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Homework Help: Galilean invariance verification (kinetic energy and momentum)

  1. Aug 9, 2011 #1
    Hi all,

    First of all, sorry for not using the template, but I think in this situation it's better to explain my problem right away:
    I'm studying for a physics test, but I think I don't really understand Galilean invariance. In my textbook there is an example in which they proof that if you consider 2 frames S and S' in standard configuration that the second law of Newton is Galilean invariant by proofing that if [itex]x' = x - Vt[/itex] than [itex]F_x = F'_x[/itex], so this law holds in both frames. So far I understand this.

    However, in the book there is one assignment in which they ask me to verify that the relationship between kinetic energy and momentum, [itex]E = p^2/2m[/itex], is Galilean invariant. I couldn't really figure it out by myself so I looked at the answers. The answer is as followed:
    Source: McComb, W. D., 1999. Dynamics and Relativity. New York: Oxford University Press Inc.

    I understand all of the equations, however I just don't understand why this verifies that this relationship is Galilean invariant.

    Can someone explain this to me?

  2. jcsd
  3. Aug 9, 2011 #2


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    Staff Emeritus
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    There is a problem with the computation of E' in the solutions. It should read:

    [tex] E^\prime = \frac{1}{2}m(\dot{x}^{\prime})^2 =\frac{1}{2}m(\dot{x}-V)^2 = \frac{1}{2}m(\dot{x}^2 - 2\dot{x}V + V^2) [/tex]

    [tex] = \frac{1}{2}m\dot{x}^2 - m\dot{x}V + \frac{1}{2}mV^2 [/tex]

    You can see that this expression for E' is the same as what is obtained if you assume that E' = (p')2/2m, showing that this relation between kinetic energy and momentum does indeed hold in S' as well.
  4. Aug 10, 2011 #3
    thank you sp much! I completely missed that. now it'd all clear to me:)
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