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  • \f0\fs28 \cf0 Imagine learning for the first eighteen years of your life that the earth

  • is flat. All through elementary school and high school you grow up hearing about the

  • flat earth we live on and doing boring flat-earth physics homework and then (if you're lucky

  • enough) you get to college and PSYCH! \f1 for the first time they show you a globe

  • and say "sorry for lying, the earth is actually round".\

  • \ Well, this is, unfortunately, exactly what

  • we do with\ \

  • 1) Gravity\ You probably learned that objects attract

  • each other based on their mass so you probably grew up thinking that light can't possibly

  • be affected by gravity because light is massless. I know I did. Well guess what? The source

  • of gravity is not mass - it's energy and momentum, which light certainly has (of course, regular

  • matter does too). So not only does light get bent passing by a star or planet or black

  • hole, but light attracts the planet or star or black hole in return (to be sure, it's

  • only a very very small amount. But a small amount is not zero). Anyway, the point is

  • that Newton's law of gravitation is just an approximation - good enough to get us to the

  • moon, but not perfect. General relativity is better.\

  • \ 2) Special Relativity\

  • Speaking of the moon, you probably also learned that if a sheep is moving 2 mph relative to

  • a train and that train is moving 2 mph in the same direction relative to the ground,

  • then the sheep is moving four mph relative to the ground. 2mph+2mph=4mph, right? FALSE.

  • Experiments in special relativity have confirmed that velocities don't simply add together

  • and so the sheep will in fact be moving very very ever-so-slightly slower than four mph

  • relative to the ground. And the formula that correctly predicts this deviation from just

  • adding the velocities is (v1+v2)/(1+v1*v2/c^2). It's not a very big effect, but then again,

  • the earth looks pretty flat, doesn't it?\ \

  • But the earth isn't flat: if I walk 10,000km away from my cat, and you continue on walking

  • 10,000km more, you're NOT 20,000km away from my cat. You're just 12,750km away\'85 in fact,

  • the farthest on earth you can get from ANYTHING on earth is 12,750km. It's the "earthly distance-limit",

  • though we normally call it the diameter of the earth. And similarly, when you try to

  • add two velocities together, there's a "cosmic speed limit" of 300,000,000m/s - that is,

  • the speed of light.\ \

  • So, just because to our eyes the earth looks flat, velocities look like they simply add

  • together, and light looks like it doesn't attract gravitationally, is that an excuse

  • to mislead ourselves and our children about the true nature of things?}

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B1 中級

常見的物理學誤區 (Common Physics Misconceptions)

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    賴亭喻 發佈於 2021 年 01 月 14 日
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