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Homework Help: Question which concerns the classic example that demonstrate the conservation of angu

  1. Aug 13, 2005 #1


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    i don't really understand question which concerns the classic example that demonstrate the conservation of angular momentum,

    a student on a free rotaing stool holds two weight, mass each 3kg, 1m from axis of rotation each, and he rotate with angular v of 0.75rad/s. moment if inertia of student +stool= 3kgm2. The student pulls the 2 weights inwards to a position of o.3m from the axis of rotation.
    1)what's the student's angular velocity
    2)what's his initial and final EK

    This question is simple as we just apply the prin. of conservation of angu. momen. Iw initial=Iw final

    however, i could not visualise the situation. Where is the axis of rotation? If merely the masses were rotating initially, how could the student possess any EK?
  2. jcsd
  3. Aug 13, 2005 #2


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    well, the student, stool and 2 masses all rotate ofcourse, not merely the masses.
    The axis of rotation is the same axis as the rotation axis of the stool.

    I am kinda surprised this student has arms 1 m in length.
  4. Aug 13, 2005 #3


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    The student's rotation posesses some kinetic energy. The formula is very similar to that for linear motion.

    Linear motion: [tex]KE=\frac{1}{2}mv^2[/tex]

    Rotational motion: [tex]KE=\frac{1}{2}I\omega^2[/tex]

    An object's mass is its linear inertia. For rotational motion, inertia depends on the distance the mass is from the rotational axis, as well.

    Angular velocity (in radians/sec) the rotational equivalent of linear velocity.
  5. Aug 13, 2005 #4


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    The axis of rotation is the vertical axis through the stool and student.

    Since you say " he rotateswith angular v of 0.75rad/s" how can you then assert that "merely the masses were rotating initially"?
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