Ages 8–12 · about 6 minutes

Why Doesn't the Book Fall?

If gravity is pulling the book down, why does it just sit there?

The big idea

Something sitting still is not a thing with no forces on it. It is a thing whose forces are balanced.

force: A push or a pull. You cannot see one, but you can see what it does.
balanced: Two forces the same size pulling opposite ways, so nothing changes.
superconductor: A material that, when it is cold enough, lets electricity flow with no resistance at all. Only some materials do this, and only when they are very cold.

If gravity is pulling the book down, why does it just sit there?

Most people say: because gravity stops at the table. It does not. Earth pulls on that book every second it lies there, exactly as hard as it pulls on it in mid-air.

Something else is happening instead. The table is pushing back.

Two forces on one book. 1 is the table pushing up. 2 is Earth pulling down. They start at the same place on the book and they are the same length, because they are the same size. This is a force diagram near the ground, not a picture of space or of anything floating.

Still is not the same as empty

A thing that is not moving is not a thing with nothing happening to it. It is a thing where the pushes and the pulls have added up to nothing left over.

That is worth saying slowly, because it is the idea the whole chapter is built on. Forces do not disappear when nothing moves. They cancel. The book is being squeezed between a pull and a push, and it sits still because they match.

NASA's own explainer for beginners says it in the language of Newton's laws: to change how something is moving, you need a leftover force, and if there is none, the motion does not change. You can read it here: Newton's laws of motion, NASA.

Which way is down?

Down means towards the middle of the Earth. That sounds obvious until you think about somebody standing on the opposite side of the planet.

Their down and your down point in opposite directions in space. Neither of you is upside down. You are both being pulled towards the same middle, and you both call it down.

The cold metal that floats

Here is a photograph you may have seen: a small piece of metal hovering above a magnet, with cold mist pouring off it.

That is a superconductor. Some materials, chilled far below anything in a kitchen freezer, let electricity run through them with no resistance at all. One of the things a superconductor can then do is hold itself steady above a magnet, with a gap you can see straight through. The US Department of Energy explains what is going on inside: DOE explains superconductivity.

Now the important part, and it is the same idea as the book. Gravity has not been switched off under that floating metal. Earth is pulling it down just as hard as ever. A magnetic push is holding it up by exactly the same amount, so it hangs there.

Two forces. Balanced. Nothing moving. The book on your table is doing the very same trick, only with a table instead of a magnet.

Please do not try this one at home. Making metal that cold needs liquids far colder than anything you should ever handle, and it belongs in a laboratory with people trained to work with it.

Your turn

Look around the room and find three things that are not moving. For each one, say what is pulling it down and what is pushing it up.

Then the harder question, and it is a real one that scientists still work on: is there any way to change the pull itself, instead of just balancing it? Chapter 11 is about the people trying to find out.

Try this at home

The book that is being pushed and pulled at once

You need: One book, a table, and an adult with you

1.Put the book flat on the table, near the middle, and keep it low — please do not drop it on anyone's toes.
2.Hold your hand flat under the book and lift it a couple of centimetres. Feel how hard you have to push up.
3.Set the book gently back down. Ask an adult: who is doing that pushing now that your hand has stopped?

Notice: Your hand had to push up to hold the book. When you put it down, the table took over the pushing — it pushes up on the book exactly as hard as Earth pulls it down. If you cannot reach the table, describe it instead: point to where the push comes from and where the pull comes from, and say which is bigger. (Neither one is bigger. That is the whole answer.)

For grown-ups and older readers

This lesson comes from Chapter 11: Gravity Control and Superconductors and the map unit Gravity control and superconductors.