Floating rotor brain teaser, Who defines sanity anyway? Options

[lateapex]

Since this reply gets slightly off topic, I decided to post it in general discussion. It is in response to Snorkelface’s question in this brake thread:
http://frrax.com/rrforum/index.php?act=ST&...=15&#entry32147
QUOTE: “OK, I have to ask what makes a "Floating Rotor" design just that?”

Answer:
Depending on the final target weight of the rotor, they disperse about 9 to 12 oz of anti-gravity granules into the mold before pouring in the molten iron. It does wonders for unsprung weight, and greatly reduces shipping costs.

This reminds me of my childhood in Nepal. My dad was a truck driver, and one of his regular jobs was delivering birds to a pet store supplier. When we were pulling on the scales to be weighed for road tax, he would have me get out and bang on the side of the trailer to get the birds to flap their wings and hover in their cages. We both thought that this would make our load weigh less. When I was older, I realized that this did not affect the weight at all. But now that I am really old, I understand that it would have some effect on the weight. Maybe some of our resident physicists would care to “weigh” in on this, since I have explained why floating rotors float.

[sgarnett]

It depends on the design of the cages.

The truck scale is measuring weight, or downward force due to gravity, not mass. No matter how much you annoy the birds, the total mass of the truck, birds, air, cages, and bird poop remains fairly constant (but see below).

However, if the cages are roomy enough to allow the bird to fly high enough to be in freely circulating air, then the force of supporting the bird is roughly balanced by overcoming friction of the air molecules and the force required to accelerate the spinning, recirculating air.

OTOH, if the birds can only get a short distance away from the floor or a shelf, then the lift is dominated by ground effects and the high pressure area under the wings presses equally up on the wings and down on the truck.

If the truck is well ventilated, then as the birds exert themselves some stored glucose and fat will be metabolized, so that some mass will be dissipated as carbon dioxide. However, the more tangible by-products will still contribute to the weight of the truck even when deposited on the floor.

If the birds are REALLY annoyed and zipping around fast enough, their mass actually increases due to relativistic effects, so if you try too hard you could end up with an infinite tax.

(IMG:http://www.frrax.com/rrforum/style_emoticons/default/tongue.gif)

[lateapex]

Great job Sean,

The size of the cages is key. I had not considered the metabolism of food components into heat lowering body weight (and therefore the requisite downward directed air pressure to stay aloft if hovering close to bottom of small cage) and the effect of the hotter interior trailer environment relative to outside temperature (hot air balloon effect). I don’t think that extraordinary ventilation would be required as the weighing of the truck would last only 20 seconds. I like your full-orbed, expansive thinking, and subtle humor.

But, no matter what their fear level, I do not believe that any of the birds would approach the speed of light, assuming that they were operating in only one time-space continuum at a time.

Maybe another member could introduce another dimension to the analysis. Remember, this is not Corner Carvers. We all play nice.

[robz71lm7]

Yes but what if the birds fly up and stop flapping their wings and are in free falling motion for a split second?

[sgarnett]

Take it a step further. What if the birds flip over near the floor and actually flap UP just as the weight is recorded?

[trackbird]

Wouldn't they have to be near the ceiling (upside down)? In "ground effect"?

[lateapex]

Yes but what if the birds fly up and stop flapping their wings and are in free falling motion for a split second?

Again, it would depend on their height above the bottom of the cage. The falling body would increase air pressure under it. The further they fell, the greater the % of that pressure would reach the cage bottom rather than being converted into heat from the air molecule friction.

[slowTA]

What if they flapped their wings while keeping a firm grasp on the cages and we're assuming the cages are infinitely large. Wouldn't this make the truck very light? Can we also assume the bottom of the truck is like the sides and top of a birdcage... so the air pressure would pass through the floor? Then I think we would be onto some drastic change in weight.

[robz71lm7]

I'm so disappointed that all of the engineers have failed to first define their problem and list a set of assumptions. I'm a student so I don't count. (IMG:http://www.frrax.com/rrforum/style_emoticons/default/tongue.gif)

What kind of birds are these?
What is their drag coeficient in various position while falling?
Associated terminal velocity?
What if the inside of the truck were in a vacuum?

There must be an ASME standard for this.

[GlennCMC70]

you guys need to think a little harder. could it be that getting you out of the truck was the weight reduction he was really after. kinda like telling a paniced person that finding a blue flower w/ pink poka dots will save your friend from the poison. its not the flower that will help him, its your absence. (IMG:http://www.frrax.com/rrforum/style_emoticons/default/beerchug.gif)

[trackbird]

I was searching for a few other things and ran across this thread from "the good old days". lol. Good stuff.