(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Ok i have the following problem. There is a pulley with radius 7.5 cm and 2 forces acting on it (2 tensions of a string) acting in opposite directions either side. One is 18.2N, the other 7.5N. I know the acceleration is 3.75 meteres per second squared.

Find the moment of inertia.

2. Relevant equations

Change in torque = Ia

3. The attempt at a solution

Ok so the torque of each individual force is r x F (F being the force and r the radius in meters). As the forces are acting perpendicular, the torque of each force is rF.

So i thought the total torque is τ = r(18.2-7.5) = Ia.

So i thought the moment of inertia would equal would equal...

I = [r(18.2-7.5)]/a

However my answer seems to be a factor of r out, i.e the equation [(r^2)(18.2-7.5)]/a sems to yield the correct answer.

Why is this? I do not undersdtand where the extra r comes from which it is multiplied by.

Thanks

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# Homework Help: Moment of inertia calculation, factor of radius out?

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