I recently solved a problem in which I used the "fact" that the momentum is conserved just before and just after a collision between two (or maybe more) objects but I am not sure whether this is true or not.
This is what I've done:
Consider a given system $A.$ Then $\sum\mathbf F=\dfrac{d\mathbf p}{dt},$ where $\sum\mathbf F$ represents the sum of all external forces on $A$ and $\bf p$ represents the momentum of the system. Thus $d\mathbf p=\sum\mathbf{F}dt$ so if we only consider a case when $dt\approx0$ then $d\mathbf p\approx0.$
Is that result true?, Is my reasoning right?