Why momentum is conserved for a jet of fluid?

[larsa]

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?

 

[phinds]

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?

 

[jbriggs444]

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.

 

[phinds]

Yes, well, I was trying to get him to think about the situation, not spoon feed him the answer.

 

[jbriggs444]

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.

 

[phinds]

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.

 

[jbriggs444]

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.

 

[phinds]

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.

 

[larsa]

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