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  • The next time you see a news report of a hurricane

  • or a tropical storm

  • showing high winds battering trees and houses,

  • ask yourself, "How did the wind get going so fast?"

  • Amazingly enough, this is a motion that started

  • more than five billion years ago.

  • But, to understand why, we need to understand spin.

  • In physics, we talk about two types of motion.

  • The first is straight-line motion.

  • You push on something, and it moves forward.

  • The second type, spin, involves an object rotating,

  • or turning on its axis in place.

  • An object in straight-line motion will move forever

  • unless something,

  • like the friction of the ground beneath it,

  • causes it to slow down and stop.

  • The same thing happens when you get something spinning.

  • It will keep on spinning until something stops it.

  • But the spin can speed up.

  • If an ice skater is gliding across the ice

  • in straight-line motion and she pulls her arms in,

  • she keeps on gliding at the same speed.

  • But if she is spinning on the ice

  • and she pulls her arms in,

  • you know what happens next.

  • She spins faster.

  • This is called the conservation of angular momentum.

  • Mathematically, angular momentum is a product of two numbers,

  • one that gives the spin rate

  • and one that gives the distance of the mass from the axis.

  • If something is freely spinning,

  • as one number gets bigger,

  • the other gets smaller.

  • Arms closer, spin faster.

  • Arms farther, spin slower.

  • Spin causes other effects, too.

  • If you are riding on a spinning merry-go-round

  • and you toss a ball to a friend,

  • it will appear to follow a curving path.

  • It doesn't actually curve, though.

  • It really goes in a straight line.

  • You were the one who was following a curving path,

  • but, from your point of view,

  • the ball appears to curve.

  • We call this the coriolis effect.

  • Oh, and you are riding on a speeding merry-go-round

  • right now at this very moment.

  • We call it the Earth.

  • The Earth spins on its axis once each day.

  • But why does the Earth spin?

  • Now, that's a story that starts billions of years ago.

  • A cloud of dust and gas that form

  • the Sun and the Earth and the planets

  • and you and me

  • started to collapse as gravity pulled it all together.

  • Before it started to collapse,

  • this cloud had a very gentle spin.

  • And, as it collapsed,

  • like that ice skater pulling her arms in,

  • the spin got faster and faster.

  • And everything that formed out of the cloud,

  • the Sun

  • and the planets around the Sun

  • and the moons around the planets,

  • all inherited this spin.

  • And this inherited spin is what gives us night and day.

  • And this day-night cycle is what drives our weather.

  • The Earth is warm on the daytime side,

  • cool on the nighttime side,

  • and it's warmer at the equator than at the poles.

  • The differences in temperature

  • make differences in air pressure,

  • and the differences in air pressure

  • make air move.

  • They make the wind blow.

  • But, because the Earth spins,

  • the moving air curves to the right

  • in the Northern Hemisphere

  • because of the coriolis effect.

  • If there's a region of low pressure in the atmosphere,

  • air is pushed toward it,

  • like water going down a drain.

  • But the air curves to the right as it goes,

  • and this gives it a spin.

  • With the dramatic low pressure in a storm,

  • the air gets pulled in tighter and tighter,

  • so it gets going faster and faster,

  • and this is how we get the high winds of a hurricane.

  • So, when you see a spinning storm on a weather report,

  • think about this:

  • The spin ultimately came from the spin of the Earth,

  • and the Earth's spin is a remnant,

  • a fossil relic,

  • of the gentle spin of the cloud of dust and gas

  • that collapsed to make the Earth

  • some five billion years ago.

  • You are watching something, the spin,

  • that is older than dirt,

  • that's older than rocks,

  • that's older than the Earth itself.

The next time you see a news report of a hurricane

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B1 TED-Ed spin earth spinning straight line faster

【TED-Ed】What on Earth is spin? - Brian Jones

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    Zenn posted on 2013/10/29
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