10.7: Constancy of Momentum and Isolated Systems
( \newcommand{\kernel}{\mathrm{null}\,}\)
Suppose we now completely isolate our system from the surroundings. When the external force acting on the system is zero,
→Fext=→0
the system is called an isolated system. For an isolated system, the change in the momentum of the system is zero,
Δ→psys=→0 (isolated system)
therefore the momentum of the isolated system is constant. The initial momentum of our system is the sum of the initial momentum of the individual particles,
→psys,i=m1→v1,i+m2→v2,i+⋯
The final momentum is the sum of the final momentum of the individual particles,
→psys,f=m1→v1,f+m2→v2,f+⋯
Note that the right-hand-sides of Equations. (10.7.3) and (10.7.4) are vector sums.
When the external force on a system is zero, then the initial momentum of the system equals the final momentum of the system,
→psys,i=→psys,f