When we stir a glass or water, the dregs move into the middle of glass. Why?
If the higher pressure near the outside actually created a net movement of water towards the centre then the centre wouldn't be shallow for very long, would it?ChrisHarvey said:1)...higher pressure at the outside and lower pressure at the centre. Therefore there is a net movement of water molecules towards the centre...
Who said so? Do you think the phenomenon will disappear if I rotate the container instead of stirring it?ChrisHarvey said:2) ... at the edge of the glass the water will be stationary...
Eventually the water will form a depression, and the system will be stable again. So the cork will probably find a resting place where the centrifugal force equals the pull of gravity. So that doesn't answer the question.actionintegral said:Consider a cylindrical tub with water in it. Place a cork in the water. Now rotate the tub forever. Where will the cork go?
They're just pulling your chain, Dave. Which, in Britain, would of course cause exactly the type of vortex that they're talking about. :tongue:DaveC426913 said:Holy Jeez, why are you trying to make this more complicated than it is?
Hi Chris,ChrisHarvey said:It's just because pressure in a rotating fluid increases according to the square of the radius
Not at the edge, I'm talking about a river with shallow and deep sections along its length. Debris will be lifted and suspended in areas of high river velocity and sedimented in areas of low river velocity.ChrisHarvey said:SIDE NOTES:
Sure, they both have fast moving water in the centre which causes sedimentary particles to be suspended, but in a river at the edge, particles are deposited and there they stay...
WHAT??!! :surprised :surprisedactionintegral said:The challenge is to NOT do the experiment, except in your head. Doing the experiment is like looking in the back of the book.
Yep AI. Teeg's right. The Greek philosophers thought the secrets of the universe could be reveaad through thought experiment alone.Teegvin said:WHAT??!! :surprised :surprised
By doing the experiment, we see exactly what happens, and then we KNOW what happens, and needn't argue. Arguing about what SHOULD happen makes us no better scientists than the philosophers who argued about how many teeth horses ought to have, and it makes our conclusions no more valid than theirs.
I would bet my paycheque the following happens:ChrisHarvey said:Hey I actually haven't tried this experiment so I don't know for sure what really happens... maybe I'm getting this wrong? I was under the impression that if you stir a glass of water with sediments in it, they become suspended in the middle of the glass. Any sediments near the edge move into a position of dynamic equilibirum near the centre, rather than settling.
Well, any doubt can be removed by experiment, but...actionintegral said:Hi Dave,
Everything you are saying sounds reasonable, but I am slow to pick up on one point...wouldn't the water by the side of the glass be the FIRST to slow down because it is rubbing against the glass?
Aren't there always shear forces (if that's the correct term) involved in stirring a liquid, since the parts farther from the stirrer lag behind the closer bits? (And I don't want to hear anything about your pool and skinny-dipping. :tongue: )DaveC426913 said:For the weater to slow down in one part without slowing down elsewhere will create a lot of friction within the water itself.
Note that, except for the friction you mention, the entire mass of water can circulate while maintaining with theoretically ZERO friction. (i.e. if the entire mass of water were frozen solid where internal frictyion were maximized, you could still rotate the whole mass within the glass).