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Flywheel moment of inertia

  1. May 7, 2016 #1
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    upload_2016-5-7_21-24-29.png

    For this problem, I don't understand how can the system be at rest and immediately after placing the mass, to move with constant speed. But anyways, can you tell me please how to proceed?

    On the hanging mass, there is mg acting and T the tension of the rope. On the flywheel, the tension of the rope is acting and the friction of the bearings.

    I have no clue what the tension of the rope is. I wanted to say that opposite to the mg, is the friction and the inertial force of the flywheel, but I am not familiar with calculations of the inertial force.

    Any idea?
     
  2. jcsd
  3. May 7, 2016 #2

    Nidum

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    Last edited: May 7, 2016
  4. May 7, 2016 #3

    SammyS

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    I would not be concerned about the ' initially at rest ' phrase.

    If the 0.25kg hanging mass has constant velocity, then what is the net force on this mass ? The answer to this should give you the tension for this case.

    That should get you started.
     
  5. May 7, 2016 #4

    haruspex

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    Can you understand how a box may be at rest on a horizontal floor, yet when pushed with a constant horizontal force F moves at constant velocity?
     
  6. May 8, 2016 #5
    OK got it. But the tricky bit is what happens during the time that speed is zero and the very next moment is some constant value. I thought that in order to attain that constant speed, there must be an acceleration. It may indeed be within fractions of second, but in that time, dV/dt is not zero, it has some value!


    Anyway, as for the problem, the friction cancels the effect of the weight. But do I have to calculate them as torques? because I don't know if the friction is exerted at the same line as the weight of the falling mass
     
  7. May 8, 2016 #6

    haruspex

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    You are right that there is a small question of how the motion gets started. You have to assume it was given some nudge to get it moving.
    Otherwise, if you steadily increase the force until motion starts it will switch from static to the lower kinetic friction and therefore accelerate.
    The friction consists of a torque at the axle.
     
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