# Tricky question

mrcore64
A man flies a small light aircraft straight and level. Inside the cockpit is a fly flying. Does the fly’s weight contribute to the aircraft’s weight at all?

cesiumfrog
yes .

Homework Helper
Of course not, unless the fly is attached to the walls of the plane (probably dead in the process)

Gold Member
Of course not, unless the fly is attached to the walls of the plane (probably dead in the process)

actually, even if the fly is flying, it still contributes to the weight of the plane - because inorder to stay in the air it has to push down with it's wings which in the end exerts a downwards pressure to the floor. saying that the fly doesn't matter is like saying that a small boat floating in a bathtub doesn't increase the weight of the tub.

Staff Emeritus
mrcore64
My friend alex refutes this and asks: But what if the fly is flying directly above the air conditioning vent?

Staff Emeritus
My friend alex refutes this and asks: But what if the fly is flying directly above the air conditioning vent?

Your "friend" alex should stop making you into his patsy and ask the question himself.

Zz.

mrcore64
*making you into his patsy*:rofl: But I like frilly things
He can't be flucked to register so I'm just copy'n'pastin his replies from our e-mails at work... I think it would still add to the weight of the plane becasue it's still flying...

Now if the fly was dead and being suspended in midair by the updraft from the air conditioning vent would it add to the weight then?

Staff Emeritus
*making you into his patsy*:rofl: But I like frilly things
He can't be flucked to register so I'm just copy'n'pastin his replies from our e-mails at work... I think it would still add to the weight of the plane becasue it's still flying...

Now if the fly was dead and being suspended in midair by the updraft from the air conditioning vent would it add to the weight then?

It is bad enough trying to explain things on here to someone directly. It will be worse, as far as communication is concerned, to try and carry a discussion via a 3rd party. I certainly have no patience to carry such a thing.

Zz.

mrcore64
Fair enough but this question *If the fly was dead and being suspended in midair by the updraft from the air conditioning vent would it add to the weight then?* was mine :D Unless your stumped...

Staff Emeritus
Fair enough but this question *If the fly was dead and being suspended in midair by the updraft from the air conditioning vent would it add to the weight then?* was mine :D Unless your stumped...

2. This is now a different question and when you add complexity to the problem, you also must make the question clearer by specifying if you make any assumption about the time of air flow - laminar or turbulent. You also need to specify if you assume that the air flow couples to the rest of the air in the room, i.e. is it a continuous flow, or simply a one-way flow?

You will notice that in physics, as soon as you try to go beyond the simplest scenario, then the QUESTION itself needs to be clearly presented if you expect any kind of unambiguous answer.

It is strange that you claim now that this question is "yours", when you said this earlier:

My friend alex refutes this and asks: But what if the fly is flying directly above the air conditioning vent?

Zz.

Last edited:
mrcore64
Yeeeeea cos he said it was flying... and I am saying if it were dead....:uhh:

OK, here's the evolved form of the question...

A man flies a small light aircraft straight and level. Inside the cockpit is a dead fly being suspend in midair by the updraft from the aircrafts AC. Does the fly’s weight contribute to the aircraft’s weight at all????

Staff Emeritus
Yeeeeea cos he said it was flying... and I am saying if it were dead....:uhh:

OK, here's the evolved form of the question...

A man flies a small light aircraft straight and level. Inside the cockpit is a dead fly being suspend in midair by the updraft from the aircrafts AC. Does the fly’s weight contribute to the aircraft’s weight at all????

It doesn't matter if it is suspended by the aircraft AC or by a string. If it is a closed system, then the answer is the same. If it is suspended by the draft coming from OUTSIDE the plane (windows open), then this is different. One can already see that if one removes the airplane - the bird will still be suspended. That is why I asked about how the air was "coupled" to the system, etc.

NOW do you see what I was getting at? If you only want to reveal only bits and pieces of the question rather than the whole question (something we REQUIRE if this is a homework or schoolwork-type question), then you will get the vague answer commensurate with your vague question.

Zz.

mrcore64
:rofl: It's a stupid question that we made up at work to pass time! You really shouldn't be gettin so worked up over something so trivial, it's bad for your heart :uhh:

Staff Emeritus
:rofl: It's a stupid question that we made up at work to pass time! You really shouldn't be gettin so worked up over something so trivial, it's bad for your heart :uhh:

We (or I) can't tell when a question is "serious" or "stupid". We treat all questions the same way. Besides, you were trying to ruffle my feathers by insinuating that I was 'stumped', which forced me to point out how ambiguous your question was for a clear answer.

