Newton's 3 Laws, with a bicycle - Joshua Manley

2,254,534 views ใƒป 2012-09-19

TED-Ed


ไธ‹ใฎ่‹ฑ่ชžๅญ—ๅน•ใ‚’ใƒ€ใƒ–ใƒซใ‚ฏใƒชใƒƒใ‚ฏใ™ใ‚‹ใจๅ‹•็”ปใ‚’ๅ†็”Ÿใงใใพใ™ใ€‚

็ฟป่จณ: Takamitsu Hirono ๆ กๆญฃ: Misaki Sato
00:14
Have you ever noticed that it's harder to start pedaling your bicycle
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ใ‚ใชใŸใฏ ่‡ช่ปข่ปŠใฎใƒšใƒ€ใƒซใฏ ใ‚นใƒ”ใƒผใƒ‰ใŒใฎใฃใŸๆ™‚ใ‚ˆใ‚Šใ‚‚
00:17
than it is to ride at a constant speed?
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ใ“ใŽๅง‹ใ‚ใ‚‹ๆ™‚ใฎๆ–นใŒ ้‡ใŸใ„ใจๆ„Ÿใ˜ใŸใ“ใจใฏใ‚ใ‚Šใพใ™ใ‹๏ผŸ
00:20
Or wondered what causes your bicycle to move?
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ใพใŸ ่‡ช่ปข่ปŠใŒๅ‹•ใ็†็”ฑใ‚„
00:23
Or thought about why it goes forward instead of backwards or sideways?
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ใชใœๅพŒใ‚ใ‚„ๆจชใงใฏใชใ ๅ‰ใซ้€ฒใ‚€ใฎใ‹ ่€ƒใˆใŸใ“ใจใฏใ‚ใ‚Šใพใ™ใ‹๏ผŸ
00:27
Perhaps not, and you wouldn't be alone.
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ๅคšๅˆ†ใชใ„ใงใ—ใ‚‡ใ† ใงใ‚‚ใ‚ใชใŸใ ใ‘ใงใฏใ‚ใ‚Šใพใ›ใ‚“
00:30
It wasn't until the 17th century
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17ไธ–็ด€ใซใชใฃใฆ
00:31
that Isaac Newton described the fundamental laws of motion
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ใ‚ขใ‚คใ‚ถใƒƒใ‚ฏใƒปใƒ‹ใƒฅใƒผใƒˆใƒณใŒ ้‹ๅ‹•ใฎๅŸบๆœฌๆณ•ๅ‰‡ใ‚’่ชฌๆ˜Žใ™ใ‚‹ใพใง
00:34
and we understood the answer to these three questions.
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็งใŸใกใฏ๏ผ“ใคใฎๅ•ใ„ใฎ็ญ”ใˆใ‚’ ็†่งฃใ—ใฆใ„ใชใ‹ใฃใŸใฎใงใ™
00:37
What Newton recognized was that things tend to keep on doing
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ใƒ‹ใƒฅใƒผใƒˆใƒณใŒๆœ€ๅˆใซ็™บ่ฆ‹ใ—ใŸใฎใฏ ็‰ฉไฝ“ใฏ็พๅœจ้€ฒ่กŒไธญใฎ้‹ๅ‹•ใ‚’
00:40
what they are already doing. So when your bicycle is stopped,
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็ถšใ‘ใ‚‹ๆ€ง่ณชใŒใ‚ใ‚‹ใจใ„ใ†ใ“ใจใงใ™ ใคใพใ‚Š่‡ช่ปข่ปŠใŒๆญขใพใ‚‹ใจ
00:43
it stays stopped, and when it is going,
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ใใฎๅ ดใซๅœใพใฃใŸใพใพใง ๅ‹•ใ„ใฆใ„ใ‚‹ๆ™‚ใซใฏ
00:46
it stays going.
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ใใฎๅ‹•ใใ‚’็ถšใ‘ใ‚ˆใ†ใจใ—ใพใ™
00:48
Objects in motion tend to stay in motion
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้‹ๅ‹•ไธญใฎใƒขใƒŽใฏ้‹ๅ‹•ใ‚’็ถšใ‘
00:50
and objects at rest tend to stay at rest.
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้™ๆญขไธญใฎใƒขใƒŽใฏ้™ๆญขใ—็ถšใ‘ใพใ™
00:53
That's Newton's First Law.
