Torque

[deleted783484]

A laboratory pan balance is shown

In general when calculating the mass of the pan neglecting the mass of the rod:
A- is possible if the fulcrum is placed at the center of mass of the whole balance
B- is possible if the center of mass of the rod is the pivot point
C- is not possible
D- is possible if the chemical mass is larger than the mass of the rod

Answer is B but I picked A. If the fulcrum is at the center of mass then isn't the lever arm 0? why are they saying that the lever arm is 0 if the center of mass is the pivot point?

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[DaAlienist]

A laboratory pan balance is shown

In general when calculating the mass of the pan neglecting the mass of the rod:
A- is possible if the fulcrum is placed at the center of mass of the whole balance
B- is possible if the center of mass of the rod is the pivot point
C- is not possible
D- is possible if the chemical mass is larger than the mass of the rod

Answer is B but I picked A. If the fulcrum is at the center of mass then isn't the lever arm 0? why are they saying that the lever arm is 0 if the center of mass is the pivot point?

Is torque on the new MCAT? Did this question come with an image or figure?

I would agree with B since if the fulcrum is at the CoM for the rod, then equal weight from the rod is on either side, and only the weight of the objects placed on the scale will affect the net torque.