Ahhhhhhhhhhhh, the message I’ve been waiting to post for three days now. sailor, I was going to collect all the rude comments you’d made to everyone who didn’t agree with you over the last couple of pages, but I don’t feel like going to the effort.
Last night I stopped by the grocery store and picked up a mylar birthday balloon with a pretty clown fish on it - very cheerful! I suggest you all do the same by the way, they’re about $3. I took it home and used the ribbon already attached to it (VERY inelastic I might add) to add a small steel nut to it about 2’ under the balloon. At first I just held it in my hand and moved it up and down slowly until LOW AND BEHOLD I discovered that Isaac Newton was RIGHT!!! MOMENTUM IS CONSERVED!!! Imagine my suprise???
Yes, you guessed it, if I moved my hand/arm up relatively slowly (almost as if I was <gasp> :eek: JUMPING SLOWLY!!!) and then stopped suddenly, the nut, with balloon attached, even with the whole system’s negative buoyancy, would FLY through the air with the greatest of ease - very nearly like Chitty Chitty Bang Bang! Going anywhere from a few inches to a foot or more depending on how fast I was moving my hand! No slack in the ribbon yet mind you, which I knew would cause a great stir here because as everyone knows it’s virtually impossible to jump THAT SLOWLY. So I started just gently tossing the nut up so that slack was created, and DO YOU KNOW WHAT I SAW??? YES, that’s right, it would go up, create slack, come down slightly while the balloon continued to rise, and when the slack was taken up, it would <gasp>:eek: KEEP RISING FURTHER!!! I was terribly afraid! Had I broken the laws of physics??? Would Mr. Nichols, my high school Physics teacher be knocking at my door with Stephen Hawking behind him with angry looks on their faces?? Fear Not! As I said 2 pages ago, the balloon rises much faster than you think it does! Sorry Achernar, I haven’t looked at your calcs yet, but there’s obviously something amiss there.
Do I have pictures of all this??? YES! Unfortunately I thought my camera cable was here at work, and it is not, so you’ll have to wait, or I may never get around to it. Tough darts if you don’t believe me, try it yourself.
To quell the further skepticism that I know will STILL be out there, I even set up the little experiment with the see-saw, the dropped weight, etc., and did it many times, because I am a scientist and I wanted good results. With no balloon attached the nut would rise about 3"-4". With balloon attached it would rise at least double that.
So, before I hear another argument from the doubters, I would encourage you to spend the $3 (you can even get the cheap balloon for only $0.98!) and do it yourself. It takes maybe 5 minutes to convince yourself.
sailor, please email me for the address to send your $500 to
Just as a final answer to the OP, YES, they can.
By the way, just received this from the nice folks at Parabounce! Please note his mention that 5# positive or negative is the difference between floating away and coming back down, and especially the last paragraph…
"Your question is a good one and the answer is a bit
involved, but here 'tiz.
The balloons we make will lift a person as heavy as 260 lbs
so that when he
pushes off, he will go up appx. 120 feet and return under
his own weight.
Therefore he is about 2-5 pounds positive. For lighter
people, we add ballast in 5
to 10 pound increments. Under light wind conditions, the
balloon w/rider is
very easy to maneuver from the ground via a tether. My 5
year old can drag me
through the air! I say this because being at plus or minus
5 pounds of zero
gravity is the difference in going up or down, but it’s
fairly gentle in either
direction, so we ‘fly’ people either way.
As for the tether, as a person rises, more tether is used
so therefore more
weight is gradually added. More weight means the balloon
will not rise as high
as it would untethered, but the difference will be minimal
since we keep
everyone under 150 feet. 100 feet of line only weighs about
2.5 pounds.
You can duplicate the same situation by tying a wad of
paper to a string
attached to a child’s helium balloon. If properly weighted,
it only takes a gentle
push to send the balloon to the ceiling before it returns
under the slight
weight of the paper. Now add a length of maybe fishing line
and give it a push
to see the difference.
Hope this helps.
Bests,
Steve at Parabounce®
PeeWee, when you’ve calmed down, could you weigh the nut with and without the balloon attached and post the figures here.
