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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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