Breaking up the semi-circle into infinitesimally small rings

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The discussion focuses on calculating the moment of inertia for a semi-circle by breaking it into infinitesimally small rings of mass dm and width dr. The user initially derived dm as pi*r*dr and integrated using the formula di=integral(dm*r^2) from 0 to R, resulting in .25*pi*R^4. However, this value is incorrect for a semi-circle, as the moment of inertia for a complete disk would yield .5*pi*R^4. The oversight identified in the discussion is the omission of a constant factor for density, which is crucial for accurate calculations.

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I attempted to solve this problem by breaking up the semi-circle into infinitesimally small rings of mass dm and width dr a distance r away from the center [0<r<R]. I then wrote dm in terms of the area: dm=pi*r*dr. Then, I plugged into the formula di=integral(dm*r^2) and integrated from 0 to R giving me .25*pi*R^4. This is not what I would have gotten for a complete disk though I would have gotten .5*pi*R^4 using the same process. Can someone please help I don't even know if I started correctly.
 
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spencerw105 said:
I plugged into the formula di=integral(dm*r^2) and integrated from 0 to R giving me .25*pi*R^4. This is not what I would have gotten for a complete disk though I would have gotten .5*pi*R^4 using the same process. Can someone please help I don't even know if I started correctly.
You have left out a constant factor for density. What happens when you include that and express it in terms of the mass and area of the object?
 

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