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Homework Help: Impulse of a ball hitting the wall

  1. Jun 16, 2011 #1
    1. The problem statement, all variables and given/known data
    Calculate the change in momentum of a 120g pool ball hitting at 9m/s and rebounding at 6m/s. It rebounds at an equal and opposite angle of 45 degrees.

    Pi = 9m/s
    Pf = 6m/s
    angle = 45 degrees

    2. Relevant equations



    3. The attempt at a solution
    I have done all my calcs in the picture below which shows a diagram

    I have concluded that the impluse was 10.8166 N, somehow I don't think I have done this right
     

    Attached Files:

  2. jcsd
  3. Jun 16, 2011 #2
    Well, you're right that you're wrong. Given that the impulse [itex]I = m\Delta v[/itex], consider breaking the velocities into orthagonal components. Which component will the normal force change? :wink:
     
  4. Jun 16, 2011 #3

    tiny-tim

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    welcome to pf!

    hi miniradman! welcome to pf! :wink:
    looks fine :smile:, except that you've not yet multiplied by the mass :wink:

    (and do you need to specify the direction of the https://www.physicsforums.com/library.php?do=view_item&itemid=340" also?)
     
    Last edited by a moderator: Apr 26, 2017
  5. Jun 16, 2011 #4
    Re: welcome to pf!

    I don't think so...

    Also, I have trouble remembering which one to flip around and make a negative? Is there are certain rule of guideline I have to follow to do this?

    Thanks ;)
     
    Last edited by a moderator: Apr 26, 2017
  6. Jun 16, 2011 #5

    tiny-tim

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    hi miniradman! :smile:
    i'm not sure i follow :confused:

    if the momentum before is p = (a,b), and the momentum after is q = (c,d),

    then the change in momentum is q - p = (c-a,d-b) :wink:
     
  7. Jun 17, 2011 #6
    What I did, was I flipped the 6m/s around so it becomes -6m/s (beause I took them way like vectors). Because it is on an angle of 45 degrees, I added the vectors head to tail

    is this right?
     
  8. Jun 17, 2011 #7

    tiny-tim

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    hi miniradman! :wink:

    yes, you can either subtract the original vectors, tail-to-tail, or flip one so that it becomes adding, which is head-to-tail :smile:
     
  9. Jun 17, 2011 #8
    awesome! Thanks mate ;)
     
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