When we write the Newton's second law equations for each mass in a system of interacting masses and add them up, we get the result that Newton's second law applies to the entire system as if it were considered to be a single mass moving at the velocity of its center of mass, because the internal forces between the interacting masses cancel due to Newton's third law. Gangopadhyaya and Harrington have cleverly run this argument in reverse to show that Newton's third law is a consequence of this interpretation of the second for systems whose masses share a common velocity and acceleration.1 I would suggest that by beginning their argument with the Newton's second law statement that the acceleration of the center of mass of a system of interacting masses is the net external force acting on them divided by the sum of their masses, they would not be so limited in "deriving" the third law from the second.
NSTL
