How about the motion if the system has net external forces? Suppose there is an external force on each particle in above expt, then n,,+m a 2=∑F+∑ f=F+ftf+e 2 2r F+F2=0, and if write∑Fa+∑F2a=∑F ∑Fn n1a+mmoa ntn Newton's second law for ∑ Fen=(m,+m2)acm systems of particles This looks very like a particle of mass m, +m, located the center of massHow about the motion if the system has net external forces? Suppose there is an external force on each particle in above expt., then This looks very like a particle of mass located at the center of mass. → → → → → → → → m1 a1 + m2 a2 = F1 +F2 = F1ext + F1r + F2ext + F2r 0 1 + 2 = → →  F r F r    → → → F1ext + F2ext = Fext , and if write → →  ext = + cm F (m m )a 1 2 m a + m a =Fext    1 1 2 2         + + = 1 2 1 1 2 2 m m m a m a acm    m1 m2 + Newton’s second law for systems of particles