In the lecture of Wed 3/31 we did a problem in which
a fixed wheel is rotated by a falling mass attached
to a rope which is wound around the wheel. In this example,
I make a mistake by saying that the normal force at the
axis of the wheel is equal and opposite to the weight
of the wheel, when in fact that force is equal and opposite
to the sum of the weight of the wheel and the tension of the
rope. Here is a correct explanation. This is also available
as a pdf file. I apologize for any
confusion this might have caused. GBA.