Simple, everyday science

[Truronian]

Ignoring diffusion. One standard light that shines in all directions. The celing and floor absorb all light so the problem is effectively two-dimensional.

SPOCK, Truronian: I thought this thread was about everyday science?

This is everyday science if you live in Versaille

EDIT: No, do not ignore diffusion. Ignore difraction. The fact that light can eminate in a full circle is what makes the problem challenging.

 

[mrtn]

I want to throw billiard balls at Bozo too!
Can I?! Can I?!

 

[El_Machinae]

Okay. Design a room where you can not change location, but are allowed to throw billiard balls at BozE. The walls are perfectly reflective, but the floor and ceiling are perfectly sticky. Assume zero friction or gravity for the balls.

Now, design this room such that BozE has a place to hide without getting hit. If he moves - pelt him.

 

[Renata]

Does mine make sense? Remember that the explosion is actually gazillions of atoms being repelled from each other. Some of them go out the back, some of the hit the rocket and move it forward.

The rocket doesn't get hit by anything. It's basic Newtonian mechanics, otherwise known as the principle of making things go forward by throwing stuff off the back. m1v1=m2v2. Think of yourself standing on the aforementioned skates with a baseball in your hand, but forget about anybody else being present. Throw the baseball forward as hard as you can (without twisting your skates, or you'll fall ), and I promise you you will move backwards. It's the same reason a gun recoils when you fire it: it's just thrown a small mass very very fast out the front ... and you wouldn't say that part of the bullet hitting you is the reason your arm gets flung back!

Rockets are great in space because there there is no air to get in the way of the exhaust mass and no friction to get in the way of the rocket itself; that's where they're most efficient.

Edit: oh, and it is incorrect to refer to rocket propulsion as an explosion. If the reaction mass *did* go every-which-way as in an explosion, the rocket would go nowhere. It's simple (though very fierce) burning, with a massive volume of exhaust gas forced by the nozzle to all go in one direction. The rocket then moves the opposite way.

 

[Perfection]

Because the amount of liquid you're pulling up the straw is very little, normally. Anything below the water line essentially weighs nothing.

However, to drink from a third story floor would require you to suck up the whole 500g of water, when we're used to sipping only about 15g at a time.

If you sucked for a bit, lifted the straw above the glass, and continued sucking (so you only had 15g at a time), you'd be fine.

Incorrect. I can just as easily suck 100kg of water up a meter as I can 100mg. The only difference is the amount of air I would have to evacuate (so basicly it'd take longer). It's harder to do higher distances (you need a better vacuum) because the outside air pressure is lower (which is what pushes the water up). At around 34 feet the air pressure cannot push anymore, and that represents the maximum one could suck. If we had a thin enough straw (assuming no capillary action) that could hold 15g it would be just as hard to suck water as a thick one.

the last two responses have now finaly settled in my mind that whole mess with "trees shouldnt be able to grow over 34 feet", "but they do".

"Thank You!" to perfection and el machine.

Trees can grow over 34 feet because they have capillary action which uses the cohesive properties and adhesive properties of water to partially support the wieight.

Ignoring diffusion. One standard light that shines in all directions. The celing and floor absorb all light so the problem is effectively two-dimensional.

You might be able to do it with angled walls but otherwise it would seem quite doubtful.

 

[Renata]

This is the correct explanation. You can observe the opposite effect by putting some ice cubes in a glass of water, then putting the glass in the freezer. Some lumps (the ice cubes) will stick above the flat surface of the ice, because as the water freezes, the already frozen cubes rise and float above it.

See, this is why I needed a picture. I thought you were describing not just a crater depressed relative to the surrounding ice, but a crater with a rim that was itself *above* the surrounding ice.

 

[Truronian]

You might be able to do it with angled walls but otherwise it would seem quite doubtful.

All walls are upright, and it is possible

 

[Perfection]

All walls are upright, and it is possible

Well, please share!

Edit: I got it!