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ใ“ใ‚ŒใŒใƒ‹ใƒฅใƒผใƒˆใƒณใฎ็ฌฌ๏ผ‘ๆณ•ๅ‰‡ใงใ™
00:55
Physicists call it the Law of Inertia, which is a fancy way of saying
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็‰ฉ็†ๅญฆ่€…้”ใฏใ“ใ‚Œใ‚’ๆ…ฃๆ€งใฎๆณ•ๅ‰‡ใจๅ‘ผใณใพใ™ ใ€Œ้‹ๅ‹•ไธญใฎ็‰ฉไฝ“ใฏ
00:59
that moving objects don't spontaneously speed up, slow down, or change direction.
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ๅ‹ๆ‰‹ใซใฏๅŠ ้€Ÿใ‚‚ๆธ›้€Ÿใ‚‚ๆ–นๅ‘่ปขๆ›ใ‚‚ใ—ใชใ„ใ€ ใจใ„ใ†ไบ‹ๅฎŸใฎใ“ใชใ‚ŒใŸ่จ€ใ„ๆ–นใงใ™
01:03
It is this inertia that you must overcome to get your bicycle moving.
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่‡ช่ปข่ปŠใ‚’ใ“ใŽๅง‹ใ‚ใ‚‹ใจใใซใฏ ใ“ใฎๆ…ฃๆ€งใซๅ‹ใŸใชใ‘ใ‚Œใฐใ„ใ‘ใพใ›ใ‚“
01:09
Now you know that you have to overcome inertia to get your bicycle moving,
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ใ•ใฆ ๆ…ฃๆ€งใซๅ‹ใคใจ่จ€ใฃใฆใ‚‚
01:12
but what is it that allows you to overcome it?
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ๅ…ทไฝ“็š„ใซใฉใ†ใ™ใ‚‹ใฎใงใ—ใ‚‡ใ†ใ‹๏ผŸ
01:14
Well, the answer is explained by Newton's Second Law.
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็ญ”ใˆใฏใƒ‹ใƒฅใƒผใƒˆใƒณใฎ็ฌฌ๏ผ’ๆณ•ๅ‰‡ใซใ‚ใ‚Šใพใ™
01:18
In mathematical terms, Newton's Second Law says
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ใƒ‹ใƒฅใƒผใƒˆใƒณใฎ็ฌฌ๏ผ’ๆณ•ๅ‰‡ใฏ ๆ•ฐๅญฆ็š„ใช็”จ่ชžใงใฏ
01:20
that force is the product of mass times acceleration.
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ใ€ŒๅŠ›ใฏ๏ผˆๆ…ฃๆ€ง๏ผ‰่ณช้‡ใจๅŠ ้€Ÿๅบฆใฎ็ฉใงใ‚ใ‚‹ใ€ ใจ่กจ็พใ•ใ‚Œใพใ™
01:24
To cause an object to accelerate, or speed up,
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็‰ฉไฝ“ใ‚’ๅ‹•ใ‹ใ—ใŸใ‚Š ๅŠ ้€Ÿใ•ใ›ใŸใ‚Šใ™ใ‚‹ใซใฏ
01:27
a force must be applied.
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ๅŠ›ใ‚’ๅŠ ใˆใ‚‹ๅฟ…่ฆใŒใ‚ใ‚Šใพใ™
01:29
The more force you apply,
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ๅคงใใชๅŠ›ใ‚’ๅŠ ใˆใ‚‹ใปใฉ
01:31
the quicker you accelerate. And the more mass your bicycle has,
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ใ‚ˆใ‚Šๆ—ฉใๅŠ ้€Ÿใ™ใ‚‹ใ“ใจใŒใงใใพใ™ ใ‚‚ใ—่‡ช่ปข่ปŠใŒ้‡ใŸใ‹ใฃใŸใ‚Š
01:34
and the more mass you have too,
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ใ‚ใชใŸ่‡ช่บซใ‚‚้‡ใŸใ‘ใ‚Œใฐ
01:36
the more force you have to use to accelerate at the same rate.
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ๅŒใ˜ๅ‰ฒๅˆใงๅŠ ้€Ÿใ•ใ›ใ‚‹ใŸใ‚ใฎๅŠ›ใฏ ่ณช้‡ใซๆฏ”ไพ‹ใ—ใฆๅคงใใใชใ‚Šใพใ™
01:39
This is why it would be really difficult to pedal a 10,000 pound bicycle.
