# Simple question about linear momentum

1. Sep 14, 2008

### fluidistic

I'm just learning about linear momentum in classical mechanics and I have a question about it. I'm not sure I understand the concept well thus my question can be senseless.
It is said that $$\frac{d\vec{P}}{dt}=\sum F_{ext.}$$.
Say I observe a group of $$1000$$ bodies in about $$50 m^2$$.
--1--Is it possible to consider the linear momentum of $$2$$ bodies separated by say $$765$$ bodies? And then to apply the formula $$\frac{d\vec{P}}{dt}=\sum F_{ext.}$$?
--2--Or have I to consider the linear momentum of all the group because the bodies are very close to each other?
Note that in the first case the external forces would include the force exerted by the 998 other bodies.
If the answer of my question is an affirmation to my question 2, I have another question that I will post in that case.

2. Sep 14, 2008

### Staff: Mentor

In this case the answer to your question 1 is "yes." You can choose any combination of objects and consider their total momentum, provided that you include the forces exerted by all other objects in $F_{ext}$.

3. Sep 14, 2008

### fluidistic

Thanks a lot jtbell! That "kills" my possible other question and more important to me: that makes it a beautiful law of nature.