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Homework Help: Motion and collision questions

  1. Sep 15, 2008 #1
    hi im taking grade 12 physics and i have 2 quesions which im not sure about.

    1) a 45 kg student stand on a staitionary 33 kg raft. the student walks with a velocity of 1.9m/s [E] relative to the water. what is the resulting velocity of the raft, relative to the water, if fluid friction is negligible?
    for this i thought it would be that the raft would be moving 1.9 m/s [w] because it will stay staitionary while the student walks [E]. am i right?

    2) There is an elastic collision of two balls on a frictionless table. ball A has a mass of .5 kg and ball B has a mass of .3 kg. Ball A has an initial velocity of 4 m/s in the positive x-direction and a final velocity of 2 m/s in an unknown direction. ball B is initially at rest. Determine the final speed of Vb2 ball B and the angles Alpha and Bata.

    the ball goes straight hits on and they both go forth but in different directions.
    -- O
    ball a ball b -- ball a
    -----O>------o ----------
    -- ball b
    -- o

    i actually dont have enoguh physics background to do this one. so if neone knows how please help me on this.
  2. jcsd
  3. Sep 15, 2008 #2
    Use conservation of momentum on the first one. If they are both at rest at the beginning, and the student starts walking, then the boat will get a momentum that is exactly
    as large as the student's, but in the opposite direction (inverted signs).

    As for the second one, I'm not really sure what you are asking, so all I can really tell you is that you should keep in mind that both momentum and energy is conserved in an elastic collision.
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