A mobile is constructed from a thin rod of mass 50 g and length 50 cm. Two hanging masses are positioned at each end of the rod. The mass of one of the hanging masses is 100 g. What must be the mass of the other hanging mass so that the mobile is in static equilibrium with the rod horizontal? The whole assembly hang from a ceiling support with a cord attached at the rod's center. What is the tension in the suspension cord?
The masses at the ends must be equal in order balance, if the rod is held at the middle.
Add up total mass and multiply by g for the cord tension.