Law of conservation of momentum in a non isolated system.

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Discussion Overview

The discussion centers on the validity of the law of conservation of momentum in non-isolated systems, exploring definitions of isolated systems and the implications of external forces. Participants examine examples such as a gun recoiling and a tennis ball in the air to illustrate their points.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions whether the law of conservation of momentum is valid in non-isolated systems and seeks clarification on what constitutes an isolated system.
  • Another participant defines an isolated system as one with no external forces and no mass crossing its boundary, using a tennis ball as an example of a non-isolated system due to gravitational forces.
  • There is a discussion about the conservation of momentum in the direction of a bullet fired from a gun, suggesting that while external forces act on the gun, momentum can still be conserved in the bullet's direction.
  • A participant emphasizes the need to define the boundaries of the system to include both the gun and the bullet for momentum calculations.
  • One participant directly asks for a conclusion regarding the law's applicability in non-isolated systems.
  • Another participant asserts that the law does not hold in non-isolated systems, referencing an earlier post for clarification.

Areas of Agreement / Disagreement

Participants express differing views on whether the law of conservation of momentum applies in non-isolated systems, with some arguing it does not hold while others suggest it can be applied under certain conditions.

Contextual Notes

Participants reference external forces such as air resistance and gravity, indicating that these factors complicate the application of conservation laws. The discussion does not resolve the implications of these forces on momentum conservation.

Ali Hamaiz
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Is the law of conservation of momentum valid in non isolated system . What is an isolated system? My teacher confuses me that the law is valid in an non isolated system . They even say that when we say about the isolated system (meaning they are present ) we do not consider the air resistance or frictional forces is that true? They give me the example of a gun recoiling , I am confused .
 
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Isolated in this case means no external forces and no mass crossing the boundary. A tennis ball in the air is not an isolated system: it will fall to the ground. A gun also, but if you look in the direction of where it's aiming then the momentum in that direction is conserved.
 
@BvU what are you talking about the direction?
 
You need a constant force to keep the gun from dropping to the floor. But to calculate the kickback you can use conservation of momentum in the direction the bullet flies.
 
BvU said:
You need a constant force to keep the gun from dropping to the floor. But to calculate the kickback you can use conservation of momentum in the direction the bullet flies.
[and make sure that you place the boundaries around the "system" so that both gun and bullet remain inside]
 
What will be the conclusion , will the law of conservation of momentum hold in an non isolated system .
 
No. See #2.
 

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