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Homework Help: Solve for torque? Torque = Moment of Inertia * Angular Acceleration?

  1. Jan 31, 2009 #1
    I have a wheel with a diameter of 26 inches that weighs 3900 lbs that is spinning at 100 MPH....
    How much torque is needed to accelerate the wheel to 109 mph in 2.0 seconds

    answer must be in ft-lbs
    convert any units as necessary

    please show me the math so I can learn how to do this..I have a few problems similar to this one and I am trying to learn how to do them

    Torque = Moment of Inertia * Angular Acceleration
    Angular Acceleration = Difference in Angular Velocity / Difference in Time

    please help me.. :( I'm feeling so lost....
  2. jcsd
  3. Jan 31, 2009 #2


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    Homework Helper

    Convert MPH to ft/s.
    Convert linear velocity to angular velocity by using w = v/r
    Calculate moment of inertia.
    Calculate angular momentum and rate of change of angular momentum.
  4. Jan 31, 2009 #3
    ok...MPH to feet/sec
    100mph = 146.66666667 ft/sec
    109mph = 159.86666703 ft/sec

    Linear velocity = (159.86666703ft/sec-146.66666667ft/sec)/2 = 6.60000018ft/sec
    angular velocity = 6.60000018/3.403392041=1.939241768revolutions = 698.1270365° = 12.18461538radians/sec


    Moment of inertia = Mass * R²(Distance from center of wheel to outside?)? = 3900lbs*(1.08333333ft)² = 4577.083333 lb-ft²

    then I'm lost.....

    do I need to convert lbs to slugs? moment of inertia in slug-ft²?
    lbs * 0.03108095004
    so 3900* 0.03108095004=121.2157052
    Moment of Inertia = 142.2600984 Slug-ft²

    or was I right above this?
    Last edited: Jan 31, 2009
  5. Jan 31, 2009 #4
    Yes, convert the weight to mass (slugs) to determine the moment of inertia. Since



    [tex]\alpha=\mbox{angular acceleration}[/tex]

    You can determine the torque with units of ft-lbs.
  6. Jan 31, 2009 #5


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    Linear velocity = (159.86666703ft/sec-146.66666667ft/sec)/2 = 6.60000018ft/sec
    How can this be a velocity?
    Change in velocity/time = acceleration. = a.
    Anguler acceleration = a/r
    Moment of Inertia of the wheel = 1/2*M*R^2
  7. Jun 3, 2011 #6
    hello friends i like all your post but unit also have its own important so keep given unit at each places so will be more helpful
  8. Sep 30, 2011 #7
    have u any information about robotic control
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