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The torque needed to produce a unit angle of twist in a solid cylindrical bar of length L and radius r is:

1. directly proportional to r²
2. directly proportional to r⁴
3. inversely proportional to L
4. inversely proportional to r²

Correct answer: directly proportional to r⁴

Solution

The restoring couple per unit twist is C = pi*eta*r⁴/(2L), so it varies as r⁴ (and inversely as L).

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