I have a general question about how to apply Newton's Second Law. When we write Fnet=ma, I am sometimes unsure of myself what the 'm' is in the problem. For instance, when two masses (m1 and m2) are connected by a pulley is 'm' equal to m1+m2, or just the mass of interest? How can I be absolutely sure of myself when I am solving problems using Newton's Second Law? Does this have something to do with the technique of cutting (i.e. if two people are pulling opposite ends of a rope, then by cutting the rope, one can find the rope's tension.)?