Physics

A circular disk of mass 0.4 kg and radius 36 cm, initially not rotating, slips down a thin spindle onto a turntable (disk) of mass 1.7 kg and the same radius, rotating freely at 3.4
a) Find the new angular velocity of the combination;
b) The change in the kinetic energy?
c) If the motor is switched on after the disk has landed, what is the constant torque needed to regain the original speed in 2.2 s?

I figured out a) and b), but I'm not sure what c) is asking or how to solve it.

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asked by Ana
  1. 3.4 WHAT??
    I assume radians/second
    but hard to say. I will use that but if RPS or something, convert

    I disk = (1/2)m r^2
    so
    Id = .5*.4*.36^2
    and
    It = .5 * 1.7 * .36^2


    initial angular momentum = It*3.4
    final angular momentum (the same of course)
    = It omega +Id omega = omega(It+Id)

    so
    It * 3.4 = (It+Id)omega
    but It =1.7/.4 * Id
    so It = 4.25 Id
    so
    3.4 (4.25 Id) = (5.25 Id) omega
    omega = 3.4 (4.25/5.25)
    omega = 2.75 whatever your units are
    ----------------
    initial Ke = .5 It(3.4)^2
    final Ke = .5 (It+Id)(2.75)^2

    -------------------

    Torque = moment = change in I*omega /time
    [just like force =change in m*v/t]

    = (It+Id) (3.4-2.75) / 2.2

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    posted by Damon
  2. Thanks Damon you're the real MVP!
    It was rad/s sorry.

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    posted by Ana

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