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A thin circular metal disc of radius 500.0 mm is rotating about its central axis perpendicular to its plane. When its temperature is raised slowly, the radius grows to 507.5 mm. Assuming angular momentum is conserved, what is the approximate percentage change in its rotational kinetic energy?

1. 1.5%
2. -1.5%
3. 3%
4. -3%

Correct answer: -3%

Solution

With L conserved, KE = L²/(2I) and I proportional to r², so delta_KE/KE = -2*(delta_r/r) = -2*(1.5%) = -3%.

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