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What is gravity?

Gravity is a concept that comes up throughout secondary science teaching - but what is it? Watch this short video to find out.
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What goes up, must come down, right? We all know this simple rule of gravity and how to break it with rockets. But for thousands of years of human history, we didn’t really understand why. It was Galileo Galilei and Sir Isaac Newton whose experiments would really start to crack this mysterious force. Galileo experimented with the speed of falling objects, including the definitely real Leaning Tower of Pisa experiments. He learned all objects fall at the same rate when air resistance is factored out. Newton later suggested every object with mass attracts every other object with mass, and for several hundred years, this seemed right. After all, it explained a lot. An attraction between you and the Earth keeps you bound to the surface.
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And a similar force keeps the Moon in orbit. The Moon, in turn, pulls on the Earth. This pull actually bunches the water up on the Earth together, producing the tides on the oceans. In Newton’s view, the Sun’s gravity holds the entire solar system together, from the largest planets, to the smallest asteroids. Inside the Sun the combined gravity of the million trillion trillion kilogrammes of its mass, heat at the core to the point where hydrogen fuses with itself to form helium. This powers almost every star in the universe. When the Sun runs out of fuel in a few billion years time, its core will collapse under its own gravity into a tiny white dwarf. But Newton’s view of gravity wasn’t perfect.
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A simple attraction force couldn’t accurately predict the path of Mercury’s orbit, nor explain the bending of light by massive objects, known as gravitational lensing. Albert Einstein realised gravity was instead a bending of the fabric of space and time. This bending distorts space, causing objects to fall towards the source of this bend, looking like an attraction force. Einstein’s theories are still our best explanation for what gravity is, but they predict a strange object with high mass and almost zero size, a black hole, from which there is no escape. So, at least somewhere in the universe, what goes down never comes back up.

As the old saying goes, what goes up must come down. But why does that happen? And does that always happen? Our understanding of what gravity is has changed from being a mysterious force that no one could explain, to the mind-blowing realisation by Einstein that gravity is the curvature of spacetime caused by massive objects.

How can you explain gravity?

A great way of explaining a tricky topic like gravity is to split your explanation into three levels – beginner, intermediate and advanced – and use the explanation most appropriate for your students.

Here, we’ve split the answer to the question “What is gravity?”, beginning with the basics of gravity and ending with the more advanced, and currently accepted, the idea of what gravity is.

What is gravity? – Basic

Imagine you are holding a ball in your hand – what happens when you let go of the ball? It falls to the ground because the Earth’s gravity attracts the ball to the Earth. Gravity is a force that attracts objects towards each other, so not only is the Earth attracting the ball, the ball is attracting the Earth too.

What is gravity? – Intermediate

Every object that has mass creates a gravitational field that surrounds it and attracts other objects. The Earth has a gravitational field and any object inside the Earth’s gravitational field will experience an attractive force towards the Earth.

If I have a ball in my hand and let go of the ball, even though the ball has a gravitational field of its own, it will be attracted towards the Earth because the Earth’s gravitational field is stronger.

What is gravity? – Advanced

Our current understanding of what gravity is comes from the brilliant mind of Albert Einstein. Imagine four people holding a sheet of cloth tautly, with each person holding a different corner of that cloth. That cloth represents space and time. If we place a ball on the sheet near its centre, what would happen? The ball would cause the cloth to sag, in other words, the ball would distort or bend the sheet of cloth.

What happens if we roll another ball along with that sheet? That ball will follow the curvature of the cloth and will appear to accelerate towards, or be attracted to, the ball lying at the centre of the cloth.
Einstein realised that objects with mass bend space and time, the fabric of our Universe, and that gravity is the curvature (or bending) of spacetime caused by those objects.
If you’d like to learn more about teaching physics, astronomy and space to secondary students, check out the Royal Observatory Greenwich online course, below. 
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Physics, Astronomy, and Space: Teaching Secondary Science

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