Your experiment, BTW, seems to confirm what I believe I have myself observed (as mentioned earlier), but I’m still going to try it for myself.
Did you get video footage or stills?
Mangetout, sorry, I get a little burned when people speak to me like I’m an idiot when they’re the ones who are wrong. :mad: I’m over it now, just had me pissed me for awhile, sorry to sound so, “I told you so!” Apologies to you too sailor, I know you were just feeling frustrated trying to explain your point of view, I shouldn’t have taken it so personally. :smack:
As for the weight of the nut and balloon, I’ll try to check it this weekend, but it’s probably a 3/8" nut, and your standard round pillow-shaped mylar birthday balloon about 15" diameter.
I don’t have a video camera, so I set my digital on rapid fire and took several pics in sequence.
A slight hijack – as a former physics major, I can’t quite understand why a couldn’t balloon accelerate upwards with an acceleration more than G.
In ideal physics-thought-experiment-land, with massless containers and arbitrarily dense filler, you could make a balloon with far more than 9.8 N of upwards force per kilogram of balloon mass, so the forces work.
And gravity can certainly accelerate things faster than G. If you hang a weight from a block and tackle, and let go, the end of the rope will accelerate upwards faster than G. Likewise the projectile on a trebuchet accelerates faster than G, though it’s all gravity-driven.
And air pressure certainly seems to be capable of accelerations beyond G (BB guns?)
So really, what is stopping my exotic-but-theoretically possible- materials- balloon from shooting upwards at 11 m/s/s ?
Probably nothing. sailor and Achernar kind of had us convinced for awhile, but the more I think about it the more I agree that the balloon may be able to accelerate faster than gravity, although I couldn’t find a problem with that part of Achernar’s calculations… Check those out if you haven’t yet.
They were saying the air it displaced couldn’t fall faster than gravity, but I think I disagree with that, because as the balloon goes up, its surfaces act in a similar manner to a wing and create areas of higher and lower pressure as the air moves past. I can thrust my fist into the air at a greater acceleration than gravity - why not a balloon, or a rocket for that matter. I think a lot of the problems everyone was having was picturing a balloon that could actually pull with a great force - it’s not something you encounter everyday. Party balloons don’t exert much force on you.
PeeWee says:
The difference is that your fist and the rocket have their own upward forces accelerating them. The balloon is pushed up solely by the falling air displacing it. Air can’t fall any faster than g, so it makes intuitive sense that a volume of falling air can’t displace a balloon of the same volume any faster than that.
PeeWee: Here is a page that explains the relationship between the acceleration upward of a buoyant object and the masses of the object and the displaced fluid. As an analogy, it suggests thinking of a balance scale: the balloon goes up, like one pan of the balance, while the surrounding air goes down, like the other pan. In both systems, the acceleration of the two components are linked.
I have an idea here (as yet untested, but it seems sound to me):
I reckon that the centre of mass of the whole system (the weight, string and balloon) will describe a smooth parabola modified by air resistance; buoyancy and gravity can be assumed constant, so even when the string is slack, the nut is decelerating as the balloon accelerates; but all that happens when the string goes taut is that energy is transferred between balloon and nut so that their vel;ocities become equal.
A little difficult to describe, but what I am hoping to do is to take video footage of my experiment and plot the paths of weight and balloon frame-by-frame, I believe that the plot will end up looking something like this crude and probably inaccurate diagram - the black line represents the height of the weight against time, the red line the height of the balloon against time and the blue line the height of their common centre of mass.
Of course all this is as yet unproven hypothesis, but I have hopes, I have hopes…
(or in other words, the nut rises again as the string goes taut because the centre of mass of the system has not yet reached the apex of its parabola).
FYI the terminal velocity of a sphere immersed in a fluid is:
v[sub]t[/sub] = ( [(4/3)(p[sub]fluid[/sub] – p[sub]object[/sub])r*g] / [ (1/2)p[sub]fluid[/sub]C[sub]d[/sub]] ) [sup]1/2[/sup]
p = density
r = radius of sphere
g = 9.8m/sec[sup]2[/sup]
C[sub]d[/sub] = drag coefficient = 1.2 for a sphere
Enjoy
One other thing, the guy in Zut’s link is confusing what he thinks of as the downward acceleration of the fluid element with drag.