Let me whip up a picture

Edit:

Note that the the two curved surfaces are parts of concentric circles around the light. And the straight segments that connect the two curved portions are colinnear with L

 

[Bozo Erectus]

Im sticking with my Lapping Waves Theory dammit:grumble:

 

[El_Machinae]

Incorrect. I can just as easily suck 100kg of water up a meter as I can 100mg. The only difference is the amount of air I would have to evacuate (so basicly it'd take longer). It's harder to do higher distances (you need a better vacuum) because the outside air pressure is lower (which is what pushes the water up). At around 34 feet the air pressure cannot push anymore, and that represents the maximum one could suck. If we had a thin enough straw (assuming no capillary action) that could hold 15g it would be just as hard to suck water as a thick one.

You only solved the light question cause you were imagining pelting Bozo Erectus with billiard balls, admit it.

I don't get your answer above, though. The air pressure on the surface of the water hasn't changed, you're saying that the air pressure inside the straw (under the water) has changed?

I don't think that you could suck up 500g of water 1 meter easier than you could 5 g ... there's still the adding of potential energy.

 

[Syterion]

The problem, El Machinae, I think is that you don't realize that you are not providing the force pushing the water up the straw. The atmospheric pressure is. You are merely sucking air out at the top to create a vacuum so as to cancel air pressure on one side, allowing the air pressure on the glass to proceed unhindered. With a long straw, you must create a more perfect vacuum. And the limit of air pressure is 33 feet of water, which is why you cannot suck water in a straw past it. Understand?

 

[ainwood]

Well you can... but the water vapourises (boils).

 

[ainwood]

Convection. It's the efficiency of the convection processes that make the conduction the slow step that by and large determines the heat flow out of the house.

Half right.

The main resistance to heat transfer is at the boundary between the solid and the fluid - the laminar boundary layer.

Here:

While convection is the medium for the exterior of the house losing heat to the environment, conduction is the medium for the interior to the exterior. That's why we have insulation.

You actually imply that conduction is the slow part - adding insulation to reduce the rate of conduction.

Insulation doesn't do this - it actually stops bulk convection by being an obstruction to fluid movement. Also, it provides another set of laminar layers that the heat must flow across from the fluid to the solid again.

 

[Renata]

Well you can... but the water vapourises (boils).

This reminds me ... has anyone ever seen a triple-point demonstration for water? It's pretty cool. You put a little puddle of water in a watch glass under a bell jar, with some dessicant next to it to trap vapor. Then you turn on the vacuum pump. As the air pressure gets lower and lower, the rate of evaporation of the water increases until the water is boiling. But it also is getting colder all the time, since the evaporative process removes energy. (Same reason rubbing alcohol feels cold.) So you have a boiling liquid that's getting colder and colder.

Ultimately, it freezes mid-boil, usually with a nice little curlicue trapped in mid-spatter.

 

[ainwood]

This reminds me ... has anyone ever seen a triple-point demonstration for water? It's pretty cool.

Another similar one is taking a multi-component mixture from liquid to vapour via the cricondenbar. You start out with liquid, then the miniscus goes a bit hazy and disappears, leaving vapour - you never see any signs of boiling.

 

[El_Machinae]

The rocket doesn't get hit by anything. It's basic Newtonian mechanics,

I was using more than Newtonian mechanics.

Throwing the ball at BozE (who's awfully tired of it) does move me back, but that's only because my arm is pushing against the ball. When it comes to rockets, the "arm" and the "ball" are atoms exploding away from each other. Some go out the nozzle, some hit the back of the rocket, pushing it forward. A whole bunch hit the sides (and nearly evenly, too, thanks to thermodynamics) and they cancel each other out.

 

[Pontiuth Pilate]

Answers

1. My water straw question: nobody got the exact correct response (edit: I see now that Perfection was very close, and Syterion explained more fully). The answer is that the vacuum created in the straw does not "suck up" the water; rather, the vacuum represents a pressure inequality. Outside atmospheric pressure pushes down on the water in the glass, forcing it up into the straw. However, atmospheric pressure can only support its weight in water, which is 34 feet per unit area (10 meters). Thus, no matter how hard you suck, the pressure difference can be no more than 1 atm, and thus using an ordinary straw you cannot suck water up to the third floor.

