# Understanding gravity's pull

02 March 2015

## Interview with

Dr Hugh Hunt, University of Cambridge

## apples.jpg

We are all affected by gravity, every day. It keeps the planets in orbit and ensures objects fall downwards. But before the 1600s, no one had any idea it existed. Then, in 1687, Isaac Newton published his laws of universal gravitation, and changed everything, as Cambridge engineer Hugh Hunt explains to Chris Smith, with the help of an apple...

Hugh - I have 1 Newton. You probably know that an apple weighs about 100 grams and 100 grams is 1 Newton of force.

Chris - Is it true that Newton did fall asleep under this apple tree and had an apple drop on his head and this is how he had the breakthrough intellectually that something is pulling the apple down?

Hugh - No.

Chris - And we can all go home now.

Hugh - In fact, Newton himself said that wasn't true. But the story was told so often that later on in life, he just accepted that it was a good story. So later on, he said it was true.

Chris - What was people's concept of the idea of gravity before Newton came along and actually put some numbers on it then?

Hugh - One of the things that people had trouble with was the idea of a force because everybody thought that things naturally just slow down. If you slide something along the floor, it stops. The laws of motion were that things naturally stop. But then Newton came up with this idea that actually, things will keep moving unless there's a force.

Chris - It is quite groundbreaking sort of idea really isn't it because obviously, here on Earth, everything does stop. So, how did he make that intellectual leap then?

Hugh - Well, he started thinking about motion in terms of rates of change of position and acceleration and he had this idea of momentum. If we think of an apple which drops, it starts off not moving and it gets faster and faster. And it makes that lovely apple sound when you catch it. The idea then is, there must be a force because the apple is accelerating. The thing that he started thinking about was, well, Kepler and others had realised that astronomical bodies orbited. There's a force, the force of gravity. Could the force that's holding the moon into orbit be the same as the force that causes the apple to fall to the ground?

Chris - How is this received by people when he proposed this?

Hugh - Well, he did a nice experiment, a thought experiment which made it rather easy for people to perceive. I wonder if I could get a volunteer from the audience to help with experiment. And your name is?

William - William.

Hugh - Now, what I want you to do is I want you to hold this apple and you just drop it with force straight down. Now, what I want you to do is to throw it towards me. Now, I want you to throw it towards me again. I'm stepping further away.

Chris - We're now about 8 feet away.

Hugh - I want you to throw it harder.

Chris - We're about 10 feet away.

Hugh - Okay now, the further I am away from you, the further you throw it, the curve is less. Now, what Isaac Newton did was to say, "Well, if I throw this really hard, it's going to curve around and it might land somewhere on the other side of Cambridge or throw it harder, it might land in New York. Throw it harder, it could get all the way around the Earth and come back and hit me on the head."

Chris - So, what are you saying is that because the Earth is curving away from the apple because the Earth is a ball, and the apple is falling towards the Earth, if you throw the apple hard enough, eventually, what's going to happen is that the Earth is going to curve out of the way of the path of the apple. So, the apple never actually is going to hit the ground.

Hugh - That's absolutely right.

Chris - So, when we put something into orbit, we are effectively firing or throwing these apples sufficiently hard that it is continuously falling towards the ground but never quite hitting it.

Hugh - And this absolutely transformed the way that we think of motion of astronomical bodies. And the next thing he did was, he worked out an equation which fitted in exactly with Kepler's Laws.

Chris - Let's give William a round of applause. Thank you very much, William. So, tell us about this equation then. What was that equation?

Hugh - For motion in a circle, there is a force required, a centripetal force to cause a motion to go in a circle. It's perhaps best if I demonstrate it really.

Chris - He's getting a brick out of his bag. I'm worried now. It looks like something you ram-raid a shop with.

Hugh - What I have here is - now, let's have another volunteer.

Lutsi - Lutsi

Hugh - I have a tennis ball and Lutsi, that's a tennis ball. It weighs about 50, 60 grams. It's regular.

Chris - Except it's got a lump of rope going through it.

Hugh - It's got a rope attached to it. Well the rope passes through a tube which is convenient to hold on to.

Chris - It's an aluminium tube with the rope running through the middle of it, ball on one end and a house brick on the other.

Hugh - And I've got a house brick on the other end.

Chris - And the rope is a meter, a meter and a half long?

Hugh - The house brick, it's a heavy house brick, isn't it?

Lutsi - Yeah.

Chris - If you drop the brick, obviously, that's just going to pull directly.

Hugh - There we go. One brick dropped out of the ground and the tennis ball is very light. There's no way the tennis ball can hold up the weight of the brick. Well, I'm going to make the tennis ball lift up the brick by using the acceleration of circular motion, the centripetal force that Isaac Newton talked about when he was thinking about apples and gravity, and the moon.

Chris - How are you going to do that?

Hugh - So, let's see. I'd like you to hold the brick, just flat on your hands. Now, I've got a tennis ball and then I'm going to start swinging it around our heads like this and I want to swing it faster and faster, and faster. And there we go, the house brick is up in the air. The remarkable thing about that Chris, is that we can all see just how big that force is and it makes you realise that when things are going around in orbits around the planets that we might think of gravity as being a quite weak force. But it holds the whole of the universe together.

Chris - So, in other words, that is the demonstration practically of what is actually holding our planets, our little clutch of planets our sun, our moon around our Earth and other planets around other stars in other solar systems.

Hugh - Absolutely right.