# I Why momentum is conserved for a jet of fluid?

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1. Feb 23, 2017

### 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?

2. Feb 23, 2017

### phinds

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?

3. Feb 23, 2017

### jbriggs444

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.

4. Feb 23, 2017

### phinds

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

5. Feb 23, 2017

### jbriggs444

The difficulty I had with that was with the claim that energy is conserved. It isn't.

6. Feb 23, 2017

### phinds

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.

7. Feb 23, 2017

### jbriggs444

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.

8. Feb 23, 2017

### phinds

Fair enough. I rushed my reply because I really just wanted him to think about it, and consequently I messed it up. Thanks.

9. Feb 23, 2017

### larsa

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