# Everything a Primary School teacher (or student) needs to know about gravity. And then some.

This post is in the context of a question posed by a primary teacher on a forum recently. Rather than reply there I thought it safer to do so where I could offer a more comprehensive answer.

We tend to associate the concept of gravity with the English scientist Isaac Newton who lived in the seventeenth century.
But he didn’t ‘invent’ gravity; objects were falling to earth long before Newton arrived on the scene, so what exactly did he do?

1.
He did what so many other kids do; he asked asked a silly question. ‘Why do things fall down?’
It does seem like a silly question, which is why nobody took it seriously before, but when you think about it it’s actually quite profound; how does the apple in an appletree ‘know’ which way to fall? How does the earth ‘know’ (if it pulls the apple down) that the apple is there in the first place ? Newton was never able to answer that question. He famously said  “Hypotheses non fingo” (Latin for “I feign [frame] no hypotheses,” or in other words, “I haven’t a clue why this works the way it does”). It’s not like there’s a string connecting the two objects, but yet the apple acts as though there were indeed an invisible string pulling it downwards.
What form does that invisible string take?

I don’t know the answer, but I do know that scientists haven’t fully worked it out yet either.
It has been suggested that all objects exchange particles called ‘gravitons’ and it is as part of this exchange process that the objects come together. The problem is that these gravitons have never been detected.

Another possibility is gravitational waves. These were postulated by Einstein in his Theory of General Relativity. There has been some indirect evidence for these but again they haven’t yet been detected directly. We know we don’t know all there is to know about gravity, and to suggest otherwise would be to do a disservice to your students. In fact the same holds for a lot of science. Gravity does seem to be a little like magnetism, yet the rules which govern gravity don’t work for magnetism and vice versa. The holy grail of physics is to show how the rules that govern the motion of very large objects like planets is connected to the rules that govern the operation of very small objects like atoms. And there’s absolutely no reason why one of your students can’t be the one to make this connection and win their very own Nobel Prize (with a bit of luck they will acknowledge  you  in their acceptance speech as the spark which ignited their passion for Science).
Matthew is a former student of mine and is currently doing a PhD with NASA on this topic. I asked him to explain it to me:

“In the Einsteinian framework, however, gravity is not a force but a curve in space-time. So any object with mass induces a curve in the spacetime around it. Any other object no longer travels along a flat spacetime, but along a curved path. That’s essentially what’s happening to the apple. Instead of hovering at the end of the branch as it would in a flat spacetime, the ‘forward direction’ of spacetime is curved due to the Earth, so the apple just follows that curve, which in three spatial dimensions is just a straight line down.”

Watch the following clip for a wonderful demonstration of a curving space-time –  imagine doing this with your kids in class: you can tell them you are studying Einstein and doing Rocket Science.

But while Newton couldn’t say why gravity worked, he was able to quantify the force of gravity, i.e. he was able to devise a formula which now enables us to say how big the force of attraction will be between any two objects. It depends on how big the objects are (or more specifically their masses) and the distance between them.

It turns out that any two objects will exert a gravitational pull on each other. Now this is mad. It means that there is a force of attraction between you and your biro, and if it was just the two of you floating in space with no other objects or planets in existence, that force of attraction would result in the biro moving towards you and you moving towards the biro. Similarly there is a force of attraction beween each student and the student next to them (cue lots of giggles) and the bigger the size (or mass) of either student, the bigger will be the force.

2.
Newton also established that the force that kept the planets in orbit around the sun was the same force as that which pulled the apple to earth. This idea was a big, big deal at the time. It meant that the planets followed the same laws of physics as objects on earth. Prior to this ‘the heavens’ were thought to be the realm of the gods or God and therefore not subject to our analysis but after Newton they were seen as fair game for anybody to study. I don’t think there’s any way we can really appreciate how big a deal this was. And while Newton wasn’t the very first to realise this, he was the first to demonstrate it mathematically.

The following is a nice video which outlines the significance of Newton and Einstein to our understanding of gravity. You only need the first ten minutes.

The bottom line for me is that you have an incredible audience who will lap this stuff up. Please, please don’t play down the mystery or the wonder. That, unfortunately, is what happens at second level and I have been trying to get teachers to fight it my entire professional career, with very limited success (it doesn’t seem to bother many other teachers, but I have it bad).

Don’t allow your lack of technical knowledge to put you off engaging with the material. Remember when it comes to Science nobody, and I mean nobody, has all the answers. If we’re looking to turn some of these kids into scientists then what they need more than anything else is curiosity and a good old-fashioned sense of wonder. If you can help develop that then everything else will follow.

# Look out! It’s Newton I!

You know you’re a physics geek when the first thought that comes in to your head when watching all those car crashes on the news is: Now that’s what I call Newton’s first law of motion!

First law:
Every body remains in a state of rest or uniform motion (constant velocity) unless it is acted upon by an external unbalanced force.

Now most people think that this force simply slows the car down, but in the world of physics the word velocity covers two quantities; speed and direction. So it could be that the force merely changes the direction of the moving object, and has (little or) no effect on its speed. In fact that is what is happening when an object is moving in a circle (like a stone tied to the end of a rope circling over your head; the stone is moving at constant speed yet we still say that it is accelerating because its direction is changing). In this case the force is provided by the tension in the rope acting inwards.

We demonstrate this in class with an air-track, which is a bit like an elongated air-hockey table. A nice way to re-inforce the concept is to discuss why, when moon-bound rockets leave our gravitational field they can turn off their propulsion system and will remain moving at that speed untill they reach the moon’s gravitational field (although technically both gravitational fields are infinite – but that’s for another day).
Galileo himself (for it was he and not Newton who first promoted this) had great difficulty persuading others of the importance of this discovery.
The response of the students to the air-track demo is a reminder of how strange this idealised world of no friction actually is to us.

But every now and again we get to experience it for ourselves.