Why Is the Mass of the Equal-Arm Balance Neglected in Torque Calculations?

[Ginny Mac]

In my lab assignment, we used an equal-arm balance to demonstrate rotational equilibrium by balancing torques using different mass quantities. A 100 g mass hangs from one end of the EAB, and a 185.18 g mass hangs from a thread at a 50 degree angle on the other end. The EAB is horizontal and we achieve rotational equilibrium. Here is my question:

When we sum all of the torques acting on the EAB, we neglect the mass of the equal arm balance (EAB). Is this simply because the torque acting on the EAB is zero? (Since the torques are balanced and in equilibrium). I understand this, but I suppose I am simply trying to really know "why."

Also, if the EAB were not horizontal, and we still were able to achieve rot. equil., its mass would still be negligible, correct? I believe this is because this only means the EAB's center of mass is in a different location. But where? I am confused about this concept. Where is the fulcrum located then?
Thank you for your time and help!
Ginny

 

[member 553083]

Yes, the mass of the EAB is negligible because the torques are balanced and in equilibrium. The torque acting on the EAB is zero because the masses on each side of the EAB are equal and opposite. Therefore, the sum of all the torques acting on the EAB is zero. If the EAB were not horizontal and we still achieved rotational equilibrium, its mass would still be negligible. This is because the center of mass of the EAB would be located at the point of equilibrium. In this case, the fulcrum would be located at the center of mass of the EAB.