My question has to do with something Leonard Susskind, a professor of theoretical physics at Stanford University and director of the Stanford Institute for Theoretical Physics, said in one of his lectures. Basically that he didn't know why Newton wrote three laws of motion, when they could all be summarized as [itex] F=ma [/itex]. I can see why you would think that of Newton's first two laws, seeing as they are the same observation: force is proportional to acceleration. But I can't say the same about the third law. Which states that, in a closed system, the center of mass suffers no acceleration. Isn't the third law a separate/additional observation?