Torque and Angular Acceleration

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r_swayze
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The pulley shown in the illustration has a radius of 2.70 m and a moment of inertia of 39.0 kg∙m2. The hanging mass is 4.20 kg and it exerts a force tangent to the edge of the pulley. What is the angular acceleration of the pulley?

All I have so far is the force exerted on the pulley by the block

mg = 4.2kg * 9.8m/s2 = 41.16N

which means it exerts a torque of r * F = 2.7m * 41.16N = 111.13Nm

I know net torque = moment of Inertia * angular acceleration, but I think I am missing another component of the net torque. What is that component and how do I find it?
 
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  • Diagram of a grey pulley hung from a T-shaped support on a yellow background, with a rope running over its edge down to a dark red trapezoidal block hanging below.
    block.JPG
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damnit I'm working on this same problem right now... having the SAME issue ><