Originally Posted by toly
The law of conservation of momentum
If the velocities of the particles are U1 and U2 before the interaction, and afterwards they are V1 and V2, then
M1U1 + M2U2 = M1V1 + M2V2 (1)
This law holds no matter how complicated the force is between particles.
M1 and M2 are constants, but U1, U2, V1, and V2 are not. They depend on an arbitrarily chosen coordinate system. If we change coordinate system, we have to rewrite equation (1), but physics of collision process would be the same.
That is OK, but I wanted someone to point out what was wrong in my argument