Hey, quick question. Is there any way that we can sort of "explain" Newton's Third Law as a corollary of Newton's 2nd (consv. of momentum)? I mean, if object A is interacting with object B, and initially the total momentum is p_A + p_B, then change in p_A = -change in p_B for momentum to be conserved. This means that the integral of F_AB over the period of interaction is the negative of the integral of F_BA. But can we go from saying that those two integrals are negatives of each other to saying that the two forces *themselves* are negatives of each other? Or does that not follow in general? Also, is it sensible to try to "explain" Newton's Third Law, or is something best just explained as an experimental fact? P.S. I am aware that in more advanced formulations of classical physics, fundamental laws such as the conservation laws for energy, angular momentum, and momentum follow from certain symmetries. And I seem to recall that using the Lagrangian formalism and the principle of least action, you can *arrive* at Newton's 2nd law (cons. of momentum) in terms of generalized coordinates. I need to go and brush up on that stuff, but *that's* why I'm wondering whether there is some sort of more underlying and fundamental theoretical explanation for Newton's Third, or whether I should just accept it as being true.