:dubious: I know that drag coefficient is a hard thing to nail down, but this is higher than I’ve ever seen listed for a sphere. Is that right?
You guys kick some serious ass! I never dreamed I’d get such a thought-out, experimented and informed set of replies. I hate to waste server space with gratuitous thank you’s, but you guys have seriously raised the bar, and I feel that’s noteworthy. If this isn’t the greatest single resource on the web, I don’t want to know what is. [/gushing]
The guy in zut’s link is right and not confusing anuthing and you can see his calculations coincide with mine which I posted further up and which nobody has bothered to refute.
A body immersed in a fluid is not pushed upwards by magic, it is pushed upwards by the pressure of the fluid falling and this is caused by gravity. The fluid has to fall in order for it to lift the body. If we have a sphere filled with helium and neglect the mass of the sphere itself at what speed will it rise? The equations of the question are exactly the same as for the following question: In a large room the air has been extracted and there is vacuum. hanging from the ceiling we have a pulley and string of neglectable mass. From one end of the string we hang a sphere of Helium under normal conditions (1 atm, 20C) and from the other end we hang a sphere of the same volume full of air under normal conditions. The sphere full of He weighs less so it will rise but with what a? As they are linked by the string and pulley both spheres move with the same a but in opposite directions. What makes the He sphere rise is the air falling. The air sphere by itself would fall at g (9.8 m/s2) and that is the max it could possibly achieve. Now it is slowed by the inertia (not drag) o the mass of He.
The acceleration of the masses will be a= F/m. What is the F? it is the difference in weights between the two spheres. What is the mass? it is the sum of the masses. That is why the acceleration
Clearly the max a for a freely falling body on earth is g and air is no different. It will be slowed down by anything it wants to push out of the way. And all this neglects friction which would slow things even more.
The observation is easy: a child’s balloon does not accelerate upwards faster than a brick falls. Release a balloon from the floor and simultaneously drop a brick from the ceiling and see what happens. Which one reaches the other end first?
For a balloon to rise with a=7.35 m/s^2 the overall density of the balloon, including everything, not just the gas, and neglecting friction, would have to be 0.142 that of air or about 1/7th that of the surrounding air. I have found quotes saying the density of air is 1.239 grams per liter so the density of the balloon would have to be under 0.176 g/l. I have found the density of helium 0.164 g/l so we have a margin for the balloon itself of 0.012 g of balloon per liter of helium contained. For example, 1 m^3 of helium, would have a buoyancy of 1063 g. If the balloon itself had a mass of 12 g then it could rise with a=7.35 m/s^2 but if the mass of the balloon is greater it will rise with less acceleration. I doubt it is possible to build such a light balloon and even if it is possible then we would have to consider air friction which would be very substantial.
Mangetout, you are correct, of course, that if the balloon has sufficient mass and speed it can jerk the body upwards. I am assuming this is not the case and the momentum of the person’s body downwards is greater than the balloon’s upwards momentum. I can do any calculations with any data you care to present. Give me a hypothetical balloon (volume, mass etc) and I will do the math.
By the way, I have been spelling it wrongly and I believe the correct spelling is buoyancy (or so my spellchecker says).
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I read in the news that the US government is predicting some terrorists will hijack my computer unless I go to the Microsoft site and install a few patches. So I did and it totally screwed up my computer in several ways, my DSL connection being one of them and I am still dealing with the mess two days later. I just wish the terrorists, Microsoft and the US government would sort everything out among themselves and leave me out of their quarrels.
I have a friend who has a regular video camera (not high speed) and I thought I’d ask him if he was up for a little experimenting. He’s a good friend of mine he never smoked, always had a healthy lifestyle but in his late 30s got cancer and has been battling it for some years now but now he is 40 and probably will not live long. He is undergoing some experimental chemotherapy and feels like shit so I do not know if or when I may get his interest to play with the camera. On the other hand I tell myself it would distract him from his pain.
