how many helium balloons would it take to defy gravity????

You can never defy gravity, only cancel out its force vector with another force vector. In this case, you want the force of buoyancy to cancel out the force of gravity. This would take many balloons. I would estimate apx. 20000 party sized balloons. There was a mythbusters with this concept in mind. It took like 5000 balloons to lift 60 pounds.

You can't defy gravity with helium. It still works.

If you want to lift yourself, then it depends on the size of the balloon. Each cubic metre can lift a little over a kg, so if you double the figure above (to say 400 lb, or 200 kg) to take the weight of the balloon into account, you'd need about 200 cubic metres. A single balloon this size would do, but I'd go for something larger to make you more comfortable, so that you'd have something to sit on, and extra clothing for warmth. And some sandwiches, and the like.

Is this supposed to be a trick question? If so 0 because the way to defy gravity is not currently known to man. If it's not then I saw an episode of 'Mythbusters' once where they used 20,000-40,000 balloons filled with Helium to lift the wieght of a 46 lbs. girl, so I would guess 60,000-200,000 balloons would do the trick for someone in that weight range. I also saw another one similar to that in which they hooked around...42 (can't remember) weather balloons to a lawn chair, but it worked, and he was in that range.

It depend on the volume of each balloon.. Think of it this way - In order to generate lift, a helium filled balloon displaces a volume of air equal to the volume of the balloon. Taking into consideration the difference in density between Air and Helium, you can then calculate lifting power based on volume.

So, in order to life a 200 lb man, the volume of Helium would have to displace an amount of air equal to the weight of the human and the weight of the Helium...

Hope that helps...

They did this on Mythbusters with Adam sitting in a lawn chair. It took a lot. Pilot 3 episode

Don't know, but I would use multiple balloons if I were you. Then you could pop one at a time to descend gradually. Here's two sites about this subject.

http://home.earthlink.net/~quade/lawnchair.html

http://abcnews.go.com/GMA/story?id=3365726