WEBVTT - What would happen if you fired a gun on a moving train?

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<v Speaker 1>Welcome to brain Stuff front House, Stuff works dot Com

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

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<v Speaker 1>what would happen if you fired a gun on a

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<v Speaker 1>train moving as fast as a bullet. This is a

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<v Speaker 1>good question because it involves the concept of reference frames.

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<v Speaker 1>The quick answer is that relative to you, the bullet

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

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<v Speaker 1>unexpected things can happen. You may have heard of Newton's

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<v Speaker 1>first law. Everybody persists in its state of rest or

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<v Speaker 1>of uniform motion in a straight line unless it's compelled

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<v Speaker 1>to change that state by forces impressed upon it. We

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<v Speaker 1>could rephrase this a little and say that a body

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<v Speaker 1>in motion tends to stay in motion, and a body

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<v Speaker 1>at rest tends to stay at rest unless acted on

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<v Speaker 1>by an external force. Imagine that you are on a

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

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<v Speaker 1>not accelerating or turning. In a car with no windows,

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<v Speaker 1>you would have no way of knowing how fast you're going,

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<v Speaker 1>or if you were moving at all. If you throw

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<v Speaker 1>a ball straight up in the air, it will come

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<v Speaker 1>straight back down, whether the train is sitting still or

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<v Speaker 1>going a thousand miles an hour. Since you and the

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<v Speaker 1>ball are already moving at the same speed as the train,

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<v Speaker 1>the only forces acting on the ball are your hand

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<v Speaker 1>and gravity, so the ball behaves exactly as it would

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<v Speaker 1>if you were standing on the ground and not moving.

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<v Speaker 1>So what does this mean for our gun. If the

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<v Speaker 1>gun shoots bullets at a thousand miles per hour, then

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<v Speaker 1>the bullet will always move away from the gun at

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<v Speaker 1>a thousand miles per hour. If you go to the

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<v Speaker 1>front of the train that's moving at a thousand miles

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<v Speaker 1>per hour and shoot the gun forward, the bullet will

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<v Speaker 1>move away from you and the train at one thousand

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<v Speaker 1>miles per hour, just as it would if the train

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<v Speaker 1>were stopped, but relative to the ground, the bullet will

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<v Speaker 1>travel at two thousand miles per hour the speed of

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<v Speaker 1>the bullet plus the speed of the train, So if

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<v Speaker 1>the bullet hits something on the ground, it will hit

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<v Speaker 1>it going two thousand miles per hour. If you shoot

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<v Speaker 1>the bullet off the back of the train, the bullet

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<v Speaker 1>will still be moving away from you and the gun

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<v Speaker 1>at a thousand miles per hour, but now the speed

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<v Speaker 1>of the train will subtract from the speed of the bullet.

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<v Speaker 1>Relative to the ground, the bullet won't be moving at

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<v Speaker 1>all and it will drop straight to the ground. That's

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<v Speaker 1>true for bullets, but it's not always true for some

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<v Speaker 1>other things that you might shoot from the front of

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<v Speaker 1>the train. A great example is sound waves. If you

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<v Speaker 1>turn on the stereo in your living room, sound waves

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<v Speaker 1>shoot out of the speakers at the speed of sound,

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<v Speaker 1>something like seven miles per hour. The waves propagate through

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<v Speaker 1>the air at that fixed speed, and they can go

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<v Speaker 1>no faster. So if you put a speaker at the

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<v Speaker 1>front of a thousand mile per hour train, the sound

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<v Speaker 1>waves will not depart the train at miles per hour.

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<v Speaker 1>They can't go faster than the speed of sound. This

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<v Speaker 1>is the reason why planes traveling faster than the speed

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<v Speaker 1>of sound creates sonic booms. For more on this and

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