If you wish to continue participating in this forum, keep that in mind.

Zz.

mrcore64
Unless you're stumped... :rofl: It was a joke dude! Are you going to stop me posting on the forum for having a sense of humor?

Anyway back to the question... In light of your answer some aspects need to be cleared up...

Where did you get the ridiculous notion of a bird being suspended when we were talking about a fly? Is it possible on a metaphysical level for a fly to spontaneously transform into a bird? If so could its weight be supported by the air conditioning system?

Last edited:
Staff Emeritus
Unless your stumped... :rofl: It was a joke dude! Are you going to stop me posting on the forum for having a sense of humor?

Anyway back to the question... In light of your answer some aspects need to be cleared up...

Where did you get the ridiculous notion of a bird being suspended when we were talking about a fly? Is it possible on a metaphysical level for a fly to spontaneously transform into a bird? If so could it's weight could be supported by the air conditioning system?

Does it really matter if it is a fly, a bird, a bowling ball, etc, and if it is suspended via floating in air, by a string, or by an air balloon?

All we care about is (i) if the object is suspended in air (i) by what mechanism and (iii) if this mechanism is part of the system under consideration or not?

You'll notice that I spent considerable effort whereby I didn't just TELL you the answer, but I spent time trying to explain why it is so.

And note the POINT I was making when I brought up your statement about me being stumped. It wasn't about your posting, but rather in response to YOUR assertion that I shouldn't be "so worked up". I made no connection between your "joke" and your ability to post on here.

Zz.

mrcore64
Oh ok...:yuck:...
As long as we are both on the same page that this is a TRIVIAL question, I'm not out to make air travel safer or create the next type of fuel...

Do you feel that you are doing a public service by answering an immensely banal question with the utmost authority and seriousness? It tests me to respect you as an "intellectual" if you can't see the funny side of this...you have a mother!:surprised

Staff Emeritus
Oh ok...:yuck:...
As long as we are both on the same page that this is a TRIVIAL question, I'm not out to make air travel safer or create the next type of fuel...

Do you feel that you are doing a public service by answering an immensely banal question with the utmost authority and seriousness? It tests me to respect you as an "intellectual" if you can't see the funny side of this...you have a mother!:surprised

Some of the most profound insights into physics came out of trivial observations! I can give you tons of examples. You'd think that the "http://www.aip.org/isns/reports/2007/006.html" [Broken]" that you'd find in gift packages is utterly trivial. Yet, the physics could have an impact in manufacturing and nanotechnology! So no, any physics question in this forum is not taken to be trivial, unless it is obtuse, vague, and plain silly.

Furthermore, this IS a physics forum, and you asked a physics question in the physics section of the forum. If you had intended this to be nothing more than a joke, I would have suggested that you post this in the General Discussion forum, not in here, where the posts are more heavily moderated to stick closer to the topic.

Zz.

Last edited by a moderator:
Gold Member
Oh ok...:yuck:...
As long as we are both on the same page that this is a TRIVIAL question, I'm not out to make air travel safer or create the next type of fuel...

Do you feel that you are doing a public service by answering an immensely banal question with the utmost authority and seriousness? It tests me to respect you as an "intellectual" if you can't see the funny side of this...you have a mother!:surprised
Zapper, he's got you there. You are obviously out-of-touch with the spirit and intent of the Physics Forum - what with your mere 6000+ posts and four year+ attendance. mrcore64 clearly has his hand on the pulse of the PF forum, having been here for several days now. Clearly, he knows why PF members come here every day.

mrcore64: Seriously dude, you've virtually stepped off the elevator into the office on your first day of work, and by 9:02 you're telling people that you know how the place runs and insulting people.

Last edited:
Gold Member
Where did you get the ridiculous notion of a bird being suspended when we were talking about a fly? Is it possible on a metaphysical level for a fly to spontaneously transform into a bird? If so could its weight be supported by the air conditioning system?
Your question is answered by the well-known logic puzzle that he refers to - birds flying inside a container truck.

If your puzszle can't be abstracted to this level and you can't see the principles involved, then you're not trying.

