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B2 ScienceOct 1, 2026

What stops the Moon from colliding with the Earth? A physicist explains #space #newton #physics

月が地球と衝突しないのはなぜか？物理学者が解説 #宇宙 #ニュートン #物理

The Royal Institution15 videos Open on YouTube

Transcript

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Translation

So let's go 300 years back in time and see what Newton looked at.
では、300年前にさかのぼって、ニュートンが何を見たのか見てみましょう。
One of the questions that Newton was trying to solve was: Well, if there's gravity between any two sets of kilos, for instance, the planet Earth and the moon, then sure, okay, then the moon is orbiting the Earth.
ニュートンが解決しようとしていた問題の一つは、こうでした。例えば、惑星である地球と月の間のように、2つの質量の間に重力があるなら、そうですね、月は地球の周りを公転しているわけです。
But why doesn't it fall down?
しかし、なぜ落ちてこないのですか？
Why doesn't it crash into each other?
なぜ衝突しないのですか？
I mean, if the Earth is pulling, why doesn't it drop down?
つまり、地球が引っ張っているのに、なぜ落ちてこないのですか？
And again, this problem was solved.
そして、この問題は解決されました。
This is how Newton and his contemporaries did it.
ニュートンと彼の同時代の科学者たちは、こうして解きました。
Newton said, "Well, listen, there is gravity between Earth and moon.
ニュートンは言いました。「聞いてください、地球と月の間には重力があります。」
That gravity here is depicted by the blue arrow.
ここでの重力は青い矢印で表されています。
So it is being pulled inwards.
つまり、内側へ引っ張られているのです。
Again, question remains: Why doesn't it drop down then?
再び、疑問が残ります。なぜ落ちてこないのですか？
Then Newton realized: Wait a minute, things really love to go into a straight line with constant velocity.
そこでニュートンは気づきました。待ってください、物は等速直線運動をしたいのです。
That's what we call the inertia of an object.
それが物体の慣性です。
If you push something, it tries to move straight forward without being pushed or pulled in any direction.
何かを押すと、押されたり引かれたりせずに、まっすぐ進もうとします。
Inertia.
慣性です。
So that would be this arrow in dark blue that you see right now.
つまり、今見えている濃い青の矢印がそれです。
But then the following happens: Listen, if I'm running in that direction, that's my inertia, the dark blue arrow.
すると、こうなります。聞いてください、私があの方向に走っているとします。それが私の慣性、濃い青の矢印です。
And somebody over there is pulling on my arm.
そして、あそこにいる誰かが私の腕を引っ張っています。
That's gravity.
それが重力です。
I won't go in that direction.
私はあの方向には行きません。
I also won't go in that direction.
あの方向にも行きません。
I will go through the middle.
中間を進みます。
I will be pushed to the side.
横へ押し出されます。
That's the orange arrow that you see here.
それがここにあるオレンジ色の矢印です。
And the moon then ends up at this new position over there.
そして月は、あちらの新しい位置に到達します。
But because nature likes to repeat itself, the same trick will happen at this new position.
しかし、自然は同じことを繰り返すのが好きなので、この新しい位置でも同じことが起こります。
There's an arrow, gravity, trying to pull it in towards the Earth.
重力の矢印があり、地球へと引っ張ろうとしています。
It still wants to go into a straight line.
月はまだ直線に進もうとします。
It's inertia, but now it's slightly slanted, and it will again go through the middle, orange arrow.
それが慣性です。しかし今や少し傾いているので、再び中間を進みます。オレンジ色の矢印です。
And if you keep this up for every second, you get a nice circular orbit of the moon around the Earth.
これを毎秒続けると、月が地球の周りをきれいな円軌道で回るようになります。
This is how Newton solved the question: Why doesn't the moon drop on the Earth despite their strong pulling towards each other?
ニュートンはこうして、互いに強く引き合っているにもかかわらず、なぜ月は地球に落ちてこないのかという疑問を解きました。
It's this interplay between inertia and gravity.
これが慣性と重力の相互作用です。

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