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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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B1 中級

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
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