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Homework Help: Final velocity of this hockey puck

  1. Nov 14, 2012 #1
    1. The problem statement, all variables and given/known data

    In the last second of a hockey game, a player slaps the puck in the empty net 32.5 m away. The puck leaves the stick with the speed of 41.5 m/s. Calculate the final velocity of the puck if its mass is 0.17 kg.

    2. Relevant equations

    μK = 0.005
    μK = FK / FN
    F = ma
    v22 = v12 + 2ad

    3. The attempt at a solution

    Force normal of puck:
    FN = Fg = m x g
    = 0.17 kg x 9.8 m/s2
    = 1.67 N​

    μK = FK / FN
    FK = μK / FN
    = 0.005 / 1.67 N ​
    = 0.00835 N​

    a = (Funbalance/ m)
    = (0.00835 N / 0.17 kg​
    0.049 m/s2

    Finally, I solve for v2:

    v2 = √(v12 + 2ad)
    = √(41.52 + 2(0.049)(32.5)​
    = 41.5 m/s2

    My problem is that my final velocity is the same as my initial velocity? Where did I go wrong?
  2. jcsd
  3. Nov 14, 2012 #2
    Well, you did do something wrong, but it won't affect your answer that much.

    What direction is the acceleration going in?
  4. Nov 14, 2012 #3


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    You went wrong twice: first, your algebra error when calculating friction force, which should be uk(N). Then , watch your signage for acceleration when using the kinematic equations.
  5. Nov 14, 2012 #4
    I feel dumb, thanks.
  6. Nov 14, 2012 #5


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    You have two problems.
    The acceleration is so small that you have lost the change in velocity in the rounding error. On the other hand, if you show more digits in your answer then you will be guilty of gaining precision: you'll have more significant digits in the answer than in the provided data. So the short answer is the change in speed is negligible.
    Your second problem is that you have acceleration going the wrong way. The dynamic friction seems to be making your puck go faster.
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