Thread: Bug in a balloon...

I can't seem to get a clear answer on this question...


There's a table in a room. On that table is a helium balloon. The helium balloon is weighted with just enough sand that it still floats away. When a bug is placed in the balloon, it's just enough extra weight so that it doesn't float. My question.. if the bug begins to fly/hover inside the balloon, will it begin to float away?


[of course, there are problems with the experiment. a) the bug would probably die in the helium and b) it might not be able to fly in the thinner gas.

so either look at it theoretically, or simply assume the experiment is the same,.... but there's regular air inside the balloon & the surrounding air is made heavier.]


I think it wouldn't float, but my mind still pictures the balloon floating away.

Am I right in assuming that the bug, whether flying or sitting, is still part of the mass of the balloon? Because even if it's flying, it's still creating a downward force equal to its own weight?

The balloon will not float away. Look at it this way: when the insect is flying it's pushing down on the air in the balloon (pushing enough to lift itself). That air is then pushing the balloon down. Of course, it's not entirely that simple, but it's basically true.

I don't think so. When the bug flies, you could as well argue that while it stays in the air, it pushes the gas up that is above it. What is the difference for the balloon between the bug being flying in it and flying above it? The air above the balloon pushes it down as well. And this is actually, why it hovers.

The air pressure from above is balanced out by the buoyant force of the helium gas. The weight of the bug is compensated by its own buoyant force during flight. If it didn't, the bug would crash. So, there is no additional weight force produced by the bug while it flies.

I guess a much simpler experiment, that would answer the same question, would be..


Put a fly inside a clear balloon. Put the balloon on a scale/balance. See if the weight changes when he takes off & lands.
It would have to be a pretty sensitive balance though, like the ones that are in a wind-proof box.
Or just use a bigger bug.

The balloon will not float away. Look at it this way: when the insect is flying it's pushing down on the air in the balloon (pushing enough to lift itself). That air is then pushing the balloon down. Of course, it's not entirely that simple, but it's basically true.

i would tend to agree but idk

I believe that Dishmaster got it right with the buoyant force.

Am I right in assuming that the bug, whether flying or sitting, is still part of the mass of the balloon?

Totally! But there’s a difference between mass and weight. The bug uses energy to keep the mass of itself in the air, so this mass shouldn’t be for the rest of the balloon to take care of. It’s not quite as simple though. If the bug is not flying, the bug’s weight will be as you would expect. If it starts flying, the weight of the fly will now be as if the bug was made up of the surrounding gas! So, if the bug has a volume of 1 mL, it now has the same weight as 1 mL helium. And it doesn’t matter if the bug has an actual mass of 1 gram or a ton!
That’s the way I see it, anyway :P

Originally Posted by DifferentDrummer

I can't seem to get a clear answer on this question...


There's a table in a room. On that table is a helium balloon. The helium balloon is weighted with just enough sand that it still floats away. When a bug is placed in the balloon, it's just enough extra weight so that it doesn't float. My question.. if the bug begins to fly/hover inside the balloon, will it begin to float away?


[of course, there are problems with the experiment. a) the bug would probably die in the helium and b) it might not be able to fly in the thinner gas.

so either look at it theoretically, or simply assume the experiment is the same,.... but there's regular air inside the balloon & the surrounding air is made heavier.]


I think it wouldn't float, but my mind still pictures the balloon floating away.

Am I right in assuming that the bug, whether flying or sitting, is still part of the mass of the balloon? Because even if it's flying, it's still creating a downward force equal to its own weight?

You have a closed system being acted upon by buoyant forces and gravity, in static equilibrium. Any forces exerted by the bug internal to the balloon are compensated by reaction forces within the balloon. Therefore the net force on the balloon/gas/bug plus gravity is zero. That in turn means that the center of mass of the balloon/gas/bug system is constant.

The gas can safely be assumed to be uniformly distributed within the balloon. So, if the table were removed, the balloon would just hover until the bug does something. With the table removed the balloon would actually fall a bit if the bug were to fly up, so as to maintain the location of the center of mass. But since the table is there the balloon will not fall due to reaction forces from the table. However, after things are in equilibrium, and the bug flies back down again, the balloon will rise slightly.

I think (yes it does happen) that the balloon would actually move downward as the bug flies up

supposing a balloon containing a bug is in total equilibrium, all the forces are balanced and it all just hovers there. when the bug flies up, it creates downward pressure greater than it's weight enabling it to fly up, therefore the balloon will go down, when the bug hovers the balloon will stop going down, and when the bug goes down the balloon remains stationairy.

I think



why not try it?

I agree with Booms. If the bug flies circles, the balloon will gyrate.

Why talk about it when you could try it?
I might try this one out.

Originally Posted by Pong

I agree with Booms. If the bug flies circles, the balloon will gyrate.

Of course it will. This is simply the fact previously noted that as the bug flies about, if you neglect the reaction forces of the table and air (let the balloon float and neglect air resistance) then all forces are internal to the balloon-bug system and the center of mass does not move.

Originally Posted by Pong

I agree with Booms. If the bug flies circles, the balloon will gyrate.

haha I read this and imagined a fly in a balloon flying round in circles going "hah this will confuse those bloody scientists that stuck me in here!"

I think this experiment is pretty similar to what the Mythbusters have done based on the myth below -

Story: A guy is travelling behind a big truck. Everytime the truck comes up to a bridge, the driver gets out, bangs on the side of the truck, and then proceeds to drive across. He asks the driver at a gas station why he was doing this -- the driver explained that he was over the weight limit for most of the bridges, so he was banging to make the birds fly and make the truck lighter.

The result was that, the mass didn't change, and I suppose Newton's Third Law applied here? What I understand is, the birds that flied would exert force which is equivalent to their mass, which result in the same mass for the whole system.

I like Physics but I am a real amateur, sorry if there is any mistake.

Originally Posted by Wilcon

I think this experiment is pretty similar to what the Mythbusters have done based on the myth below -

Story: A guy is travelling behind a big truck. Everytime the truck comes up to a bridge, the driver gets out, bangs on the side of the truck, and then proceeds to drive across. He asks the driver at a gas station why he was doing this -- the driver explained that he was over the weight limit for most of the bridges, so he was banging to make the birds fly and make the truck lighter.

The result was that, the mass didn't change, and I suppose Newton's Third Law applied here? What I understand is, the birds that flied would exert force which is equivalent to their mass, which result in the same mass for the whole system.

I like Physics but I am a real amateur, sorry if there is any mistake.

What you said is correct, and that is one way to look at it.

The easiest way is to note that the since forces are equal and opposite among individual point masses, that all forces internal to a system cancel out and you are left with only the externally applied forces to consider -- in this case gravity.

I interpreted his scenario to be that the weight of the fly causes the balloon to sink (the balloon was neutrally buoyant before the fly was added), and he wants to know if the fly taking off will send the balloon back to neutral buoyancy, and stop it from sinking. The answer is no, the overall density of the balloon/fly/etc. system will stay the same when the fly is flying, so its buoyancy will stay the same. And when the fly is moving upward the balloon will actually sink faster , due to conservation of momentum.

I'm going to go with none of the above, unless of course the bug can breath in such a helium rich environment.

Originally Posted by DrRocket

What you said is correct, and that is one way to look at it.

The easiest way is to note that the since forces are equal and opposite among individual point masses, that all forces internal to a system cancel out and you are left with only the externally applied forces to consider -- in this case gravity.

Thanks for your comment! I guess I still have a lot to learn anyway.