# Closed Systems and Isolated Systems

Within the scope of classical mechanics, what exactly is the definition of a closed system, and of an isolated system? Also, do these definitions differ in thermodynamics?
And does the law of conservation of linear momentum apply to a closed system or an isolated system?

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Andrew Mason
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Within the scope of classical mechanics, what exactly is the definition of a closed system, and of an isolated system? Also, do these definitions differ in thermodynamics?
And does the law of conservation of linear momentum apply to a closed system or an isolated system?
In thermodynamics, the distinction between closed systems and open systems is whether the system can lose or gain matter. Closed systems can't. Open systems can.

The law of conservation of momentum is a fundamental physical law that applies to every interaction involving matter. There is no distinction between open and closed systems relating to the applicability of this law. The only thing that matters is the interaction. The momentum of all the interacting bodies before the interaction must be equal to the momentum of all the interacting bodies after the interaction. One has to include the momentum of massless particles as well (photons).

AM

In thermodynamics, the distinction between closed systems and open systems is whether the system can lose or gain matter. Closed systems can't. Open systems can.

The law of conservation of momentum is a fundamental physical law that applies to every interaction involving matter. There is no distinction between open and closed systems relating to the applicability of this law. The only thing that matters is the interaction. The momentum of all the interacting bodies before the interaction must be equal to the momentum of all the interacting bodies after the interaction. One has to include the momentum of massless particles as well (photons).

AM
Yes, I am familiar with the idea that momentum is always conserved, regardless of whether or not the system under study is closed/isolated. Perhaps I should've made my question more clear: which statement is correct?
1) The total momentum of a closed system remains constant.
Or
2) The total momentum of an isolated system remains constant.

Yes, I am familiar with the idea that momentum is always conserved, regardless of whether or not the system under study is closed/isolated. Perhaps I should've made my question more clear: which statement is correct?
1) The total momentum of a closed system remains constant.
Or
2) The total momentum of an isolated system remains constant.
The second one.

D H
Staff Emeritus