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Homework Help: Momentum, elastic collision

  1. Dec 13, 2007 #1
    The drawing shows a collision between two pucks on an air-hockey table. Puck A has a mass of 0.023 kg and is moving along the x axis with a velocity of +5.5 m/s. It makes a collision with puck B, which has a mass of 0.048 kg and is initially at rest. The collision is not head-on. After the collision, the two pucks fly apart with the angles shown in the drawing.


    (Puck A is 65 degrees north of east, puck B is 37 degrees south of east.)

    I used MaVa = MaVa(cos 65) + MbVb(cos 37), which gives me
    .1265 = .0097202Va + .03833Vb. Since the total momentum in the Y direction was zero, I also used
    Va(sin 65) = Vb(sin 37)
    Va = .6640Vb or Vb = 1.506Va.

    Every time I plug in those numbers I get a wrong value! Am I missing some important concept, or am I just making an algebraic error?
  2. jcsd
  3. Dec 13, 2007 #2

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    > I used MaVa = MaVa(cos 65) + MbVb(cos 37)

    You Va on RHS should be different from initial Va on LHS.
  4. Dec 15, 2007 #3
    Aah, thanks for pointing that out. I had the Va on the LHS as Vo when I did the problem, though, so it didn't change my answer.
  5. Dec 15, 2007 #4

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    > Va(sin 65) = Vb(sin 37)

    Why? It should be MaVasin 65 = MbVbsin 37.
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