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Is it possible to use L= r x p to find angular momentum here

  1. Dec 3, 2015 #1
    1. The problem statement, all variables and given/known data

    upload_2015-12-3_13-6-0.png



    2. Relevant equations

    L = I ω
    L= r x p

    3. The attempt at a solution

    For b)

    My proffesor found the moment of inertia at the point at which the wheel touches the ground, and used the formula L = I ω
    upload_2015-12-3_13-7-47.png


    What I dont understand is why can't one use L= r x p to solve the problem.
    Wouldn't L then simply be L=RMV?
     

    Attached Files:

  2. jcsd
  3. Dec 3, 2015 #2

    gneill

    User Avatar

    Staff: Mentor

    If the wheel were sliding without friction along the road then that would be true. Then the wheel could be modeled as a point mass M moving past the point of interest at a perpendicular distance R.

    But the wheel is rotating (about the instantaneous contact point on the road). So the wheel's moment of inertia and the parallel axis theorem is called for.
     
  4. Dec 3, 2015 #3
    I see, thank you. So if the object is not rotating, I can use L=MRV, where R is the distance to the center of mass of the object?
     
  5. Dec 3, 2015 #4

    gneill

    User Avatar

    Staff: Mentor

    Yes. Then it's just a non-rotating rigid body undergoing linear motion.
     
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