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Homework Help: Basics True or False, Momentum Problem

  1. Oct 17, 2013 #1
    1. The problem statement, all variables and given/known data


    This problem has analogies in the game of pool. Puck 1 slides on a horizontal, frictionless surface with velocity v and collides elastically with stationary puck 2; each puck has the same mass, m. The pucks move off with velocities v1 and v2, as shown. The angles θ1 and θ2 are measured relative to the initial direction of puck 1.

    Which of these statements is correct?

    1) v^2 = v1^2+v2^2
    2) This problem has analogies to pool because pool balls have equal mass and roll without friction. [As long as rotational KE of pool balls is neglected, the analogy is perfect.]
    3) In this problem, linear momentum is conserved but kinetic energy is not conserved.
    4) 0 = v1sinθ1−v2sinθ2
    5) If b+c = a for three non-zero vectors, and b^2+c^2 = a^2, then the angle between a and c is 90°.

    2. Relevant equations

    mv1 + mv2 = mv1' + mv2'

    3. The attempt at a solution

    1) False
    2) True
    3) False
    4) False
    5) True

    Thinking why Im wrong here, maybe it is number 1? Using the momentum formula, I find that v1 = v1' + v2', not the equation described.

    best regards,
    Last edited: Oct 17, 2013
  2. jcsd
  3. Oct 17, 2013 #2
    funny how everyone is looking for an answer. not to answer someone help. people are very greedy. or at least most of them. I never took the stuff above so I cant answer
  4. Oct 17, 2013 #3
    Sorry I would help you but i'm still learning first year physics
  5. Oct 17, 2013 #4
    maybe i should explain further,

    1) False, because v = v1 + v2 because of conservation of momentum

    2) True, because it makes sense that a pool table would be similar and even says in the question

    3) False, linear momentum is of course conserved because of the conservation of momentum

    4) False, they go off at completely different velocities right? even in y direction

    5) True, The picture is at a 90 degree angle and I read somewhere that this is true, because of right triangles and c and b are the legs of the triangle
    Last edited: Oct 17, 2013
  6. Oct 17, 2013 #5
    Why do you think kinetic energy is not conserved??
  7. Oct 17, 2013 #6
    oh woops, typo
  8. Oct 17, 2013 #7
    still looks wrong guys. maybe wat is going on
  9. Oct 17, 2013 #8
    Kinetic energy and linear momentum both are conserved in elastic collisions.
  10. Oct 17, 2013 #9
    I heard that was true about energy but maybe not momentum,?

    also, the question asks one or the other right? one has to be conserved and the other cannot be conserved. I want to say that kinetic energy is defentiely conserved because where else does the energy go right.
  11. Oct 17, 2013 #10
    please let me know if anyone understands
    Last edited by a moderator: Nov 7, 2013
  12. Oct 17, 2013 #11
    if that was not an elastic collision then Kinetic energy would not have been conserved because there will be energy loses in form of sound energy,change in shape of ball,heat energy, etc
  13. Oct 18, 2013 #12
    You should try to actually compute a few answers. You can compute everything there is to compute with conservation of energy and conservation of momentum (both in the x and y direction)

    1) Since the collision is elastic, kinetic energy is conserved. If you would write down what that means, it would be easy to see the right answer.

    2) ok

    3) ok, but you seem to have misread the question, because the part about energy conservation is wrong.

    4) try to compute this.

    5) "because of right triangles" isn't very sound reasoning. Have you heard of the cosine rule?
  14. Oct 18, 2013 #13
    dont worry dudes, i just asked my teacher and he told me the answers
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