Yesterday I called his home and his wife, who I do not particularly like, answered the phone. When I mentioned the video camera she said, “oh, by the way and now that you mention this, we plug it into the computer but we cannot get it to work, can you help?” (The camera can be plugged into the USB.) So, told her the drivers would be in a CD that came with the camera. She said she had no idea where it was and they probably got rid of it as they did not know what it was for :rolleyes: I had to find the drivers online and then guide her step by step installing them and she just could not follow my instructions so the whole thing took over 3 hrs. And I still don’t know if or when I can borrow their camera. I think they prefer to not lend it but rather do the filming themselves. Anyway, we shall see. If I can get their help I will post some pictures and/or video.
Oh, one point I forgot to mention and which I feel stupid for assuming everybody would agree and know so I felt no need to mention it. A body can have a small positive bouyancy and remain on the ground until disturbed. Conversely, a body can have a small net positive weight and remain in the ceiling until a small disturbance causes it to drop. That is the experimet described with the balloon in that article: the balloon and weight have a small positive bouyancy but the weight remains on the table until a small tap makes it start to move upwards and then it keeps going. I guess this is sort of like water being able to go over 212F and not boil until it is disturbed.
Someone might remember that toy which came in cereal boxes. It was a figure which you put in a bottle filled with water. By pushing the cork in a bit you could make the figure sink and by pulling the cork out a bit you could make the figure float. The trick was that the figure held a tiny bubble of air and the pressure change wa enough to compress it so the figure had positive or negative bouyancy. I remember you could take it to the bottom and then slowly pull the cork to where the figure would rise but it stayed in the bottom. Then you could tap the bottle and the figure would rise.
Iwas out shopping today and my kids just happened to be given a promotional helium balloon each, I snatched the balloons, threw the kids to the ground and RAN! (actually no, I politely asked them to help me with a science experiment).
I tied a small plastic toy chicken (8 grammes) to the bottom of the string (which was a plastic ribbon with very little elasticity, with the balloon in place, the scale only read 2 grammes.
1). Sharply throwing the chicken upwards, the string went slack, the chicken rose to the apex of its trajectory, fell a little and was then tugged upwards considerably by the rising balloon as the string went taut.
2). By throwing the chicken a little more gently, it was possible to get the string to go taut as the chicken decelerated, but before it began to fall
3). Pushing the chicken upwards slowly, it was possible to make it ‘jump’ very slowly, but it should be noted that the ‘pushing’ force here was applied over a longer span of time.
I tried to capture it with the ‘movie’ mode on my digital still camera, but it the results were very poor - only five frames/sec at 640x480 and so much compression that the whole thing was unviewable. I will try to borrow a proper video camera.
I didn’t have the chance to set up a consistent launching mechanism, but in any case, there is absolutely no doubt in my mind that experiment 1 demonstrates that an upward trajectory can be resumed even if the point at which the line goes taut happens after the weight has begun falling and that this can carry the weight to a second apex that is higher than the first.
A few ponderings:
-At rest, the weight is already subject to the force created by the buoyancy of the balloon, so maybe the launching force affects it more than it would if the balloon were not attached (i.e. even after the line goes slack, maybe the first apex is higher than the natural one would be)
-The effect becomes more pronounced as the overall buoyancy of the system approaches neutral.
-I wonder whether the effect will be as pronounced when the system is scaled up to human size (although the parabounce thing would seem to indicate that the effect is still achievable, but perhaps at the expense of approaching very close to neutral buoyancy.
-The whole ‘slow jump’ thing - however slowly you jump, you only have the length of your own legs to apply force, after your feet leave the ground, you can’t push any more (sounds obvious, but with experiment 3, I was applying the force over a distance of 6 inches or so. - I suspect that the parabounce folks tell you to do the slow jump thing to prevent you setting up an uncomfortable rythmic bouncing scenario.
If you use the terminal velocity equation from either of these sites you can readily see that the acceleration due to buoyancy forces can easily exceed g. (notice that v[sub]t[/sub] is proportional to the radius), and that the retarding force is drag.
Achernar the drag coefficient I posted is from the second site.