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Simple MCQ

  1. Oct 24, 2009 #1
    1. The problem statement, all variables and given/known data
    A ball whose momentum is p strikes a wall and bounces off, The change in the balls momentum is:
    A- 0
    B- P
    C- 2p
    D- p/2
    2. Relevant equations

    3. The attempt at a solution
    It's simple question but I want to make sure if my answer is true.

    The change in momentum = 0

    According the momentum conservation principle.

    Right ?
  2. jcsd
  3. Oct 24, 2009 #2
    The problem is asking for the change in momentum of the ball, not the ball+wall system.

    Remember that momentum is a vector quantity--if the momentum is p before bouncing, what is the momentum when the ball bounces?

    You are missing a bit of information here in your description--is the ball bouncing back the direction it came from, or is it being bounced at an angle?
    Last edited: Oct 24, 2009
  4. Oct 24, 2009 #3
    That's not mentioned in the problem, but I thing it bounced in the same direction it came from.

    momentum after bouncing would be -p

    so change in momentum is p - (-p) = p + p = 2p is the answer then.

    BUT how did we know the the momentum after bouncing is equal to momentum after bouncing since there could be change in the velocity, and velocity affect on the magnitude of the momentum because momentum = mass × velocity
  5. Oct 24, 2009 #4
    You can't know, unless you specify the constraint that the bounce is completely elastic and hence total mechanical energy is also conserved.
  6. Oct 24, 2009 #5
    Thanks for your replies.

    This problem is from past exam paper, it asked to choose the most suitable answer.

    The most suitable answer is 2p isn't ??


    Another problem:

    Which of the following automobile safety features uses the Impulse Principle to reduce injury during Road Traffic Accidents?

    a. Lap-shoulder seat belt.
    b. Active head rest.
    c. Collapsible steering column.
    d. All of the above.
    e. None of the above.


    I get confusion !!!!
    Last edited: Oct 24, 2009
  7. Oct 24, 2009 #6
    We assume the wall to have a significantly larger mass compared to the ball, such that the wall essentially remains stationary, and thus the ball rebounds with the same speed in the opposite direction. (Try to prove it using COLM when M>>m and assuming elastic collision)

    As for your next problem, firstly, what do you understand about the "Impulse Principle"?
  8. Oct 24, 2009 #7
    OK Thanks,

    Impulse = Force × Time taken

    If we increase the time we will decrease the force applied on the body so that we will reduce injuries.

    a. Lap-shoulder seat belt. >> It use impulse principle by increasing time collision
    b. Active head rest. >> It use inertia priciple
    c. Collapsible steering column. >> It use impulse principle by increasing time collision
    d. All of the above. >> False because B is not correct
    e. None of the above. >> False

    So ?


    Another problem:

    In gunshot injuries, which of the following is correct?

    a. Shock-wave damage can temporarily compress tissue up to a maximum of 10 atm.
    b. The shock-wave is transmitted mostly along skeletal bone.
    c. Only those tissues in direct contact with the bullet will be damaged.
    d. Enclosed liquids within the body are particularly sensitive to cavitation damage.
    e. None of the above.

    The answer is d isn't it ?

    P.S: sorry, I posed a lot of question cuz tomorrow I've quiz
    Last edited: Oct 24, 2009
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