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Coefficient of Friction Between Axe and Grindstone (Torque & MI)

  1. Jun 3, 2006 #1
    A grindstone in the shape of a solid disk with diameter 0.510 and a mass of m= 50.0kg is rotating at w = 900rev/min. You press an ax against the rim with a normal force of N= 160N, and the grindstone comes to rest in 7.00sec.

    Find the coefficient of friction between the ax and the grindstone. You can ignore friction in the bearings.

    ----
    Simply put, I have no idea how to connect the information here to Coefficient of friction.
    I have found Torque using t = I*(Angular Acceleration)
    Where Moment of Inertia = 1/2 MR^2 = 1.63kg*m^2
    And Angular Acceleration was found using w = w(initial) + angular acceleration*t.
    I found the angular acceleration to be -2.14 rev/sec^2 (I converted the angular velocity to rev/sec in finding this).
    And torque was equal to -3.5N...

    I have all this info so far, but have no idea how to connect it to Coefficient of Friction, I thought maybe there would be some ratio with the Normal Force, but I don't have Friction Force so I'm pretty much stuck...

    Any help would be greatly appreciated.

    -L.
     
  2. jcsd
  3. Jun 3, 2006 #2

    arildno

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    Well, the torque must be the radius times the frictional force..:smile:
     
  4. Jun 3, 2006 #3
    That makes perfect sense...

    But...
    I solved for Friction using torque/radius or (3.5)/(.255) = 13.7N

    I then tried to solve for the coefficient of friction using the relationship:
    Friction = Mu*Normal Force
    So 13.7/160 = 0.0856 for coefficient of friction... which is wrong.

    Is that last relationship incorrect in this case?

    -L.
     
  5. Jun 3, 2006 #4

    arildno

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    Well, but you must use angular acceleration measured in radians per second per second in your standard torque equation.
    This is where you've gone wrong; multiply your coefficient of friction with [tex]2\pi[/tex] to get the right value.
     
  6. Jun 3, 2006 #5
    Ohhh I see...
    I have to use radians for angular acceleration otherwise my Torque value is wrong.
    Cool, thanks a lot man.

    -L.
     
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