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Finding Distance -- Hockey Puck Velocity

  1. May 10, 2017 #1
    1. The problem statement, all variables and given/known data
    An ice hockey puck leaves a hockey stick with a velocity of 45 m/s, how far will it travel in 3.0 seconds?

    2. Relevant equations
    D = v/t or

    ## \Delta \vec d= \frac 1 2 \vec a \Delta t^2##

    with ## \vec a = v_2 / \Delta t##

    3. The attempt at a solution

    D = v/t
    = 45 m/s / 3 = 135 meters

    or ##0.5 (15 m/s^2) (3)^2## = 67.5 meters

    Which one is it, and more importantly... how do I tell the difference on when to use each formula?

    Thank you
  2. jcsd
  3. May 10, 2017 #2


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  4. May 10, 2017 #3
    Since no friction is mentioned, I believe for this problem you should assume there is no friction, which means the velocity is constant. In other words, a = 0.
  5. May 10, 2017 #4
    Thank you both very much. Very helpful.
  6. May 10, 2017 #5
    Actually a follow up question to this adds friction. The puck is hit with a force of 15.3 N and the friction slowing it down is 0.75 N.
    Same time (3.0 s) same velocity (45 m/s)

    So a = f/m then plug that into the second formula mentioned above?
  7. May 10, 2017 #6


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    Those equations are not quite right. It is ##\Delta d= v\Delta t+ \frac 1 2 a \Delta t^2## and ##a =\Delta v/\Delta t##.
    You are not told the puck stops in 3 seconds. If you assume it keeps going at v then Δv=0 and a =0, so you end up with the same equation as d=vt.
  8. May 10, 2017 #7
    Extremely helpful. Thank you
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