1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Momentum in Galilean transformation

  1. Jan 29, 2006 #1

    Pengwuino

    User Avatar
    Gold Member

    I need to show that the definition of linear momentum p=mv, has the same form p'=mv' under a Galilean transformation. What does it mean to "show" such a thing? I have no idea where to start :(
     
  2. jcsd
  3. Jan 30, 2006 #2
    It means that you have to apply the Galileo transformation to your momentum p and end up with p' in your transformated system.
     
  4. Jan 30, 2006 #3

    Pengwuino

    User Avatar
    Gold Member

    How would I go about doing that...
     
  5. Jan 30, 2006 #4
    Say you have a primed system moving with a velocity v_0 relative to your unprimed system. What is the Galileo transformation then? What does that tell you about the velocity transformation? The mass m is a constant, so what does that tell you about the momentum?
     
    Last edited: Jan 30, 2006
  6. Jan 30, 2006 #5

    Pengwuino

    User Avatar
    Gold Member

    I'm not sure what the velocity transformation is. I know x'=x-vt but i don't konw where to go from there if that's even the right place to start.
     
  7. Jan 30, 2006 #6
    It is. Recall that x=x(t), now what do you get when you apply d/dt to it?
     
  8. Jan 31, 2006 #7

    Pengwuino

    User Avatar
    Gold Member

    Ok my professor gave me a little and I got here…

    [tex]\begin{array}{l}
    x' = x - ut \\
    p = mv \\
    p = m\frac{d}{{dt}}(x' + ut) \\
    p = m(v' + u) \\
    p = mv' + mu \\
    p = p' + mu \\
    \end{array}[/tex]

    So what exactly does this tell me? What does p' mean in actuality? If K is the stationary frame, is it the momentum as seen by the person in the reference frame K'? And is p simply the momentum from the K frame?
     
  9. Feb 1, 2006 #8

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    To answer your questions: yes and yes.

    What you are trying to show is that if a person in the K frame sees an object with momentum p = mv, where v is the velocity of the object in the K frame, then will a person in the K' frame see the object as having a moment p' = mv', where v' is the object's velocity as measured in the K' frame? If he doesn't, then the laws of physics (specifically the equation for momentum) are not *invariant* under a Galilean transformation. But it turns out they are.:

    If the object has velocity v = dx/dt in K, then in K' its velocity is dx'/dt = d(x - ut)/dt = dx/dt - u = v - u. So we have established that v' = v - u in a Galilean transformation.

    And since in your result you got p' = m(v-u) = mv', you know you're ok.
     
  10. Feb 1, 2006 #9

    Pengwuino

    User Avatar
    Gold Member

    So p is the momentum in the rest frame? And is the p' the momentum as seen from the K' frame?
     
  11. Feb 1, 2006 #10

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Umm...the notation does seem pretty unambiguous to me. Primed quantities are measured relative to the primed coordinate system. Unprimed quantities are measured relative to the unprimed coordinate system.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Momentum in Galilean transformation
  1. Galilean transform (Replies: 1)

Loading...