Does a fly in a jar increase the mass of the jar?

Suppose you have an airtight jar. You weigh it.
Then you put in a fly and reseal it. Does the fly increase the mass of the jar when it is flying around in the jar?

Worded as it is, no. Would the jar be harder to pick up? Depending on what the fly was doing at the time, as air circulations could cause it to feel heavier or lighter at any given moment. However, on average, it will make it heavier than a non-fly jar by exactly the weight of the fly minus the air it displaces.

Assuming you were able to keep the transfer of air to zero while you put the fly in, then yes. Depending on how your thought experiment is set up, you have either added the mass of a fly, or replaced a certain volume of air with fly. Since fly is denser than air, there is a net increase of mass.

(I assume we’re speaking of the mass of the fly-air-jar system, not just the jar itself.)

To be clear, I’m agreeing with Ludovic. I assume the question is comparing non-fly to fly, not non-moving-fly to moving-fly.

Is this one of those situations where the distinction between weight and mass is important?

No, because the whole experiment is under standard earth gravity.

The question is trivial when talking about mass. Obviously, the jar has increased its mass by the mass of one fly minus the mass of the volume of air the creature displaced. Whether it would weigh more is at least slightly less obvious.

Remember, the difference between weight and mass doesn’t only show up on different planets. Relative density between objects and the surrounding medium can effect weights, too, as one can tell by jumping in a swimming pool.

I think the OP is trying to re-raise the old (and now solved) puzzle about the trcuk full of birds, where the driver tries to get the truck lighter by makeing the birds fly.

The* flight *of the fly makes no difference.

Now suppose the fly has a twin, and the twin is in an identical jar aboard a spaceship accelerating to .99c . . . .

Missed the edit window, but I wanted to add that downward forces will also change at least the measured weight, as you can tell by jumping up and down on a scale. I think that’s what the OP is getting at, like that question about a bird flying around in an airplane. Does it make the plane heavier or lighter?

I don’t know the answer but DrDeth says it makes no difference.

…and put it on a treadmill.

If you jump up and down on a scale, the measured weight will fluctuate. However, if I understand it right, the average weight after landing will still be the reading when the object is motionless.

DrDeth is right. Think about it this way.

When the fly is flying what is holding the fly up?


What is holding the air up?

The bottle

IANAPhysicist though.

What about Brundle-fly?

And what if Brundle-fly is drenched in yak jizz?

The fly doesn’t replace any of the jar’s molecules, so the jar remains the the same mass.

To be a bit pedantic, I’ll amend DrDeth’s statement to: On average The* flight *of the fly makes no difference.

When the fly zooms up you could theoretically measure a slight increase in weight (the result of changing momentum) and conversely measure a slight decrease when the fly zooms down.

IOW, then, the truck full of birds would weigh less if you could get all the birds to coordinate. I.e., have all the birds fly up to the top of the truck, then have them all drop at the same time, and, until they land, the truck weighs less than it would when they land.

Or something like that.


Just to be clear, the truck only weighs less while the birds are dropping; while they’re hovering in the air at the top of the truck, it weighs (on average) the same as if they’re perched.

Correct, and when they land or stop falling, the weight of the truck goes up by a sufficient amount to average out. And then rebounds back to the average.