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Homework Help: Momentum at the speed of light

  1. Nov 16, 2009 #1
    1. The problem statement, all variables and given/known data
    Why is the value of the electron's momentum according to special relativity larger than that predicted by Newtonian mechanics?

    A. At high speeds, the electron responds to forces and collisions as if its mass
    were greater than the rest mass
    B. At high speeds, the total momentum of two colliding particles is not conserved
    C. Special relativity only applies at speeds close to the speed of light

    2. Relevant equations
    p= m(Vo)/sqrt(1-beta^2)

    3. The attempt at a solution
    I believe that the answer is A because as the velocity of an object approaches the speed of light, the momentum increases because its mass begins increase. So therefore at high speeds it would act as if its mass is greater than if it were at rest. Please let me know if this is the correct answer. Thank you
  2. jcsd
  3. Nov 16, 2009 #2
    you're correct, at high speeds it's mass energy is greater than that of it's rest mass, so it's mass (and therefore momentum) increases, also seen by [tex] E = \frac{mc^2}{root(1-B^2)} [/tex]
  4. Nov 16, 2009 #3
    Well it is true but there is something that is not fully clear here;

    Statement C is also right, if an object is not close to the speed of light the change in its mass can be neglectible,it is just so little.But it is still appliable.So i guess the answer is A but in my opinion statement C should be clearer anyway...
  5. Nov 16, 2009 #4
    thanks a lot for everyone's help!
  6. Nov 17, 2009 #5


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Glad it worked out. I solved this by process of elimination; without thinking much about it B and C are false statements.
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