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ใ“ใ‚ŒใŒ4.5ใƒˆใƒณใฎ่‡ช่ปข่ปŠใ‚’ใ“ใใฎใŒ ใจใฆใ‚‚้›ฃใ—ใ„็†็”ฑใงใ™
01:43
And it is this force, which is applied by your legs pushing down on the pedals,
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ใใ—ใฆใ‚ใชใŸใฎ่„šใŒใƒšใƒ€ใƒซใ‚’ ๆŠผใ—ไธ‹ใ’ใ‚‹ใ“ใจใงๅŠ ใˆใ‚‰ใ‚Œใ‚‹ๅŠ›ใŒ
01:49
that allows you to overcome Newton's Law of Inertia.
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ใƒ‹ใƒฅใƒผใƒˆใƒณใฎๆ…ฃๆ€งใฎๆณ•ๅ‰‡ใซๅ‹ใคใ“ใจใ‚’ ๅฏ่ƒฝใซใ™ใ‚‹ใฎใงใ™
01:52
The harder you push down on the pedals, the bigger the force
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ใ‚ˆใ‚Šๅผทใใƒšใƒ€ใƒซใ‚’ใ“ใ’ใฐ ๅŠ›ใ‚‚ๅคงใใใชใ‚Š
01:54
and the quicker you accelerate.
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ใ‚ˆใ‚Šๆ—ฉใๅŠ ้€Ÿใ™ใ‚‹ใ“ใจใŒใงใใพใ™
01:56
Now on to the final question:
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ใงใฏๆœ€ๅพŒใฎ่ณชๅ•ใงใ™
01:58
When you do get your bike moving,
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ใ‚ใชใŸใฎ่‡ช่ปข่ปŠใฏ
02:00
why does it go forward?
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ใชใœๅ‰ใซ้€ฒใ‚€ใฎใงใ—ใ‚‡ใ†ใ‹๏ผŸ
ใƒ‹ใƒฅใƒผใƒˆใƒณใฎ็ฌฌ๏ผ“ๆณ•ๅ‰‡ใซใ‚ˆใ‚Œใฐ ใฉใ‚“ใชๅŠ›๏ผˆไฝœ็”จ๏ผ‰ใซใ‚‚
02:03
According to Newton's Third Law, for every action,
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02:05
there is an equal and opposite reaction.
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ๅๅฏพๅ‘ใใงๅŒใ˜ๅคงใใ•ใฎๅŠ›๏ผˆๅไฝœ็”จ๏ผ‰ใŒ็”Ÿใ˜ใพใ™
02:07
To understand this, think about what happens when you drop a bouncy ball.
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ใ“ใ‚Œใ‚’็†่งฃใ™ใ‚‹ใŸใ‚ใซ ใ‚ˆใๅผพใ‚€ใƒœใƒผใƒซใ‚’่ฝใจใ—ใŸใจ่€ƒใˆใฆใใ ใ•ใ„
02:12
As the bouncy ball hits the floor,
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ใ‚ˆใๅผพใ‚€ใƒœใƒผใƒซใŒๅบŠใซๅฝ“ใŸใ‚‹ใจ
02:13
it causes a downward force on the floor.
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ๅบŠใซๅฏพใ—ใฆไธ‹ๅ‘ใใฎๅŠ›ใ‚’ๅŠ ใˆใพใ™
02:15
This is the action.
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ใ“ใ‚ŒใŒไฝœ็”จใงใ™
02:17
The floor reacts by pushing on the ball with the same force,
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ๅบŠใฏใƒœใƒผใƒซใ‚’ๅŒใ˜ๅคงใใ•ใฎๅŠ›ใง
02:21
but in the opposite direction, upward,
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ใŸใ ใ—ๅๅฏพใฎไธŠๅ‘ใใซๆŠผใ—่ฟ”ใ—
02:24
causing it to bounce back up to you.
