1. The problem statement, all variables and given/known data The bar has a mass of 3 grams/cm. When a mass of 15 grams is hung on the left side, the system is balanced. Calculate the length of the bar. 2. Relevant equations F1*r1 = F2*r2 3. The attempt at a solution I thought that the moment on the left should be equal to the moment at the right, so it should be mleft * g * rleft = mright* g *rright But apparently it should be that the moment of the mass on the left side is equal to the moment of the whole bar. Can someone explain why this is?