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Force/distance/velocity help please

  1. Dec 30, 2012 #1
    A hockey player sends a .4kg puck at 80 m/s. A stationary golie cathces the puck. If the coefficient of friction of the golie on the ice is .05 and his mass is 90 kg how far does he slide on the ice before he comes to a stop.

    I have found
    normal force = mg
    = 885.92N
    and then friction = .05(885.92N)
    = 44.296N
    and using conservation of momentum i found the velocity after the collision will be 0.35m/s
    but now im stuck
    please help.
     
  2. jcsd
  3. Dec 30, 2012 #2

    cepheid

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    Welcome to PF Loading!

    It's good that you've shown an attempt, but in the future please use the template provided.

    So, you have the initial velocity. Given the frictional force that acts, what is the acceleration? So, how far will he get before he slides to a stop?

    Hint: you can solve this just using kinematics. You have vf, vi, a, and you want d. Can you think of any equation with these things in them?

    Note: haven't checked your actual numbers.
     
  4. Dec 30, 2012 #3
    okay so final velocity will be 0 and initial is .35m/s but then i dont have acceleration.
    i also have to account for the friction.

    eqn: Vfinal^2 - Vinitail^2 = 2a x d
     
  5. Dec 30, 2012 #4

    haruspex

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    You know the mass, so you know the normal force. You know the coefficient of friction, so you know the frictional force. Doesn't that give you the acceleration?
     
  6. Dec 30, 2012 #5
    but thats giving me the acceleration due to gravity...man im so confused abt this question
     
  7. Dec 30, 2012 #6

    cepheid

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    No, you computed the friction force in your original post. The friction acts horizontally. It accelerates the player horizontally, to a stop. We're not talking about acceleration due to gravity. We're talking about acceleration resulting from that frictional force.

    So, for friction: you have the force (F), you have the mass (m). How can you find the acceleration (a)? It's like one of the most fundamental laws of mechanics that will let you do so...
     
  8. Dec 30, 2012 #7
    Friction= (m x a)x the coefficient ?
     
  9. Dec 30, 2012 #8
    okay so this is what i have upto now:
    mass of puck = .4kg
    Speed of puck = 80m/s
    Mass of goalie= 90kg and he is at rest
    μ = 0.05

    i found the common velocity after the colision to be 0.35 m/s
    due to vertical equilibrium i stated that Fg=Fn
    Fn=mg
    Fn=885.92N
    and then Friction = μ x Fn
    Friction = 44.296N
     
  10. Dec 30, 2012 #9

    haruspex

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    So the acceleration is?
     
  11. Dec 30, 2012 #10
  12. Dec 30, 2012 #11
    This is good so far. Now, make a list of the horizontal forces acting on the guy. There should only be one item on this list. This is the net force acting on the guy. So, if you apply Newton's second law to the guy, what is his mass times horizontal acceleration? What is his initial velocity? You need to assume that, when his horizontal velocity slows to zero, static friction takes over, and he remains stopped.
     
  13. Dec 30, 2012 #12
    oh ok...ahah wow...thanks for all the help guys :D
     
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