Topic: Dynamics
Concepts: Circular motion, Newton’s laws

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Solution:

Looking at one of the masses, we see that the tension provides the centripetal acceleration,

so the answer is C.

Topic: System of Masses
Concepts: Conservation of angular momentum, Work-energy theorem

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Solution:

There is no net external torque on the system of two masses since the tensions pulling them inward cancel out. Thus, we have conservation of angular momentum. Choosing the center of the rod as our pivot point, the initial moment of inertia $I_i$ is

The final moment of inertia $I_f$ is

The angular momentum $L$ is

By the work-energy theorem and using the fact that angular momentum is conserved,

so the answer is D.