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Tension and acceleration of a pulley sytem attached to a block

  1. May 27, 2009 #1
    A 10kg block is attached to a massless cord that doesnt stretch or sag which is wrapped around a disc-like wheel for which m=5kg, I=(1/2)mR^2. If the block is allowed to drop straight down held back only by the rotation of the wheel
    a) What is the tension in the rope
    b) What is the acceleration of the block

    I know that the tension isnt just F=mg and so am thinking it is combined with the mass of the block but dont know how to go about it
     
  2. jcsd
  3. May 27, 2009 #2

    LowlyPion

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    Welcome to PF.

    Consider the torque on the wheel.

    What equations do you know for Torque?
     
  4. May 27, 2009 #3
    I know that torque = r x F or rFsintheta
    i think it needs to use the ones related to angular momentum where torque= dL/dt= Id omega/ dt= I alpha
    there was a similiar question but it just said the things and didnt really progress or confirm if it was right
     
  5. May 27, 2009 #4

    LowlyPion

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    OK. So you have

    T = F * R = I * α

    m*g*R = 1/2*M*R2 * α

    but α = a/R

    So ... m*g = 1/2*M*a
     
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