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Unwinding a string around a spool, what is the angular acceleration?

  1. Dec 3, 2003 #1
    Problem 5. given:g=9.81m/s^2.
    A light string 1.20 m long is wrapped around a solid cylindrical spool with a redius of 0.0195 m and a mass of 0.380 kg. A 8.40 kg mass is then attached to the free end of the string, causing the string to unwind from the spool.
    a. What is the angular acceleration of the spool? Answer in rad/s^2.
    Note : I worked on the problem and I got 22241.14162 by using the formula (moment of inertia* angular acceleration= mass*g*radius. However my answer was wrong what did I do wrong?
  2. jcsd
  3. Dec 4, 2003 #2

    Doc Al

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    Staff: Mentor

    Re: Physics

    You are assuming that the tension in the string (which is what is pulling on the spool) equals the weight of the mass.

    To do this problem correctly, treat each object separately. Consider the forces acting on each body, and write down the equations describing each. You'll have two equations and two unknowns. And the right answer.
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