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Homework Help: Conservation of angular momentum of an an engine flywheel

  1. Feb 20, 2015 #1
    1. The problem statement, all variables and given/known data
    Figure 10.30 shows two disks: an engine flywheel (A) and a clutch plate (B) attached to a transmission shaft. Their moments of inertia are and initially IA and IB, they are rotating with constant angular speeds ωA and ωB, respectively. We push the disks together with forces acting along the axis, so as not to apply any torque on either disk. The disks rub against each other and eventually reach a common angular speed Derive an expression for ω

    2. Relevant equations
    Conservation of angular momentum

    3. The attempt at a solution
    I'm confused about why angular momentum is conserved here. When the flywheel comes in contact with the clutch, isn't there friction before the two begin moving with the same speed, which is an external force that contributes to the torque of the system; thus, momentum would not be conserved? Is friction ignored because I am only looking at two moments in time: when the clutch and flywheel aren't in contact and after the two are rotating together?
  2. jcsd
  3. Feb 20, 2015 #2


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    Staff: Mentor

    The friction is a force that's internal to the system under consideration, so no external torques are involved.

    The linear analogy is the perfectly inelastic collision where energy is lost to heat, sound, etc., yet momentum is still conserved.
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