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A solid circular shaft of diameter 8 cm and length 5 m transmits 8 kW of power at 300 revolutions per minute. If the modulus of rigidity of the shaft material is 8 * 10¹¹ dyne/cm², find the angle of twist (relative angular shift) between the two ends of the shaft.

1. 3.96 * 10⁻³ rad
2. 1.98 * 10⁻³ rad
3. 7.92 * 10⁻³ rad
4. 2.27 * 10⁻² rad

Correct answer: 3.96 * 10⁻³ rad

Solution

Transmitted power equals torque times angular speed, so the torque follows from P and N. The angle of twist of a shaft of length L under torque T is theta = T*L/(G*J), where J is the polar second moment of area.

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