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Physics LibreTexts

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,

Δpsys=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=m1v1,i+m2v2,i+

The final momentum is the sum of the final momentum of the individual particles,

psys,f=m1v1,f+m2v2,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


This page titled 10.7: Constancy of Momentum and Isolated Systems is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Peter Dourmashkin (MIT OpenCourseWare) via source content that was edited to the style and standards of the LibreTexts platform.

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