TBR CBT 2 - Pendulum center of mass

[LuminousTruth]

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Assuming that the combined mass of the balls is greater than the combined mass of the cords, but that the cords do have some mass, where is the center of mass for the system shown below?

A. Point a
B. Point b
C. Point c
D. Point d

I chose answer C but TBR says the answer was "D". Was there a mistake in TBR's explanations? I thought while the two masses will have a center of mass closer towards the heavier (bottom) ball, the mass from the cords will bring that center of mass at point d a bit higher and to the right, which the closest would be point C.

 

[advair250]

Think of how the tension forces are behaving in the system, and what the mass of the cords really mean. With or without the mass of the cords being considered as a factor, the answer in my mind would still be the one indicated. Usually the cord is mass-less in pendulums, and since they've made a point to indicate that the combined mass of the balls > the combines mass of the cords, that would lead me to treat it no differently than a mass-less cord system.

 

[milski]

It's a lame questions. Whether the answer is C or D depends on the relation between the mass of the cord and the difference between the masses of the balls. It could have been a great question, if one of the answers was "not enough information."