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Homework Help: 3-Object Elastic Collision

  1. Nov 24, 2008 #1
    1. The problem statement, all variables and given/known data

    Matthew and Amanda have challenged Trevor and Julie to a curling match, played with perfectly elastic rocks sliding on level frictionless ice. Three rocks are lined up all in a row (perfectly straight line). B is a junior rock and its mass is exactly half of A or C. B and C are stationary while A approaches. Calculate the time interval between the collision of A&B and the collision of B&C. The distance between B and C is 4.5m. Rock A has an initial velocity of 1.5 m/s.

    Diagram: 2M -----> M __________ 2M
    Distance between M and the right-most 2M is 4.5m.

    2. Relevant equations

    m1v1+m2v2=m1v1^prime + m2v2^prime
    d=vt, where v is equal to v2^prime
    Probably some equation that I missed (main problem of the question that I can't figure out)

    3. The attempt at a solution

    Variables: m1=2 kg
    m2=1 kg
    v1=1.5 m/s
    v2=0 m/s


    Then I tried doing v2^prime=d/t, which got me to:


    I tried subbing that into the other equation to make it 3=2v1^prime+v2^prime, but that got me nowhere. So basically I'm stuck trying to find v2^prime, which is what I need to figure out the time interval between the 2 collisions. I tried to think of the impulse&momentum equations, but they didn't seem to help (can't find the force for the impulse equation).

    Any help is appreciated! Thanks :).

    -ElementUser (first post in these forums)
  2. jcsd
  3. Nov 25, 2008 #2


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi ElementUser! Welcome to PF! :smile:

    (no need to write ^prime … just write ' :wink:)

    In exam questions, "perfectly elastic" means you can use conservation of energy. :wink:
  4. Nov 25, 2008 #3
    Thanks tiny-tim :)

    Took me a long time because my physics teacher today added a portion for this question, that you also have to find the time interval for the second timer Rock B will hit Rock A (after B bounces off of C).
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