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Chapter 1: Q21DQ (page 328)

In example 10.10 (Section 10.6) the angular speed changes, and this must mean that there is no torque about the rotation axis if the forces the professor applies to the weights are directly, radially inward. Then by Eq. (10.7), must be zero. Explain what is wrong with this reasoning which leads to an apparent contradiction.

Short Answer

Expert verified

There is no contradiction, as torque is coming due to increased moment of inertia.

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

Torqueτ is given by

τ=ddtlαwhere I is moment of inertia andα is angular speed.

So, torque does not comes from force only, it can come from increased Moment of Inertia as well.

03

Conclusion

Hence, there is no contradiction, as torque is coming due to increased moment of inertia.

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