G is for Gravity

By Siobhan Fairgreaves

“Gravity pulls everything down”

Wrong!

Now we know what gravity doesn’t do- let’s take a look at what it does.

Before I get too stuck in I’m going to clarify- we’ve all heard of gravity right? Just in case you missed that class here is a quick reminder. Gravity is one of the many invisible forces acting upon us and our environment every minute of every day. We learn about it in the early days of school, but gravity is still hugely important to physicists. The famous Sir Isaac Newton had a ‘eureka!’ moment concerning the force – more on that later though.

Every object with mass has some gravitational pull (that includes you!) That means your own body is exerting its own force, however tiny, both on the clothes you are wearing, and on distant stars. But for gravity, a bigger mass means a bigger force. So you never need to worry about walking around with lots of small objects stuck to you simply because the gravitational pull created by the Earth is much, much stronger.

For humans, animals and absolutely everything we share our planet with, gravity pulls us towards the centre of Earth. It might seem pedantic but there is an important difference between pulling towards the centre of the Earth and pulling ‘down’. An important law in physics, called Newton’s Third Law, states that “for every action, there is an equal and opposite reaction”.

Applying the law to gravity; the force you exert on the Earth (pulling the Earth towards your body), and the force the Earth exerts on you (pulling your body to the Earth’s centre), are equal and opposite, keeping you firmly in place on the planet’s surface. Understanding this principle helps us understand how gravity works when we scale things up.

So how does this come into play for the rest of the Solar System? Well, the Earth has a bigger mass than our moon – just as all the other planets have bigger masses than their moons. But the circular orbits the bodies make around each other mean that gravity doesn’t simply cause the bodies to crash into each other. Instead, the force of gravity maintains the circular paths.

Another one of Newton’s laws – the second this time – states that the acceleration of an object is equal to the force acting on it, divided by its mass. For planets and the moons, the force acting on both of them will be the same, but the mass of the planet will always be much larger. This means that the acceleration of the moon is much larger, making the path its’ circular orbit much longer than that of the planet around it.

Fortunately there is one very important celestial body holding this all together (for our Solar System at least) and that is the Sun. The Sun has a by far the biggest gravitational pull which allows it to hold Earth and the other 7 planets (sorry, Pluto) in position. Due to their respective size and distance each of the planets will experience the gravitational pull from the Sun differently.

Newton summed this all up nicely with yet another law: the law of gravitation. The equation calculates the gravitational force between two bodies using their two masses, the distance between them, and a gravitational constant Newton discovered himself. The law is still hugely important to astronomers today.

Hopefully this makes it clear why we shouldn’t say that gravity pulls everything ‘down’! I don’t know exactly what is ‘down’ from our position in the cosmos but I’m not so sure I want to find out. In a way, it would be more correct to say that gravity pulls everything ‘in’.

I’ve mentioned our old friend Sir Isaac Newton a lot now, and we heard about his famous apple at the very start. Legend has it that an apple fell on Newton’s head and just like that, he understood gravity- but did it really happen? Well it certainly seems to be true that a falling apple did inspire Newton’s work on gravity but there was no instant moment of understanding as the story seems to suggest.

The truth is that, whilst sheltering from the wrath of the bubonic plague at his family home in the countryside, a young Newton witnessed an apple fall from a tree. This sight led him to wonder, “Why should that apple always descend perpendicularly to the ground?” However it was not until around 20 years later that Newton first published this principle.

So that’s one myth busted, and another classic example of science, or scientists, being inspired by nature.

See you next time for a futuristic post all about holograms!