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Homework Help: Momentum: Collisions; Unable to get correct answer, using wrong formula?

  1. Jul 29, 2012 #1
    Hello again everyone!

    I've been plowing through yet another worksheet and thought I'd been okay until my solutions and formulas stopped working :( I was wondering if someone could tell me what I'm doing wrong. Thanks for reading!

    1. The problem statement, all variables and given/known data
    - A 225g ball moves right with a velocity of 30cm/s. (.225kg and .3m/s)

    - This ball collides with a 125g ball also moving to the right with a velocity of 10cm/s.
    (9.125kg and .1m/s)

    - After they collide the velocity of the 125g ball is 24cm/s(.24m/s) to the right. What is the velocity of the 225g ball after the collision?

    2. Relevant equations & 3. The attempt at a solution
    I showed all of my work (information and relevant equations) in the attached document. What I don't understand is why it's all wrong and why the direction I got is negative (it's positive in the answer key).

    Any help would be really appreciated. I have a whole page of these to do but this one is stopping me as I am obviously doing something wrong.:confused:

    Attached Files:

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  3. Jul 29, 2012 #2


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    Hi Apollinaria. Why did you set the final total momentum equal to zero?
  4. Jul 29, 2012 #3
    Hi again TSny, I based it on notes; "If an explosion is occuring in an isolated system, the momentum is conserved but the momentum before the explosion is zero."
    We were given a formula:

    P1' + P2' = 0

    Is there something else I should be using that I'm not aware of?...
  5. Jul 29, 2012 #4


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    Well, in the case of the explosion, the bomb was just sitting there. So, it had no momentum. Thus, the final total momentum must be zero.

    But, you have a different problem. What is the total momentum of the two masses before the collision?
  6. Jul 29, 2012 #5
    So I can only use that formula for when one of the objects is stationary?


    p1= .225kg x .3m/s = 0.0675
    p2= .125kg x .1m/s = 0.0125

    p1 + p2 = 0.08
  7. Jul 29, 2012 #6
    There should be 2 P1 and 2 P2.
  8. Jul 29, 2012 #7
    So I should be using this.......

    p1 + p2 = p1' + p2'

    instead of......
    P1' + P2' = 0

  9. Jul 29, 2012 #8
    Using symbols P11, P21, P12 and P22
    or P1i, P2i,P1f,P2f make them clear of their states
  10. Jul 29, 2012 #9
    I know. However, my confusion isn't with the symbols or states but with which formula to use for that particular problem.
  11. Jul 29, 2012 #10


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    Yes, that's how to set it up. As azizlwl points out, it might be clearer if you use appropriate subscripts to denote initial and final values. But, if you are used to using primes for final values, ok.
  12. Jul 29, 2012 #11
    You find the TOTAL initial momentum and final TOTAL momentum.
    For conservation of momentum, the total initial momentum must be equal to total final momentum.
  13. Jul 29, 2012 #12
    You are amazing :) I use primes because having an obscene amount of subscripts get really confusing. Thanks for the nudge in the right direction:rofl::tongue:
  14. Jul 29, 2012 #13
    Thank you very much for both of your suggestions. I realize in some cases its easier to label initial and final values with "correct" subscripts but it gets messy and long to write out every single time. I finally got this figured out :)
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