Trees grow taller than 34 feet because the xylem (water-carrying tubes) within the wood are so narrow (just one cell diameter) that cohesion and adhesion become significant factors, enabling a column of water to be elastically extended beyond 34 feet.

2. Bozo's rocket in a vacuum question: by Newton's Third Law, the momentum of the escaping gases must be equal to the momentum of the rocket (m1v1=m2v2). You talked about "Climbing by pulling on a rope that isnt attached to anything." Great example. If you were to try to climb a rope ladder on the Space Shuttle in null g, you would climb "upwards" a few centimeters at most and the rope ladder would move downwards at a velocity opposite and nearly equal to what yours would be if you were climbing a rope ladder attached to a wall (the heavier the rope ladder, the faster it would move. If it had a mass equal to your own, it would move downward with a velocity exactly equal to your upward climb). In all cases, your collective center of gravity does not change. Similarly, the velocity of the collective center of gravity of a rocket and its exhaust in a zero-G environment must be zero. If you think about the rocket/exhaust combo as a single object with no external forces acting on it, you'll see. Just think of an astronaut trying to swim in space. It isn't possible. Thus, when part of the object goes one way the other must go the opposite direction.

3. Truorian asks: Can you design a room with mirror walls such that no light from a lamp at A reaches a camera at B (where A and B are specified points in this room)?

My intuition on this is to say that according to Feynman's interpretation of quantum tunneling, we cannot exactly predict the path of any photon and thus there is no way to build a perfectly dark container. Just like the famous "double-slit" experiment.

Hmm, thinking about it some more, you can create an elliptical chamber where all light from the source at one focus must pass through the other focus (there was a famous "fixed" billiards table built to this design). Not sure how that helps. And it is a Newtonian interp of photon motion, which we now know for sure is not correct.

I'm leaning towards this being a trick question

 

[ainwood]

Answers

1. My water straw question: nobody got the exact correct response (edit: I see now that Perfection was very close, and Syterion explained more fully). The answer is that the vacuum created in the straw does not "suck up" the water; rather, the vacuum represents a pressure inequality. Outside atmospheric pressure pushes down on the water in the glass, forcing it up into the straw. However, atmospheric pressure can only support its weight in water, which is 34 feet per unit area (10 meters). Thus, no matter how hard you suck, the pressure difference can be no more than 1 atm, and thus using an ordinary straw you cannot suck water up to the third floor.

Yes, its a pressure inequality (or actually, an equality).

The atmospheric pressure pushes down on the liquid outside the straw at around 101.325 kPa. On the straw-side, there is the 'atmospheric pressure' ( which, at least, can be reduced to an absolute vacuum) plus the static head of the water. The 32 ft (or thereabouts) of water is the static head that balances the atmospheric pressure (rho.g.h).

Similarly - if you try and use a straw to breath under-water and more than a few feet, and you will find that you can't breathe...

 

[Truronian]

Note that the the two curved surfaces are parts of concentric circles around the light. And the straight segments that connect the two curved portions are colinnear with L

Nice, I was thinking of something more along the lines of an ellipse:

Where the red dots are the foci of the semi-ellipse at the top.

But both work.

 

[Renata]

I was using more than Newtonian mechanics.

Throwing the ball at BozE (who's awfully tired of it) does move me back, but that's only because my arm is pushing against the ball. When it comes to rockets, the "arm" and the "ball" are atoms exploding away from each other. Some go out the nozzle, some hit the back of the rocket, pushing it forward. A whole bunch hit the sides (and nearly evenly, too, thanks to thermodynamics) and they cancel each other out.

No, I'm sorry, but this entire paragraph after the first sentence is wrong, and I'm not sure you understand the first sentence correctly. Maybe someone else can do a better job of explaining it?