Hello again Physics Forums! I have a question about fluid dynamics. Perhaps someone here can help me out.(adsbygoogle = window.adsbygoogle || []).push({});

I am trying to understand how a plume of water moving at some speed carries momentum within a body of water. For instance the ‘exhaust’ from a submarine propeller. I am having a hard time understanding how momentum is conserved in such a situation. I do not care about surface effects like waves.

A traveling plume of water always displaces a volume of water equal to its own, and that volume of water effectively fills the space left behind by the plume. So the net momentum of a moving plume of water and the remainder of the system is always zero.

The submarine also displaces a volume of water equal to its own as it travels. If the submarine has the same density as the water, the system also has a momentum of zero. But if the density of the submarine is different than water, that system has a non-zero momentum.

To conclude, the ( submarine + system ) have a non-zero momentum, but the ( plume of water it generates + system ) cannot have any momentum. How is momentum conserved?

I hope I have written it so that it is comprehensible. If someone could please comment I would really appreciate it. :)

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# I Puzzle: propagation of momentum in water

Have something to add?

Draft saved
Draft deleted

**Physics Forums | Science Articles, Homework Help, Discussion**