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Homework Help: Angular Speed problem

  1. Mar 4, 2010 #1
    A turntable on a frictionless bearing has an initial angular speed of 33 1/3 revolutions per minute. The turntable platter has a mass of 1.0 kg and a radius of 30.0 cm. A spider with a mass of 1.0 gram drops onto the platter at a distance of 29 cm from the center, and begins to walk, very slowly, towards the center. Assumig that the turntable is not being driven, calculate:

    a. The angular speed of the turnable and spider a short time after impat, but before the spider begins walking.

    b. The angular speed of the turntable when the spider reaches a point 1 cm from the center of the turntable.

    c. Explain why we had to specify that the spider was moving slowly. What terms could be neglected if the spider's radial velocity is small?

    a. 33.14 revolutions per minute


    c. You wouldn't have to take into account the force caused by the movement of the spider, just the force caused by the torque of the spider.

    I apologize if this isn't enough, I'm typing this up for my boyfriend while he continues working. He's been doing this stuff for hours so I offered to post it somewhere because he feels unsure of his answers and isn't sure if he's overthinking it or not.
  2. jcsd
  3. Mar 4, 2010 #2


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    Homework Helper

    The turntable is spinning initially.

    The spider then falls onto it.

    So a collision in a sense has taken place. Since everything is rotating, momentum is conserved, in particular, angular momentum is conserved i.e. angular momentum before impact = angular momentum after impact.

    Also the moment of inertia of a Ipoint mass is mr2
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