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Chapter 1: Q22DQ (page 328)

In example 10.10 (Section 10.6) the rotational kinetic energy of the professor and dumbbells increases. But since there are no external torques, no work is being done to change the rotational kinetic energy. Then, by Eq. (10.22), the kinetic energy must remain the same! Explain what is wrong with this reasoning which leads to an apparent contradiction. Where does the extra kinetic energy come from?

Short Answer

Expert verified

There is no contradiction because the work is done.

Step by step solution

01

Reference used

A physics professor stands at the center of a frictionless turntable with arms outstretched and a 5 kg dumbbell in each hand. He is set rotating about the vertical axis, making one revolution is 2 s.

02

Explanation

The work is done here.

The increase in kinetic energy comes from the positive work that the professor performs in pulling his arms and weighing toward the center.

The force and displacement are in the same direction, so the system鈥檚 kinetic energy increases

03

Conclusion

Hence, there is no contradiction, kinetic energy increases because the work is done.

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