Two rods of same mass and same length lo but having different coefficient of expansions a and 3a are joined at P (see figure). System is placed on frictionless surface. If temperature of whole system is changed by DT, the displacement of junction point is
Net force on the system is zero hence displacement of the centre of mass of the system will be zero. Let x be the shift in P, then
m(l02(1+αΔT)+x)=m(l02(1+3αΔT)−x)⇒x=l0αΔT2∴(c)