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Homework Help: Calculate the frictional torque of the record player

  1. Nov 18, 2007 #1
    URGENT! rotational kinematics

    A record player is spinning at 33 rpm when the motor is switched off. After 5.9 seconds the angular speed has decreased to 20 rpm. Calculate the frictional torque if the record player can be treated as a solid wheel of mass 5.5 kg and radius 14 cm.

    A. 1187.62712 Nm
    B.0.01244 Nm
    C. 0.04752 Nm
    D. 2375.25424 Nm
    E. 0.11876 Nm

    Which one?
  2. jcsd
  3. Nov 18, 2007 #2


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    Show us your work. We don't give out answers here. We will help you figure it out.
  4. Nov 18, 2007 #3
    omege final = 3.45 rad/sec

    omega initial = 2.094 rad/sec

    omega final = omega initial + alpha x delta t

    2.094 = 3.45 + 5.9 alpha

    alpha = -0.230

    torque = I x alpha

    mR^2 x alpha
    (0.1078) (-0.229)
    = -0.02468

    ??????????? none of these answers
  5. Nov 18, 2007 #4
    Plzz Hurrrrryyyy Folks!!
  6. Nov 18, 2007 #5


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    You are not using the right moment of inertia. For a solid wheel it should be 0.5MR^2.
  7. Nov 18, 2007 #6


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  8. Nov 18, 2007 #7
    Ok what about this????


    A solid sphere of mass 4.0 kg and radius 0.12 m starts from rest at the top of a ramp inclined 15 degrees, and rolls without slipping to the bottom. The upper end of the ramp is 2.7 m higher than the lower end. What is the linear speed of the sphere when it reaches the bottom of the ramp?

    A. 132.3 m/s
    B. 6.15 m/s
    C. 7.27 m/s
    D. 7.67 m/s
    E. 37.8 m/s

    i 've tried PE = 1/2 mv2 + 1/2 Iw2

    but nothing worked please HELP!!!!!!!!!!!!
  9. Nov 18, 2007 #8


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    What did you use for I? Did you put w in terms of v?
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