don't be silly. None of the text you quoted suggested that equal volumes of ice were created, only that evaporation of hot water resulted in less ice in the same time as a cold body of water froze completely. In no way does "hot" water freeze faster than "cold" water. What they all stated was that larger volumes of cold water takes more time to freeze than smaller volumes of warmer water, which is something that anyone could deduce. There are a number of conditions that must be met before the "hot" water can "freeze faster," including (but not limited to) reduced volume from evaporation, and water purity.

If you toss a pan of boiling water into sub-zero temps, all the water will evaporate before hitting the ground. If you toss a pot of cold tap water out into sub-zero temps, not much will evaporate. Is it magic, or does it demonstrate that evaporation of water is more rapid as the ambient temp and the water temp differ more?

If you want to see something amazing, scary, and more than slightly dangerous, using simple tap water, do this:
1. Clean a pot very thoroughly, scrubbing to ensure the surfcae is perfectly clean.
2. Fill pot with hot tap water.
3. Place pot on stove, and turn heat to high. Time
4. Once the water boils, turn off heat.
5. Once pan has cooled enough for the pot to be safe to touch, roughly the same temp as the hot tap water, turn heat back on, to high. Do not put finger or anything else into water. Make sure the watrer in the pot is not contaminated while cooling.
6. Set timer to the same time as it took the water to initially boil. If the water temp had cooled to a temp close to the hot tap water, it will not be boiling when the timer goes off. At this time, STAND BACK and watch the pot. After a few minutes, if it still is not boiling, dump a small handful of salt or any other powder into the pot. Be VERY careful if you do this. I cannot stress this enough.
7. Clean up the water that splashed all over as a result of the pot erupting in boiling water.

Another way to see this effect is to put a cup of room-temp water in a microwave, and set it on high for long enough for it to boil. once it has boiled, stop the microwave, and let it cool for an hour or two without disturbing it. Reset the microwave on high, and let it run until the water boils. When the water does finally boil, it will explode out of the cup with a fair bit of violence. You have successfully superheated tap water. Now clean it up.

The reason boiled water "might" freeze faster than cold tap waer has more to do with the boiling than the temp of the water. if you boil some water, let it cool to room temp, and then place it in a freezer, it will freeze faster than room temp tap water, depending on the purity of your tap water. Water at 80 degrees does not freeze faster than water at 70 degrees. Water at 40 degrees does not freeze faster than water at 35 degrees. Assuming the "water" in these experiments is the same substance, i.e. one sample doesn't contain more gasses and impurities than the other. It's called good science. All water is not the same.

 

Evaporation's not the only thing helping out, hotter water usually has less dissolved gas in it, which helps the freezing process. Also, convection in the hotter water helps get things frozen fast.

 

I think you forgot about this chart and convection currents supercooling....

Hot water doesn't spend as much time supercooling as cooler water.

I tried an experiement in my freezer. The first time the cold water froze first, the second time the boiled water froze first and room temperature water froze last. the third time I missed it. The amount of ice was so close that I couldn't tell a difference between the two but you can see the water evaporate from the boiling water much quicker. There's a lot of information on this effect out there, the buckets of water really demonstrate the point quicker. I think the evaporating water molecules freeze faster because well they require less to freeze so they form a layer of cooler water on top quicker. Like I said I don't know and neither does the scientific community, it appears. Either way the experiements show that the hot water just spends less time supercooling than room temp water. I can't explain it and I'm not making it up. Quite frankly I could care less, I'm not going to boil water to make ice cubes it would take longer to do that anyway.

 

I'm bored of the water question. Go back to the plane. If there is a bird inside the plane, the plane is flying, and the bird starts to fly around inside the plane, do the plane's wings still support the weight of the bird? Is the answer the same or different if the plane has an open cockpit? This was an extra credit question on an engineering test I took back in college.

 

Hmmm....I'm going to have to say no, the wings of the plane will not be supporting the weight of the bird.

When an object inside the plane is on the floor, on the seat, on any surface, gravity is acting on that object, the the force is tranlated into the structure of the aircraft. The wings of the plane create lift to counter-act that force.

The bird that is flying around is not translating any of its weight into any part of the aircraft structure, therefore the wings do not need to create that much more lift.

HOWEVER, you might be able to argue that for every action, there is an equal and opposite reaction. If the bird weighs 10 lbs, then it would have to generate 10 lbs of lift, which equate to some sort of downwash (sorta like helicopter downwash) of air that would generate forces onto the surfaces of the aircraft. But fluid dynamics would suggest that some of that downwash would dissapate outwards, so there isn't 10 lbs worth of air going straight down and hitting the floor. The amount of downwash hitting the floor would probably be negligible in registering a change in weight difference of the aircraft. So I'll stick with my original answer.

