Definition: rCM = ∑ mi ri/∑ mi = ∑ mi ri/ M → ∫ r dm/ M
We rewrite this definition as: M rCM = ∑ mi ri
The total momentum is: ptot = ∑ mi vi = M vCM
Newton's 2nd law: d ptot/dt = M aCM = Fext
Conclusion: A composite object is equivalent
to a single point of mass M that is located at the center of mass.