Why should the sum of MaVa + MbVb =0 ?

The sum of MaVa + MbVb = 0 because it represents the conservation of momentum in a system. According to the principle of conservation of momentum, the total momentum of a system before an interaction is equal to the total momentum of the system after the interaction, as long as no external forces are acting on the system.

In this equation, Ma and Mb represent the masses of two objects, while Va and Vb represent their respective velocities. When the two objects interact, their momenta are transferred between each other, but the total momentum of the system remains constant. Therefore, the sum of MaVa + MbVb must equal zero to satisfy the conservation of momentum principle.