelerates, how big it is (it's mass or in simple terms weight) and the force applied. Simply put it says that for a given force, the smaller the object the faster it will accelerate. If you hit a golf ball and a football with a golf club, the golf ball goes much further because it is smaller and lighter.

“That makes sense.”

Newton's last law sounds a bit odd but is just as important as the others. It says that for every action there is an equal and opposite reaction. Most of the time you don't see this in action but it's still there. Step out of a small rowing boat at a jetty and it becomes a little more apparent. As your front foot moves forward, the equal and opposite reaction pushes the boat away from the jetty. Ignore Newton's third law and you'll get wet.

Together with his theory of gravity, these laws create what scientists call Newtonian mechanics. Four laws to explain most of the movement in the universe.

“So if Newton got it right, what was left for you to discover Albert?”

Well these four rules seemed to explain everything about how the universe worked and for most situations they still do. It's only when things are going spectacularly fast or gravity becomes very strong that Newton's law start to stop working but I'll explain all about that one day. Even though I ending up disagreeing with Newton on some things, when they sent a man to the moon they used Newton’s equations and not mine to make sure he got there.