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A **yo-yo** (solid uniform cylinder) of mass $M$, radius $R$ unwinds under gravity. Its downward acceleration is:
A$g$
B$g/2$
C$2g/3$
D$g/3$
Answer & Solution
Correct answer: C. $2g/3$
Newton: $Mg - T = Ma$. Torque about CM: $TR = I\alpha = (MR^2/2)(a/R)$ ⇒ $T = Ma/2$. Sub: $Mg - Ma/2 = Ma$ ⇒ $a = 2g/3$. Tension $T = Mg/3$.
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