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Speaker 1: Welcome to brain Stuff front House, Stuff works dot Com

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Speaker 1: where smart Happens. Hi Am Marshall Brain with today's question,

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Speaker 1: what would happen if you fired a gun on a

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Speaker 1: train moving as fast as a bullet. This is a

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Speaker 1: good question because it involves the concept of reference frames.

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Speaker 1: The quick answer is that relative to you, the bullet

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Speaker 1: will always travel at the same speed. In other reference frames, however,

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Speaker 1: unexpected things can happen. You may have heard of Newton's

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Speaker 1: first law. Everybody persists in its state of rest or

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Speaker 1: of uniform motion in a straight line unless it's compelled

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Speaker 1: to change that state by forces impressed upon it. We

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Speaker 1: could rephrase this a little and say that a body

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Speaker 1: in motion tends to stay in motion, and a body

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Speaker 1: at rest tends to stay at rest unless acted on

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Speaker 1: by an external force. Imagine that you are on a

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Speaker 1: perfectly smooth, speeding train moving at a uniform speed. It's

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Speaker 1: not accelerating or turning. In a car with no windows,

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Speaker 1: you would have no way of knowing how fast you're going,

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Speaker 1: or if you were moving at all. If you throw

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Speaker 1: a ball straight up in the air, it will come

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Speaker 1: straight back down, whether the train is sitting still or

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Speaker 1: going a thousand miles an hour. Since you and the

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Speaker 1: ball are already moving at the same speed as the train,

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Speaker 1: the only forces acting on the ball are your hand

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Speaker 1: and gravity, so the ball behaves exactly as it would

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Speaker 1: if you were standing on the ground and not moving.

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Speaker 1: So what does this mean for our gun. If the

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Speaker 1: gun shoots bullets at a thousand miles per hour, then

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Speaker 1: the bullet will always move away from the gun at

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Speaker 1: a thousand miles per hour. If you go to the

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Speaker 1: front of the train that's moving at a thousand miles

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Speaker 1: per hour and shoot the gun forward, the bullet will

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Speaker 1: move away from you and the train at one thousand

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Speaker 1: miles per hour, just as it would if the train

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Speaker 1: were stopped, but relative to the ground, the bullet will

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Speaker 1: travel at two thousand miles per hour the speed of

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Speaker 1: the bullet plus the speed of the train, So if

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Speaker 1: the bullet hits something on the ground, it will hit

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Speaker 1: it going two thousand miles per hour. If you shoot

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Speaker 1: the bullet off the back of the train, the bullet

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Speaker 1: will still be moving away from you and the gun

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Speaker 1: at a thousand miles per hour, but now the speed

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Speaker 1: of the train will subtract from the speed of the bullet.

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Speaker 1: Relative to the ground, the bullet won't be moving at

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Speaker 1: all and it will drop straight to the ground. That's

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Speaker 1: true for bullets, but it's not always true for some

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Speaker 1: other things that you might shoot from the front of

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Speaker 1: the train. A great example is sound waves. If you

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Speaker 1: turn on the stereo in your living room, sound waves

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Speaker 1: shoot out of the speakers at the speed of sound,

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Speaker 1: something like seven miles per hour. The waves propagate through

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Speaker 1: the air at that fixed speed, and they can go

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Speaker 1: no faster. So if you put a speaker at the

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Speaker 1: front of a thousand mile per hour train, the sound

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Speaker 1: waves will not depart the train at miles per hour.

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Speaker 1: They can't go faster than the speed of sound. This

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Speaker 1: is the reason why planes traveling faster than the speed

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Speaker 1: of sound creates sonic booms. For more on this and

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