Name: 【TED-Ed】How to think about gravity - Jon Bergmann
Uploaded: 2013-05-11T06:18:24.000Z
Duration: 4 min 44 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Technically, the law of gravity is an equation.

where G is the universal gravitational constant,

M1 and M2 are the masses of the two objects,

and R is the distance between them squared.

Probably not. What does this actually mean?

Well it means that - well, everything is attracted to everything else.

What I mean by that is if you have two objects, any two objects,

Let's try and wrap our minds around this.

What happens when you drop a rock off a cliff?

You see, the law of gravity says that both objects,

the rock and the earth, are attracted to each other.

This means that the rock falls towards the earth,

Wait a second - you mean to tell me that if - the earth falls up to meet the rock?

And the reason you don't see the earth fall up to meet the rock

is because the objects move towards each other proportionate to their respective masses.

The earth is much much much more massive than the rock,

and the rock is much less massive, so it moves farther

Maybe a better way to understand gravity is to take two teenagers

in spacesuits, and place them far out in space - away from all the planets and the stars.

It turns out they will be attracted to each other.

I'm not talking about that kind of attraction.

See, they have mass, and since they have mass,

Maybe one more thing might help. Have you ever played with two magnets?

You know, the magnets with the north and the south poles?

When you take the magnets and put them closer to each other,

they move together. They are attracted to each other.

And the closer they are, the stronger the attraction.

Think of the mass of the object like the strength of a magnet

and the distance between the objects like the distance between the two magnets.

Now understand, I'm not saying that gravity and magnetism are the same,

Let's think of one other thing - astronauts.

You know, astronauts, they weigh less on the moon

Well you see, the moon is less massive than the earth.

Therefore it has a smaller gravitational pull on the astronaut.

Distance also plays a role. Think back to playing with a magnet.

The pull of the magnets towards each other

are stronger when they are closer together. The same is true of gravity.

For example, the sun is the most massive object near the earth.

It dictates most of the gravitational forces in our solar system.

It is very very massive. But it is relatively far away,

so even though the sun is a much stronger magnet, so to speak,

Therefore the attraction isn't as strong.

So let's look back at that law of gravity.

You see the force of gravity is equal to a number.

That's that universal gravitational constant G

times the mass of object one, times the mass of object two.

And then we divide by the distance between them squared.

This determines the force of attraction between the sun and earth.

You could just as easily plug in your mass

and the earth's mass and the distance between you and the center of the earth,

and find out how much you are attracted to the earth,

Everything is attracted to everything else.

just to make you wonder. What causes gravity?

Why are two objects with mass attracted to each other?

The cause of gravity remains a mystery to scientists.

We don't really know conclusively what causes gravity.

It is one of the great mysteries of science.