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Homework Help: Oblique Impact of Smooth Spheres

  1. Feb 14, 2012 #1
    Hey, I'm struggling with this question, any help would be great.

    A sphere of mass m impinges obliquely on a sphere of mass M, which is at rest. The coefficient of restitution between the spheres is e. Show that if m=eM, the directions of motion after impact are at right angles

    My attempt was:

    mUcosα = mV1 + MV2 (1)
    V2-V1 = e(Ucosα) (2)

    since m=eM
    => eMcosα = eMV1 + Mv2 then /m

    so subtracting the equations gives
    eUcosα = eV1 + V2
    - eUcosα = V2 - V1

    => 0 = eV1 + V1
    e = -1

    which is where I just dont understand what to do now :(
  2. jcsd
  3. Feb 14, 2012 #2


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    Science Advisor

    Well, if you want to show that " the directions of motion after impact are at right angles" you had better work with the directions of motion hadn't you? Yet, I see no indication of x or y components. You see to be treating V1 and V2 as numbers- that is, assuming the two masses move in a straight line.
  4. Feb 14, 2012 #3
    Hi, I used the parallel and perpendicular components and yeah the masses move in a linear direction.
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