when you’ve got a glass of water or pop with ice cubes in it, and you spin the glass…why does the ice stay at the same point in space, relatively?
“How do you know that my dimwitted inexperience isn’t merely a subtle form of manipulation used to lower peoples’ expectations, thereby enhancing my ability to effectively manuver within any given situation?”
Dewey—“Scream 2”
You mean, “why doesn’t the ice rotate?”
It will, eventually.
At first, the ice doesn’t rotate because you’re not applying a force to it. The side of the glass will start to apply force to the liquid, and eventually the spinning liquid will eventually start the ice rotating.
I remember from a class on Applied Fluid Dynamics something involving a teacup on a turntable with little corks floating in it and the corks each had an x on the top, and as fluid rotated in the cup, the x didn’t spin like you would expect (i.e. the cork was not rotating). Basically it was showing that the fluid just moves things around in circles without spinning them at the same time. I’m not sure how relevant this is to the ice question, but it seems to be a similar principle.
In one respect this question goes deeper than it seems at first glance. If all motion is relative, why does the water move up the sides of the glass at all? Why doesn’t the water in the glass stay flat and rest of the universe rotate around the glass?
Ernst Mach, who was a big influence on the young Einstein (not Yahoo Serious – the other one), spent a lot of time on this question and ended up invoking all the mass in the distant stars and galaxies, etc. I can’t say that I understand his answer. I’m not at all sure I understand the question. But spinning glasses and relativity brought it to mind.
“You have no choice but to be impressed.”
Tony Rothman and George Sudarshan
Doubt and Certainty
pluto asks a fascinating question. I think maybe what Mach was getting at was, if the whole universe were spinning around a glass of water, say at some ordinary turntable speed like 33 rpm, then that would mean a lot of galaxies travelling at incredible speeds. Their relative masses would increase, and pull the water out to the sides of the glass at exactly the right amount.
Not a great explanation on my part, but it’s the best wild-ass guess I can come up with on short notice. The point is, ordinary relativity works best with inertial motion; rotation is a constant redirection of momentum, hence “fictitious forces” (though I don’t like that term) like centrifugal force.
What part of “I don’t know” don’t you understand?
Mach rejected Einstein’s interpretation of his (Mach’s) work, and Einstein eventually moved away from interpreting general relativity in terms of Mach’s principle. At one time, though, Mach’s principle, covariance, and the equivalence (of gravity and acceleration) principle were Einstein’s guiding principles as he developed general relativity.
Einstein’s general relativity allows the water to remain stationary, while the universe rotates around it. In that reference frame, the distant stars do move very fast, but they don’t ‘gain mass’ because the characteristics of spacetime change in that situtation, and even faster than light speeds are allowed.
rocks