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Need desperate help Velocity

  1. Nov 18, 2008 #1
    Need desperate help!!! Velocity!!!

    The graph shows the change in the velocity of an object with time in a situation in which momentum is conserved. which of the following situations is most likely indicated by the graph. explain why or why not the situations is or is not the best choice..

    Hint: think about conservation of linear momentum and conservation of energy to solve the problem.

    A) The velocity of a bullet striking a ballistic pendulum

    B) The velocity of a car colliding head on with another car of equal weight and velocity

    C) The velocity of a fixed stationary object being hit by a bullet

    D) The velocity of a swinging pendulum

    E) A ball rolling across a level floor
  2. jcsd
  3. Nov 18, 2008 #2
    Re: Need desperate help!!! Velocity!!!

    I'd first cancel out E and D because the question of momentum does not apply...Do you see?I'd also cancel out C,because the object is?Fixed!Its velocity doesnt change and if it does then they would have been irregular energy losses,same applies to B in which we are not so sure the car will bounce back well off the crash,it hasnt been said it is a perfectly elastic collision so practically there would significant energy loss due to heating and friction..That means we remain with A which by me would my choice of the graph!Why do you think so?
  4. Nov 18, 2008 #3
    Re: Need desperate help!!! Velocity!!!

    is it because before the bullet strikes the pendulum the bullet can be assumed to be traveling with constant velocity and after the bullet hits the pendulum there is a huge decrease in velocity because the pendulum is heavy and the change is exponential because the the bullet transfers its energy to the pendulum-bullet system and so it starts oscillating with a small velocity because it's very heavy.
  5. Nov 18, 2008 #4
    Re: Need desperate help!!! Velocity!!!

    Sort of...But more so because in most of these cases (ballistic pendulum) conserve both linear momentum and kinetic energy...which was a condition you were required to fulfill in the question!
  6. Nov 19, 2008 #5

    Doc Al

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    Staff: Mentor

    Re: Need desperate help!!! Velocity!!!

    Without seeing the actual graph, we cannot offer intelligent advice.

    Post the graph and give us your selection and the reasoning behind it.
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