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Calculating the coefficicent of friction

  1. Dec 18, 2006 #1
    1. The problem statement, all variables and given/known data

    A hockey Puck is hit and starts moving at 12m/s. Exactly 5 s later its speed is 6m/s. What is the coefficient of friction? How far has the pick traveled during the 5 s?

    2. Relevant equations

    [tex]\vec{F}_{net} = \Sigma \vec{F} = m \vec{a}[/tex]
    [tex]f_k = \mu_k N[/tex]

    3. The attempt at a solution

    [tex]v = v_0 + a t[/tex]
    [tex]6m/s = 12m/s + a 5s[/tex]
    a= -1.2 m/s^2

    [tex]x = x_0 + v_0 t + (1/2) a t^2[/tex]
    [tex]x = 0m + 12*5s + (1/2) (-1.2m/s^2) 5^2[/tex]
    x= 45m

    Not sure how to precede. Answers are x =75m and coefficient of friction = 0.12. I apparently did the first part wrong:redface:
  2. jcsd
  3. Dec 18, 2006 #2


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    Staff Emeritus
    Science Advisor

    I think you're right. Your acceleration is definitely correct, and your substitution into the distance equation is correct. (To get x=75, one would put in 1.2 for the acceleration, but it is clearly decelerating!)

    For the next part, use your two equations concerning the force. Put the mass in as m; it will cancel. Can you go on from here?
  4. Dec 18, 2006 #3


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    Staff: Mentor

    You have the correct magnitude of acceleration, which is consistent with 0.12.

    The weight of the puck is mg, and the force of friction is mu*mg = ma, so

    a = mu * g or mu = a/g, and g = 9.81 m/s2.

    So one has to determine how far a puck travels while decelerating at a constant 1.2 m/s2, i.e. -1.2 m/s2 acceleration.
  5. Dec 18, 2006 #4
    haha thanks for the help! Forgot that the Normal force (N) can be replaced with "mg"
  6. Dec 18, 2006 #5
    dang i could helped you with this one, we just finished are test on this. Cool
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