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University Physics I - Classical Mechanics, 2019

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216 CHAPTER 9. ROTATIONAL DYNAMICS<br />

which is easily solved for a:<br />

m 2 g<br />

a =<br />

m 1 + m 2 + I/R 2 (9.55)<br />

If you look at the structure of this equation, it all makes sense. The numerator is the force of gravity<br />

on block 2, which is, ultimately, the force responsible for setting the whole thing in motion. The<br />

denominator is, essentially, the inertia of the system: ordinary inertia for the blocks, and rotational<br />

inertia for the pulley. Note further that, if we treat the pulley as a flat, homogeneous disk of mass<br />

M, thenI = 1 2 MR2 , and the denominator of (9.55) becomes just m 1 + m 2 + M/2.

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