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A steel rod of length l = 1.0 m and radius r = 10 mm has one end fixed while the other end is twisted through an angle of 60 degrees. Taking the shear modulus of steel as G = 81 GPa, find the elastic deformation (torsional) energy stored in the rod.

1. about 700 J
2. about 70 J
3. about 350 J
4. about 1400 J

Correct answer: about 700 J

Solution

Using U = pi G r⁴ phi² / (4 l) with phi = pi/3 gives about 7.0 * 10² J.

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