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Immediate Acceleration Possible?

  1. Feb 1, 2012 #1
    Is it possible that an object changes velocity immediately?

    For example, Object A is ALWAYS travelling at a constant velocity (say 100mph). Its velocity never changes.

    If Object B (which is at rest) were in the path of Object A and Object A were to slam into Object B, would Object B suddenly and immediately accelerate to 100mph in no time, since Object A never changes its velocity (at least theoretically)?
  2. jcsd
  3. Feb 1, 2012 #2
    First it would be very unlikely that object B would have a velocity equal to object A, since all of the energy from object A would have to transfer into object B. Some energy instead is dissipated as heat and some as noise, also some will remain in object A as it bounces away. The remaining energy will be what causes object B to move.

    Also, object B will not move away from object A right away, since both materials would deflect under the impact. Basically, think of a set of very stiff springs. Those springs will deflect building up potential energy until they reach a point that it will turn into kinetic energy causing object A and B to move in a certain direction, which depends on the angle of the impact.
  4. Feb 1, 2012 #3
    That is very true, but I was talking theoretical and emphasized that Object A's velocity cannot change at all. Then, would immediate acceleration be possible?
  5. Feb 1, 2012 #4


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    No. Essentially what you want is an infinitely massive (and rigid) body, which is of course impossible.
  6. Feb 1, 2012 #5
    So you're saying that if you had an infinitely massive and rigid body, immediate acceleration would be possible.
  7. Feb 1, 2012 #6


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    Well, not really. Such objects don't exist, and I don't like statements like this (similar to if you had an infinite amount of energy you could accelerate a massive object to the speed of light). To be precise, in the limit as mass and stiffness increase without bound, time of acceleration asymptotes to zero.
  8. Feb 1, 2012 #7


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    Concur with Nabeshin.

    The laws of physics (Einstein's relativity is particular) forbid infinitely rigid materials, unstoppable forces and infinite accelerations.

    If you want to ask what would happen if something did violate the laws of physics then the answer is, is always, you get pink unicorns and fairies.
  9. Feb 2, 2012 #8
    You're asking what would happen if something that couldn't happen happened.

    Not even that, but you specified MANY thing that couldn't happen. You can't have a "body that doesn't change velocity". That violates conservation of momentum, conservation of energy, Newton's laws, and who knows how many other basic physics concepts. Infinite mass? Infinite rigid body?

    Might as well ask what would happen if God himself slammed into your object. I suppose it would accelerate infinitely fast, but then that means that the force on it is infinitely large, and so the object would probably get instantly annhilated. Unless, of course, God instnatly re-created it.
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