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• So what is gravity?

• I bet most of you think it's:

• "What goes up,

• must come down!" Is that right?

• Well, sorta, but not really.

• Technically, the law of gravity is an equation.

• It is: F = G x M1 x M2 / R^2,

• 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.

• That was easy, right?

• 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,

• they are attracted to each other. OK.

• Let's try and wrap our minds around this.

• What happens when you drop a rock off a cliff?

• It falls to the earth. Right?

• Well, yes, but something else happens.

• 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,

• and the earth falls towards the rock.

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

• Yes, that's exactly what I'm saying.

• 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,

• so it moves a very very small distance,

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

• with respect to the earth.

• 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,

• they will move towards each other.

• They are attracted to each other.

• 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,

• they just behave in a similar way.

• Let's think of one other thing - astronauts.

• You know, astronauts, they weigh less on the moon

• than on the earth. Why is that?

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

• Therefore it has a smaller gravitational pull on the astronaut.

• It's like the moon is a weaker magnet.

• They aren't as attracted to each other.

• 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,

• it is a long ways away.

• Therefore the attraction isn't as strong.

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

• The equation: F = G x M1 x M2 / R ^2.

• 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.

• Think of M1 being the mass of the sun

• and M2 being the mass of the earth.

• 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,

• and the earth attracted to you.

• So, what's gravity?

• Everything is attracted to everything else.

• Everything. Oh, one last thing,

• just to make you wonder. What causes gravity?

• Why are two objects with mass attracted to each other?

• Well, the answer is -

• We don't know.

• 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.

So what is gravity?

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# TED-ED】重力の考え方 - ジョン・バーグマン (【TED-Ed】How to think about gravity - Jon Bergmann)

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VoiceTube に公開 2021 年 01 月 14 日