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Looking for equation for change in momentum

  1. Oct 9, 2006 #1
    One of the questions on my test review tells me to look for the equation in the book, but i have a different book because my backpack got stolen and I can only get my hands on a old physics book at the moment. can someone help me out and let me know what equation im looking for here? thanks,

    A pitched baseball, of mass 0.2 kg, arrives at home plate with a speed of 40 m/s and is batted straight back to the pitcher with a return speed of 60 m/s. What is the magnitude of change in the ball's momentum?

    a. 40 kg-m/s b. 8.0 kg-m/s c. 18 kg-m/s d. 20 kg-m/s
     
  2. jcsd
  3. Oct 9, 2006 #2

    Doc Al

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    Staff: Mentor

    You shouldn't need a special formula here. Do you understand that momentum is mass x velocity? The change in anything will be final value minus initial value. (Tricky part: Direction--and signs--matter.)
     
  4. Oct 9, 2006 #3
    so simply:

    (0.2*40)-(0.2* -60) = 20 kg-m/s

    is this correct?
     
  5. Oct 9, 2006 #4

    Doc Al

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    Staff: Mentor

    No. Your main mistake is getting the signs wrong. But you also got initial and final reversed. Fill this out:

    Final momentum =

    Initial momentum =

    Change (Final - Initial) =
     
  6. Oct 9, 2006 #5
    maybe im finding the momentum wrong?

    Final momentum = -12

    Initial momentum = 8

    Change (-12 - 8) = -20 since it's magnatude, just 20. I dont see how it could be anything else.
     
  7. Oct 9, 2006 #6

    Doc Al

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    Staff: Mentor

    Exactly right! (I misread your answer before :redface:--sorry about that. You didn't make the sign error I thought you did! The only mistake was reversing initial and final, but that didn't really matter.)
     
  8. Oct 9, 2006 #7
    thanks, this forum is so helpful.
     
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