It will be difficult to observe the results.
Huh? Of course we can. That part’s easy. A sealed container that has increased its volume. Like the hypodermic needle, when you pull the plunger back. The volume of 5 ccs is now 10 ccs. That produces suction.
The “suction” model is naive, but it is far from undefined. It’s a little like “impetus” in physics: it’s wrong, but it’s very clearly defined, and is a decent model for reality until you go to the effort of making precise measurements.
That’s true. For example if we said that suction is like tension it falls apart as soon as we realised that the particles of a fluid aren’t tightly coupled to one another (or a liquid couldn’t break up into drops), and also that the theoretical point of maximum ‘suction’ is when all of the particles have been evacuated (so there’s nothing there to be pulling).
Suction doesn’t exist?
Explain spaghetti.
(I thought I remembered an epic trainwreck thread about how it is possible to suck on spaghetti in a world where suction is only a fictional force, but searching turns up nothing nearly as epic as my recollection.)
Frylock: I seem to remember the same trainwreck. Spaghetti is weird to visualize, either under the “suction” or the “air pressure” model. It’s hard to figure out where the vectors line up.
Mangetout: I like the point that liquids don’t have much tensile strength, and so “pulling” a column of liquid upwards would be difficult to model. That’s a superb common-sense visualization of the situation. Thank’ee!
(Ya bum! Ya’s spoiled my whole argyment!)
You can’t suck spaghetti in the vacuum of outer space.
So, IF I could run a small open hose to outer space and leave the other end here at sea level, the hose would suck all the air away?
Be that as it may, I always took suction to be the result of pressure differentials anyway so that doesn’t surprise me.
You would be dead before you began the experiment. :dubious:
Schrodinger’s spaghetti eater.
If liquids lack sufficient tensile strength to be pulled upon, why do you suppose a “suction” composed of newly created vacuum as your plunger withdraws would have enough tensile strength to pull on anything. Yet you assert the vacuum should somehow pull fluids or solids.
I reiterate that you have no solid concept of what “suction” is. It’s simply magical pulling that somehow couples to some kinds of matter sometimes but not to other matter at other times.
ETA: On rereading your last post to me: … You may have a clearly defined notion of “suction” as a noun. But you have no conception of its mechanism, nor any way to make testable predictions about its behavior. Knowing the name of a flower or bird species doesn’t mean you know anything useful about it. All you know is the artificial label somebody applied to it.
I’m NOT trying to be hostile here, although I might be coming off that way, for which I apologize. Fuzzy thinking is at the core of “suction”. My goal is to sharpen those fuzzy lines.
Consider a siphon. Atmospheric pressure pushes on the surface of a liquid. Does the leaving fluid push or pull?
I would say that it is merely a difference in pressure, more at the input, less at the output.
nm
Yes, if not for the Earth’s gravitation well that is stronger than the pressure differential.
The vacuum does pull fluids and solids. That’s observed fact. Slurp a milkshake and you’ll see it for yourself. (Would that all physics experiments were that pleasant!)
(How does the toilet plunger pull up a column of water (and other stuff) if there isn’t any air pressure down in the sewer pipe to “push” it upwards?)
I reiterate that you’re wrong.
I’ve already conceded that the model is wrong. You, however, are absolutely wrong in trying to claim that is isn’t a model.
It’s like phlogiston: it’s a very compelling naive theory, because it explains what everyone can plainly see and experience. It requires some sophisticated arguments to get beyond. I also compared it to “impetus” in mechanical physics.
You are simply not correct here.
Your post is coming across as FURIOUSLY hostile. My first response to you was pretty damn hot, also. But, see, this is a thread about naive impressions of physics, and how to get around them. (I also intended it to remain light-hearted.)
Your job is to tell me why my impressions are wrong – and several others here have done that elegantly.
For instance, you say that the suction model “…somehow couples to some kinds of matter sometimes but not to other matter at other times.” Well, super! Give examples! Demolish the model by concrete experimental citation! Which kinds of matter does the suction model fail to explain?
(Mangetout already gave a superb example of how the “suction” model fails to explain the lifting of columns of highly non-viscous liquids such as water. He managed to do this without being insulting. Yay!)
If you say, “You haven’t got a model,” well, that’s just plain stomach-flu wrong. Of course I have a model. It’s a wrong model, but that’s not the same as “not a model.”
Now let’s stop demonstrating Boyle’s Law. (“The greater the external pressure, the greater the volume of hot air.” Flanders and Swann.)
Exactly: gravity is still pulling down on the water inside the hose.
Now, if you could open a teleportation gate to outer space…it would, indeed, suck the air right off the earth.
There was a really nifty sci-fi story on that theme, where a (unexplained) hole opened up at sea level, creating a hefty vacuum vortex. The authorities went to the guys who put out oil-well fires, and they clapped big steel hemispheres (Magdeburg revisited!) over it.
Perhaps you’re remembering Cecil’s article about it which referred to a Usenet thread.
I apologize
I’m on my phone, so won’t attempt a physics explanation now, but I did want to express my contrition while the offense was fresh.
Bless you, and I apologize also, including for the version of the post that I didn’t actually put up, because I realized (only barely in time) that it was stupid and not helpful. And certainly for the hostility in what I did post. I’ll do better!
Seriously, coming back to the one concrete question I posted…I learned that the sewer system does have air pressure in it – it isn’t just water and poop all the way from my toilet to the treatment plant. The big sewer mains aren’t wholly full of water – I had thought they were! There’s lots of air down there, so the “air pressure” model succeeds in explaining the working of my toilet plunger.
Ignorance fought. And I didn’t even have to crawl down a manhole. (I should have just watched “The Third Man” again!)
“Local science enthusiast chokes to death on a mouthful of mercury after trying to prove atmospheric pressure can’t push this toxic heavy metal up a 30 inch ‘straw’. The EPA is doing what they can to clean up the spill in the surrounding area. His last words were ‘spaghetti in space’ through a silvery cough.”