Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

When I shower I notice that the soapy water spinning around the drain often takes the form of a spinning galaxy, complete with spiral arms. Does this indicate that the water's speed around the drain is inversely proportional to the square of its distance from the drain? Actually, I have heard that spinning galaxies themselves don't follow this rule, hence the discovery of dark matter, so does the spinning water follow whatever rule that the spinning stars do?

Thanks.

share|improve this question
add comment

1 Answer

up vote 1 down vote accepted

See the free (irrotational) vortex section of the Wikipedia article on vortices. The velocity varies linearly with distance not as the square of the distance.

share|improve this answer
    
Thank you John. Linear variation sounds to me as though the soap is spinning as a solid mass, which in retrospect does seem to be the case as I don't notice the shape changing as it spins. How then do the spiral arms form? –  dotancohen Jun 29 '12 at 6:48
    
When I shower this morning I will make a point of studying the water drainage :-) –  John Rennie Jun 29 '12 at 6:49
    
Looking at the spinning soap this morning, in fact the center was spinning (angularly) faster than the outer edges. I don't have a way of measuring the linear velocity at each distance, but it certainly wasn't spinning as a solid body! –  dotancohen Jul 4 '12 at 18:21
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.