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Speed of ball changed by lengthening string

  1. Nov 30, 2009 #1
    A ball of mass 0.18 kg is whirled in a horizontal circle at the end of a massless string 75 cm long at a speed of 4.5 m/s. If the string is lengthened to 1.5 m while the ball is being twirled, then the speed of the ball will now be...

    What law or formula applies to this question?
  2. jcsd
  3. Nov 30, 2009 #2


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    I think you're supposed to assume that external torque is 0, so the conservation law is...
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