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ใƒœใƒผใƒซใฏ็š†ใ•ใ‚“ใฎใปใ†ใซ่ทณใญ่ฟ”ใ‚Šใพใ™
02:27
Together, the floor and the ball form what's called
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ๅบŠใจใƒœใƒผใƒซใฏ ไฝœ็”จ๏ผๅไฝœ็”จใจๅ‘ผใฐใ‚Œใ‚‹ใ‚‚ใฎใ‚’ไฝœใ‚Šใพใ™
02:29
the action/reaction pair. When it comes to your bicycle,
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่‡ช่ปข่ปŠใฎๅ ดๅˆใฏ ใ‚‚ใ†ๅฐ‘ใ—่ค‡้›‘ใงใ™
02:32
it is a little more complicated. As your bicycle wheels spin
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่‡ช่ปข่ปŠใฎ่ปŠ่ผชใฏ ๆ™‚่จˆใฎใ‚ˆใ†ใซๅ›ž่ปขใ™ใ‚‹ใฎใง
02:35
clockwise, the parts of each tire touching the ground
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ใใ‚Œใžใ‚Œใฎใ‚ฟใ‚คใƒคใฎๆŽฅๅœฐ้ขใฏ
02:39
push backwards against the Earth:
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ๅœฐ้ขใ‚’ๅพŒใ‚ๅ‘ใใซๆŠผใ—ใพใ™ ใ“ใ‚ŒใŒไฝœ็”จใงใ™
02:41
the actions. The ground pushes forward with the same force
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ๅœฐ้ขใฏใใ‚Œใžใ‚Œใฎใ‚ฟใ‚คใƒคใ‚’ ๅŒใ˜ๅŠ›ใงๅ‰ใซๆŠผใ—่ฟ”ใ—ใพใ™
02:45
against each of your tires: the reactions.
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ใ“ใ‚ŒใŒๅไฝœ็”จใงใ™
02:49
Since you have two bicycle tires, each one forms an action/reaction pair
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่‡ช่ปข่ปŠใซใฏ ใ‚ฟใ‚คใƒคใŒ๏ผ’ใคใ‚ใ‚‹ใฎใง ใใ‚Œใžใ‚ŒใŒๅœฐ้ขใจใฎ้–“ใซ
02:53
with the ground. And since the Earth is really, really, really big
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ไฝœ็”จ๏ผๅไฝœ็”จใฎใƒšใ‚ขใ‚’ไฝœใ‚Šใพใ™ ใŸใ ใ—ๅœฐ็ƒใฏ่‡ช่ปข่ปŠใ‚ˆใ‚Šใ‚‚้ฅใ‹ใซๅคงใใ„ใฎใง
02:56
compared to your bicycle, it barely moves
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่‡ช่ปข่ปŠใฎใ‚ฟใ‚คใƒคใŒๅพŒใ‚ๅ‘ใใซๆŠผใ™ๅŠ›ใง
02:59
from the force caused by your bicycle tires pushing backwards,
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ๅœฐ็ƒใŒๅ‹•ใใ“ใจใฏใปใจใ‚“ใฉใชใ
03:02
but you are propelled forward.
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็š†ใ•ใ‚“ใŒๅ‰ใซ้€ฒใ‚€ใฎใงใ™
ใ“ใฎใ‚ฆใ‚งใƒ–ใ‚ตใ‚คใƒˆใซใคใ„ใฆ

ใ“ใฎใ‚ตใ‚คใƒˆใงใฏ่‹ฑ่ชžๅญฆ็ฟ’ใซๅฝน็ซ‹ใคYouTubeๅ‹•็”ปใ‚’็ดนไป‹ใ—ใพใ™ใ€‚ไธ–็•Œไธญใฎไธ€ๆต่ฌ›ๅธซใซใ‚ˆใ‚‹่‹ฑ่ชžใƒฌใƒƒใ‚นใƒณใ‚’่ฆ‹ใ‚‹ใ“ใจใŒใงใใพใ™ใ€‚ๅ„ใƒ“ใƒ‡ใ‚ชใฎใƒšใƒผใ‚ธใซ่กจ็คบใ•ใ‚Œใ‚‹่‹ฑ่ชžๅญ—ๅน•ใ‚’ใƒ€ใƒ–ใƒซใ‚ฏใƒชใƒƒใ‚ฏใ™ใ‚‹ใจใ€ใใ“ใ‹ใ‚‰ใƒ“ใƒ‡ใ‚ชใ‚’ๅ†็”Ÿใ™ใ‚‹ใ“ใจใŒใงใใพใ™ใ€‚ๅญ—ๅน•ใฏใƒ“ใƒ‡ใ‚ชใฎๅ†็”ŸใจๅŒๆœŸใ—ใฆใ‚นใ‚ฏใƒญใƒผใƒซใ—ใพใ™ใ€‚ใ”ๆ„่ฆ‹ใƒปใ”่ฆๆœ›ใŒใ”ใ–ใ„ใพใ—ใŸใ‚‰ใ€ใ“ใกใ‚‰ใฎใŠๅ•ใ„ๅˆใ‚ใ›ใƒ•ใ‚ฉใƒผใƒ ใ‚ˆใ‚Šใ”้€ฃ็ตกใใ ใ•ใ„ใ€‚

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