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Homework Help: Coefficients of kinetic friction, help PLEASE!

  1. Sep 1, 2007 #1
    Here is the problem:

    A hockey puck is hit on a frozen lake and starts moving with a speed of 13.0 m/s. Five seconds later, its speed is 5.20 m/s.

    (a) What is its average acceleration?

    I used the kinematic equation V=Vo+at to find the accerlation which came out to be -1.56 m/s^2 (this answer is right).

    (b) What is the average value of the coefficient of kinetic friction between puck and ice?

    (c) How far does the puck travel during the 5.00 s interval? m

    I used X-Xo=1/2[V+Vo]t to solve this and got 45.5 m, which is right.


    So basically, I'm stuck on part B of this problem. I don't remember my professor going over how to solve this particular type of problem in class so I was hoping someone here could get me started. I'm not asking for the answer, just an explanation of how to solve Part B.
     
  2. jcsd
  3. Sep 1, 2007 #2

    hage567

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    Think about forces and Newton's second law. You have already figured out the acceleration. What's the definition of kinetic friction?
     
  4. Sep 1, 2007 #3
    I assume I would make F=ma equal to Fk=uN.

    However, I don't see how I can do this since the mass wasn't given.
     
  5. Sep 1, 2007 #4

    hage567

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    OK, but what is the normal force equal to?
     
  6. Sep 1, 2007 #5
    The normal force is equal to the weight, which is mass x gravity, but again I get lost because I don't have the mass.
     
  7. Sep 1, 2007 #6
    Wait, nevermind. I got it. I divided acceleration/gravity and came out with the answer.

    Thanks for your help though!!
     
  8. Sep 1, 2007 #7

    hage567

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    Good!

    You're welcome :smile:
     
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