Speaker 1: Welcome to brain Stuff front House, Stuff works dot Com
Speaker 1: where smart Happens. Hi Am Marshall Brain with today's question,
Speaker 1: what would happen if you fired a gun on a
Speaker 1: train moving as fast as a bullet. This is a
Speaker 1: good question because it involves the concept of reference frames.
Speaker 1: The quick answer is that relative to you, the bullet
Speaker 1: will always travel at the same speed. In other reference frames, however,
Speaker 1: unexpected things can happen. You may have heard of Newton's
Speaker 1: first law. Everybody persists in its state of rest or
Speaker 1: of uniform motion in a straight line unless it's compelled
Speaker 1: to change that state by forces impressed upon it. We
Speaker 1: could rephrase this a little and say that a body
Speaker 1: in motion tends to stay in motion, and a body
Speaker 1: at rest tends to stay at rest unless acted on
Speaker 1: by an external force. Imagine that you are on a
Speaker 1: perfectly smooth, speeding train moving at a uniform speed. It's
Speaker 1: not accelerating or turning. In a car with no windows,
Speaker 1: you would have no way of knowing how fast you're going,
Speaker 1: or if you were moving at all. If you throw
Speaker 1: a ball straight up in the air, it will come
Speaker 1: straight back down, whether the train is sitting still or
Speaker 1: going a thousand miles an hour. Since you and the
Speaker 1: ball are already moving at the same speed as the train,
Speaker 1: the only forces acting on the ball are your hand
Speaker 1: and gravity, so the ball behaves exactly as it would
Speaker 1: if you were standing on the ground and not moving.
Speaker 1: So what does this mean for our gun. If the
Speaker 1: gun shoots bullets at a thousand miles per hour, then
Speaker 1: the bullet will always move away from the gun at
Speaker 1: a thousand miles per hour. If you go to the
Speaker 1: front of the train that's moving at a thousand miles
Speaker 1: per hour and shoot the gun forward, the bullet will
Speaker 1: move away from you and the train at one thousand
Speaker 1: miles per hour, just as it would if the train
Speaker 1: were stopped, but relative to the ground, the bullet will
Speaker 1: travel at two thousand miles per hour the speed of
Speaker 1: the bullet plus the speed of the train, So if
Speaker 1: the bullet hits something on the ground, it will hit
Speaker 1: it going two thousand miles per hour. If you shoot
Speaker 1: the bullet off the back of the train, the bullet
Speaker 1: will still be moving away from you and the gun
Speaker 1: at a thousand miles per hour, but now the speed
Speaker 1: of the train will subtract from the speed of the bullet.
Speaker 1: Relative to the ground, the bullet won't be moving at
Speaker 1: all and it will drop straight to the ground. That's
Speaker 1: true for bullets, but it's not always true for some
Speaker 1: other things that you might shoot from the front of
Speaker 1: the train. A great example is sound waves. If you
Speaker 1: turn on the stereo in your living room, sound waves
Speaker 1: shoot out of the speakers at the speed of sound,
Speaker 1: something like seven miles per hour. The waves propagate through
Speaker 1: the air at that fixed speed, and they can go
Speaker 1: no faster. So if you put a speaker at the
Speaker 1: front of a thousand mile per hour train, the sound
Speaker 1: waves will not depart the train at miles per hour.
Speaker 1: They can't go faster than the speed of sound. This
Speaker 1: is the reason why planes traveling faster than the speed
Speaker 1: of sound creates sonic booms. For more on this and
Speaker 1: thousands of other topics, is it how staff works dot com.
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