Dear Experts, Trying to analyze the work done by internal forces in a very simple two particle system which is attracted to each other with a constant force, i performed calculations based on two frame of references, 1.The center or mass frame of reference and 2. Frame of reference of one of the particles say A. Computing the work done by internal forces on a system of particles, under two different frame of references, i obtain two different answers.I have uploaded the image of the derivation. The answers for the change in kinetic energy will be hence different in both frame of references. Is that normal? What are the cases where the work done by internal forces becomes 1.zero, 2. positive, 3. negative ?