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Q & A: velocity vs acceleration

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Most recent answer: 10/22/2007
Q:
A friend and I had an argument we can’t resolve. Imagine a rocket traveling through space. Inside the rocket there is a electron beam pointing at the wall, assuming the beam is in a vacuum, how much would the beam be deflected by velocity and acceleration?
- chris (age 18)
A:
Nice question.
The beam will not be deflected at all by the rocket's velocity. In other words, whatever the velocity of the rocket, the beam will head for the same point on the wall. In fact, this insensitivity to velocity is universal, for all physical effects. Galileo was the first to suggest something like that. It's now known as the principle of relativity. Of course, what it really means is that there's no physical meaning to saying what the rocket's velocity is, since there's no way to test it. All that's meaningful is to say what the velocity is relative to something else, such as a nearby planet.

Acceleration is quite another matter. If the rocket is accelerating, the wall will change its motion while the beam is heading toward the wall, changing the spot where the beam hits. So in that sense the acceleration is absolutely there. Einstein noticed, however, that the effect of the rocket accelerating up looked just like that of the rocket not accelerating but rather the beam being pulled down by a gravitational field. So what really produces the the observable physical effects is not acceleration in the normal sense but rather the difference between the acceleration and any acceleration due to a gravitational field. In other words, the acceleration due to forces such as those produced by firing the rocket has physical effects which can be measured inside the rocket, without reference to the motion of any nearby objects.

Mike W

(published on 10/22/2007)

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