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Objects cannot have a velocity > c. But what about acceleration?

  1. Mar 27, 2010 #1
    Ok I was doing a question about electrons and this question arose.

    Imagine we have a tremendous, constant mechanical force used to "push"(accelerate) an electron. At the moment the force is applied, the acceleration is easily greater than c in terms of magnitude. So what happens? Is the acceleration greater than c? But what happens after 1s? do we explain the electron's behaviour and velocity using relativity?

    What I am thinking is that it IS possible to accelerate an electron, but as its velocity goes up, so does its mass. So eventually the electron would be accelerated slower and slower and thus its velocity would never reach c. Am I right here? Have I missed anything? Cheers
  2. jcsd
  3. Mar 27, 2010 #2
    c is a velocity. Acceleration is acceleration. The question can you have an acceleration greater then c is meaningless.
    Last edited: Mar 27, 2010
  4. Mar 27, 2010 #3
    So you can have an acceleration greater than the value of c right? Sorry man the question was confusing. What I meant was can objects have an acceleration greater than the value of c. I always had the impression that if objects cannot have a velocity greater than c, so does its acceleration.
  5. Mar 27, 2010 #4

    Vanadium 50

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    Acceleration is acceleration and velocity is velocity. They are different things, measured in different units. Talking about "an acceleration greater than c" is like talking about "a sound greater than green".
  6. Mar 27, 2010 #5

    Filip Larsen

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    The theory of special relativity does not in itself limit the acceleration of a physical object, but there will of course, depending on the situation, be practical limits as to how much acceleration you can give an object (power requirements, heating, maximum shear stress, and so on). I would think that you will be able to get some of the highest practical obtainable accelerations experiments involving particle physics, or perhaps in constructions like the railgun.
  7. Mar 27, 2010 #6
    I agree to all the reserve of the preceeding posts.
    An acceleration greater than c in magnitude only means that the object would reach the spped of light from rest in less that 1 second. That's all… And that's impossible.
    Now, as a thought experiment, calculate the acceleration of an object which goes from rest to, say, 0.9c in 1/10 of a second. Well, that's trivial : the acceleration is already 9c m.s-2. Which is what you mean by “acceleration greater than c” :)
  8. Mar 27, 2010 #7


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    That's still meaningless. Doesn't matter how many times you say it.
    An acceleration of 0 to C in 1 second isn't "an acceleration greater than c", but it does make more sense: it is 300,000 km/s/s. And no, it most certainly is not impossible. Particle accelerators far exceed that value of acceleration. Heck, it wouldn't be surprising to me if the electron gun in an ordinary TV exceeds that acceleration.
    It's good that you've now clarified mathematically what you mean, but now please start describing it correctly!
  9. Mar 27, 2010 #8
    Don't fool yourself… Obviously, the question of the OP was “Is an acceleration of more than 299792458 m.s-2 possible ?”. At least, that's how I understand it…
    The answer is yes. His choice of words was poor, that's all :)

    PS : I was saying that it is impossible for a material object to reach the speed of light.
  10. Mar 27, 2010 #9

    Vanadium 50

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    Why is that more obviously correct than "Is an acceleration of more than 1 billion km/hr^2 possible ?”
  11. Mar 27, 2010 #10
    OK, ok… Maybe I misanderstood the OP… I don't know :)
    Let's wait for sakodo to clarify :)
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