Lift counteracts weight. Weight is a measurement of acceleration as the object is acted upon by gravity, but is decelerated by the force of another object. Like me standing on a floor. Gravity is trying to accelerate me, but the floor is stopping me from acclerating. The aircraft is not trying to accelerate the bird opposite to gravity. The bird is doing that himself by flapping his wings. Therefore there is no forces from the aircraft acting on the bird, therefore no change in weight, therefore no extra lift required. No difference with an open cockpit. Does another aircraft affect the weight of another while flying in close formation? No. All the bird is doing is flying in close formation, from within the cabin.

 

Actually, your "HOWEVER" is correct. Yes, the 10 pound bird will need 10 pounds of vertical lift. And even if half of the bird's downwash gets dissipated to the sides or is lost to vortex elements, 10 pounds of upward lift is required, period, meaning that the bird is actually exerting 20 pounds of force on the air. Otherwise, it'd simply fall.

Therefore, bird acts on the air in the cabin, which in turn acts on the plane's airframe and wings.

(Unless the bird goes into a free-fall, at which time the plane is (very briefly!) not supporting the weight of the bird at all.)

The open cockpit questiong cannot be easily answered - it depends on the airflow in and around it, and how far the bird is in (or not) the passing airstream.

 

Elistan, I can see why you agree with me, but even though I mentioned the "downwash" theory, I think the force of air acting on the aircraft would be negligible.

I fly a bell 412 helicopter, which at max weight, is 11,900 lbs. If I stand underneath a hovering 412, I'm not going to be feeling 11,900 lbs of air blasting down on me. If I did, i'd be crushed. THe downwash isn't originating from a single point. Its emminating from the span of the rotor disc (46 ft) diameter. So the forces are spread out, as opposed to the chopper being on the ground, where all weight is concentrated on the skids. Also, the downwash dissipates downwards, out to the sides, and even upwards. You have to find out the velocity and volome vertical component of downwash, translate that into PSI, then apply it to the area that it is blowing onto.

You mentioned 20 lbs of lift. If the bird weighed 10 lbs, and was creating 20 lbs of lift, then it would be accelerating upwards. It would require 10 lbs of lift (more or less with some drag forces) to maintain level flight, not 20.

Then the 10 lbs would be spead out over the wingspan of the bird - then you have to find the vertical volume and speed of downwash, then find the square footage of the aircraft floor. Its my opinion that the weight from downwash from a flying bird would be negligible, especially when comparing it to a bird that is just sitting there.

DAMN, THINKING AS I TYPE - but then you'd have to factor in conservation of energy. In a closed system ie airplane cabin, energy cannot be created or destroyed, its just transformed from one form to another. A 10 lb bird needs 10 lbs of lift period to remain flying. It probably produces 20 lbs of energy from its wings, 10 of which dissipates sideways etc as I mentioned before - but bottom line is 10lbs worth is going straight down. Divide that by bird wing area and you have a PSI - that PSI acts on the aircraft floor as 10 lbs total force.

Dunno, you could argue all day about this I guess.

Think of it like this - you have a scale shaped in a circle - you have a 10 lbs fan pointed straight down at the scale. Fan turned off, the fan sits on the scale and registers 10 lbs. The whole setup is surrounded by a airtight cylinder. you crank on the fan to the point where the fan lifts off the ground and hovers above the scale. Forget about ground effect and all that and keep things simple - will the scale still read 10 lbs on it? I think so.

 

[QUOTE=03_AP1,Dec 6 2005, 09:20 AM]Elistan,

 

A trucker is driving down the highway and pulls over and bangs on the back of the truck gets in and drives down the road a few miles gets out and does it again. The cop stops him and asks him why are you doing this? He said well I have a bunch of birds in there and I'm over the weight limit with them so I bang on the truck to get the flying.

Those birds flying does nothing to lighten the load in the truck because they need as much downward force as they weight to keep them up and in a closed container you're going to see that as weight.

 

OK,you guys are close and are making good, logical arguments, but haven't "shown your work" as the prof would say, although I accept verbal explanations. You can solve this with free body diagrams (the MEs in the group will remember what those are, but for everyone else, they are just pictures with arrows showing where all the forces are acting). The plane is in level flight equilibrium over the earth. The bird is in level flight equilibrium in the plane. The answer should then be obvious. Hints: Does the plane exert force on the earth as it flies? Does a rocket ship exert force on the earth as it flies? What about when it leaves the atmosphere? Draw the free body diagrams to find out. Remember, the sum of the forces has to equal zero or something will be accelerating.

When you get tired of this one, here's question number two:

You are driving at 45 mph in your car on your way to a party. In the back seat is the VCR you are giving as a gift and an unrestrained, inflated helium balloon. Something goes horribly wrong and you slam into a brick wall. What happens to the VCR is obvious. But, what happens to the helium balloon?