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Disk and Block

  1. Nov 30, 2004 #1
    A uniform disk of mass Mdisk = 4.4 kg and radius R = 0.28 m has a small block of mass mblock = 2.4 kg on its rim. It rotates about an axis a distance d = 0.18 m from its center intersecting the disk along the radius on which the block is situated.


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    a) What is the moment of inertia of the block about the rotation axis?
    Iblock = kg*m2
    2*0.18*0.18 NO

    HELP: Remember: the block is on the rim of the disk.
    HELP: The block is considered a point-mass.


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    b) What is the moment of inertia of the disk about the rotation axis?
    Idisk = kg*m2 *
    4.4*0.28^2/2+4.4*0.18^2 OK


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    c) When the system is rotating about the axis with an angular velocity of 5 rad/s, what is its energy?
    KErot = J
    (2.4*0.18^2+4.4*0.28^2/2+4.4*0.18^2)*5.0^2/2 NO


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    d) If while the system is rotating with angular velocity 5 rad/s it has an angular acceleration of 8.4 rad/s2, what is the magnitude of the acceleration of the block?
    |ablock| = m/s2
    ((5^2*0.18)^2+(8.4*0.18)^2)^(1/2) NO

    Somebody please help!!!!!
     
  2. jcsd
  3. Nov 30, 2004 #2

    arildno

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    a)
    i) What is the mass of the block?
    ii) What is the distance of the block to the rotation axis?
    c)"(2.4*0.18^2+4.4*0.28^2/2+4.4*0.18^2)*5.0^2/2 NO"
    Use the correct answer to aii)!
     
  4. Nov 30, 2004 #3
    OK, for a, here is what I did:
    I = MR square
    so,

    I = Mblock * distance square
    = 2.4 * 0.18 * 0.18 , but still the answer is wrong!!!!!
     
  5. Nov 30, 2004 #4

    arildno

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    But the distance from the BLOCK to the rotation axis is 0.28-0.18=0.10
     
  6. Nov 30, 2004 #5
    OK, got that one can u help on c & d. Some inital guidance.
     
  7. Nov 30, 2004 #6

    arildno

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    For c), you've used the wrong distance for the block.
    On d) you should use:
    [tex]||a||=\sqrt{r^{2}(\dot{\omega})^{2}+(r\omega^{2})^{2}}[/tex]
    where [tex]\omega[/tex] is the angulur velocity, [tex]\dot{\omega}[/tex] the angular acceleration, and r the radius to the rotation axis.

    Note that this is just the formula you've been using but with the wrong radius value..
     
    Last edited: Nov 30, 2004
  8. Nov 30, 2004 #7
    I got part d, need help with part c , then is the correct distance 0.10 m, if so,
    is this correct:

    2.4*0.10^2+4.4*0.28^2/2+4.4*0.10^2)*5.0^2/2
     
  9. Nov 30, 2004 #8

    arildno

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    "2.4*0.10^2+4.4*0.28^2/2+4.4*0.10^2)*5.0^2/2"

    "4.4*0.10^2"
    Why did you change this value??
    It should be, as it was initially 4.4*0.18^2
     
  10. Nov 30, 2004 #9
    The correct should be as follows: (2.4*0.10^2+4.4*0.28^2/2+4.4*0.18^2)*5.0^2/2

    I got , this and all, thank you.!
     
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