Editing Newton's laws of motion (section)

===Center of mass===
{{main|Center of mass}}
[[File:Masocentro1.jpg|alt=Fork-cork-toothpick object balanced on a pen on the toothpick part|thumb|The total center of mass of the [[Fork|forks]], [[Stopper (plug)|cork]], and [[toothpick]] is on top of the pen's tip.]]
Significant aspects of the motion of an extended body can be understood by imagining the mass of that body concentrated to a single point, known as the center of mass. The location of a body's center of mass depends upon how that body's material is distributed. For a collection of pointlike objects with masses <math>m_1, \ldots, m_N</math> at positions <math>\mathbf{r}_1, \ldots, \mathbf{r}_N</math>, the center of mass is located at <math display="block">\mathbf{R} = \sum_{i=1}^N \frac{m_i \mathbf{r}_i}{M},</math> where <math>M</math> is the total mass of the collection. In the absence of a net external force, the center of mass moves at a constant speed in a straight line. This applies, for example, to a collision between two bodies.<ref>{{Cite journal |last=Lyublinskaya |first=Irina E. |date=January 1998 |title=Central collisions—The general case |url=http://aapt.scitation.org/doi/10.1119/1.879949 |journal=[[The Physics Teacher]] |language=en |volume=36 |issue=1 |pages=18–19 |doi=10.1119/1.879949 |bibcode=1998PhTea..36...18L |issn=0031-921X}}</ref> If the total external force is not zero, then the center of mass changes velocity as though it were a point body of mass <math>M</math>. This follows from the fact that the internal forces within the collection, the forces that the objects exert upon each other, occur in balanced pairs by Newton's third law. In a system of two bodies with one much more massive than the other, the center of mass will approximately coincide with the location of the more massive body.<ref name=":2" />{{Rp|pages=22-24}}