Why momentum is conserved for a jet of fluid?

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Discussion Overview

The discussion revolves around the conservation of momentum in the context of a jet of fluid and the effects of external forces, such as friction with ambient air. Participants explore the principles of momentum conservation, contrasting it with energy conservation, and examine the implications of including external forces in the system.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants question the assumption that momentum should be conserved, particularly in scenarios where external forces, like friction, are present.
  • One participant argues that if a system is closed to include both the air and the jet, then both energy and momentum can be conserved.
  • Another participant emphasizes the need for critical thinking rather than simply providing answers, suggesting that understanding the situation is more important.
  • There is a contention regarding the claim that energy is conserved in the presence of friction, with some arguing that it is not conserved without considering all forms of energy, such as heat generated by friction.
  • Participants express differing views on whether momentum can be considered conserved when external forces act on a system.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the conservation of momentum and energy, with multiple competing views remaining regarding the conditions under which these principles apply.

Contextual Notes

Limitations include the dependence on definitions of closed systems and the treatment of energy forms in the presence of friction. The discussion reflects varying interpretations of conservation laws in different contexts.

larsa
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This is something I do not know how to explain.

Although velocity decreases due to friction with ambient air, momentum should be conserved and, therefore, it entrains mass.

What is the reason that momentum should be conserved?
 
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larsa said:
This is something I do not know how to explain.

Although velocity decreases due to friction with ambient air, momentum should be conserved and, therefore, it entrains mass.

What is the reason that momentum should be conserved?
I think the question is, why do you think momentum should be conserved? ENERGY is conserved but why would momentum be conserved? If you push a box along a floor and then let go, at the time you let go, the box has a certain velocity and momentum. Friction will slow the box down. Do you think momentum is conserved in this case?
 
phinds said:
I think the question is, why do you think momentum should be conserved? ENERGY is conserved but why would momentum be conserved? If you push a box along a floor and then let go, at the time you let go, the box has a certain velocity and momentum. Friction will slow the box down. Do you think momentum is conserved in this case?
If you have an external force on the system, neither energy nor momentum need be conserved.

If you close the system by including both the air and the jet so that their interaction stays inside the system then both energy and momentum are conserved.
 
Yes, well, I was trying to get him to think about the situation, not spoon feed him the answer.
 
phinds said:
Yes, well, I was trying to get him to think about the situation, not spoon feed him the answer.
The difficulty I had with that was with the claim that energy is conserved. It isn't.
 
jbriggs444 said:
The difficulty I had with that was with the claim that energy is conserved. It isn't.
It is in my example if you include the floor and the heat energy created by the friction that slows the box down. Again, my point was to get him to think about it.
 
phinds said:
It is in my example if you include the floor and the heat energy created by the friction that slows the box down. Again, my point was to get him to think about it.
Again, you are not listening. You claimed that energy was conserved but that momentum was not. That claim is false. Trying to explain things by making false claims is not proper.
 
jbriggs444 said:
Again, you are not listening. You claimed that energy was conserved but that momentum was not. That claim is false. Trying to explain things by making false claims is not proper.
Fair enough. I rushed my reply because I really just wanted him to think about it, and consequently I messed it up. Thanks.
 
phinds said:
Fair enough. I rushed my reply because I really just wanted him to think about it, and consequently I messed it up. Thanks.

It was simpler than I thought, thank you for your help
 

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