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Homework Help: Linear momentum translational problem

  1. Aug 27, 2008 #1
    1. The problem statement, all variables and given/known data

    A number of particles with masses m(1), m(2) , m(3),... are situated at the points with positions vectors r(1),r(2), r(3),... relative to an origin O. The center of mass G of the particles is defined to be the point of space with position vector

    R=m(1)r(1)+m(2)r(2)+m(3)r(3)/(m(1)+m(2)+m(3))

    Show that if a different Origin O' were used , this definition would still place G at the same point of space

    2. Relevant equations
    R=m(1)r(1)+m(2)r(2)+m(3)r(3)/(m(1)+m(2)+m(3))

    Possibly C+R=R'


    3. The attempt at a solution

    I think I need to translate R to a new coordinate system , which is O', and essentially show that If a vectors moves into a new coordinate system , calling constant c the distance between the new coordinate system and the old coordinate system, I have to show the magnitude of the vectors don't changed. So here it goes:

    R'=(m1)*(r1+c)+(m2)(r2+c)+(m3)(r3+c)/(m1+m2+m3)=m1r1+mc+m2r2+mc+m3r3+mc/(m1+m2+m3)
    C=R'-R=c(m1+m2+m3)/(m1+m2+m3)=c; Therefore, R=R'-C. Is that the procedure I would apply to proved That the magnitude of the vectors do not change at all as I move my position vectors into a new coordinate system?
     
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  3. Aug 27, 2008 #2

    Dick

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    I think, if I'm reading what you have written correctly, that you have shown that if you translate all of the ri vectors by c then the center of mass moves by c? If so then doesn't that show that the center of mass is translation independent?
     
  4. Aug 28, 2008 #3
    I am trying to show that the magnitude of the position vectors will not change if I move my positions vectors to a new coordinate system. Isn't that what translation independence is?
     
  5. Aug 28, 2008 #4

    tiny-tim

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    Hi Benzoate! :smile:

    Why do you keep going on about magnitude?

    Magnitude has nothing to do with it.

    As Dick says:
    You have proved (very messily, and only for n = 3 … can't you use ∑ notation?) that if R is the average of R1 R2 … Rn then R+C is the average of R1+C R2+C … Rn+C.

    In other words: "this definition would still place G at the same point of space". :smile:
     
  6. Aug 28, 2008 #5

    Redbelly98

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    The magnitude of the position vectors does change when you move the origin's location.

    |ri + c| is not |ri|

    But that is irrelevant to solving this problem.

    Moving the origin is equivalent to adding a constant vector c to each position vector. By showing that the center of mass R becomes R + c, you prove that the CM is in the same location after the origin shift. Just as Dick said:

    Edit --
    Note to self: I owe tiny-tim a beer :smile:
     
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