Now did you want an answer, or did you want to keep mocking him?

Staff Emeritus
Zapper, he's got you there. You are obviously out-of-touch with the spirit and intent of the Physics Forum - what with your mere 6000+ posts and four year+ attendance. mrcore64 clearly has his hand on the pulse of the PF forum, having been here for several days now. Clearly, he knows why PF members come here every day.

You are so right!!

<sobbing hysterically>

Zz.

Gold Member
P.S. If the fly were dead and suspended above an AC unit, yes it wouold still add to the weight. The fly's weight pushes down on the air, which pushes on the AC unit, which pushes on the plane.

Consider scaling it up to an elephant over a very powerful AC unit in a 747. It becomes pretty intuitive that a plane with an elephant in it is heavier than a plane sans elephant. In fact, you don't even need the plane to fly. It weighs more on the ground. In fact, you don't even need the plane...

If the fly suddenly dies in mid air, and then fall to the ground, then the weight would not contribute to the plane before it hit the ground.

The potential energy that contributes to the weight of the plane transform into kinetic energy on the way down, and then transforms into potential energy on impact. Isn't that right?

In an air conditioner, the air molecules constantly have to push the fly UP, pushing them DOWN, making the fly contributing to the weight of the plane. On a free fall, the fly's weight is not supported, so the weight should not be contributing to the total weight.

Chaos' lil bro Order
If the fly suddenly dies in mid air, and then fall to the ground, then the weight would not contribute to the plane before it hit the ground.

The potential energy that contributes to the weight of the plane transform into kinetic energy on the way down, and then transforms into potential energy on impact. Isn't that right?

In an air conditioner, the air molecules constantly have to push the fly UP, pushing them DOWN, making the fly contributing to the weight of the plane. On a free fall, the fly's weight is not supported, so the weight should not be contributing to the total weight.

As the fly falls to the ground, it is pushing air downwards, this air pushes on the plane's floor. There is no escaping the fact that the fly contributes to the plane's weight. Try as you may, but like a fly in a jar, there is no escape.

mrcore64
yea you're all right, I have no place to be judging a high roller like ZapperZ. Sorry dude! (T'was all in good humor tho):tongue2:

Anyways... Something I was thinkin about yesterday...science folk with the ability to harness Nuclear Power used that as a route to the Nuke (I've no idea if that's right)...my question is... If you could harness Zero Point Energy as an energy source could you make a ZPE bomb? And would it be one badass mutha?

Gold Member
...science folk with the ability to harness Nuclear Power used that as a route to the Nuke (I've no idea if that's right)...

You've got it backwards. The political (not scientific) mandate to develop an atomic bomb was the driver; whereas our increasing (scientific) knowledge of nuclear power was the result.

(Einstein himself, terrified of the implications, wrote a letter to the president pleading that he reconsider dropping the Manhattan Project.)

"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones." - A. Einstein

If you could harness Zero Point Energy as an energy source could you make a ZPE bomb? And would it be one badass mutha?)
Almost any advance in science has the potential to be used for harm, yes. It is not the knowledge that's evil, it's the abuse.

As the fly falls to the ground, it is pushing air downwards, this air pushes on the plane's floor. There is no escaping the fact that the fly contributes to the plane's weight. Try as you may, but like a fly in a jar, there is no escape.

If but when the fly is falling, the air that is pushed down is minimal, the only force the plane gets is the the force the air resists the falling fly with. Because the fly is dropping in height, means that there is no force to keep it up, making the plane not contributing to it's weight. I agree with you if the fly was flying around, but not if it is falling.

Gold Member
If but when the fly is falling, the air that is pushed down is minimal*, the only force the plane gets is the the force the air resists the falling fly with**. Because the fly is dropping in height, means that there is no force to keep it up, making the plane not contributing to it's weight. I agree with you if the fly was flying around, but not if it is falling.
* and by "minimal", you mean "equal to the weight of a fly"
** and by "the force of air resistance", you mean "equal to the weight of a fly"
They are small, but they are measurable.

Now, as far as a falling object - see attached illo.

For the duration of the fall, it is true, a falling fly would not contribute to the weight of the plane. What would actually happen is that the plane as a closed system would have its centre of mass remain in level horizontal motion. As the fly fell, the plane would rise slightly (very slightly).

However, when the fly hit bottom, the plane would stop rising. If you could measure the trajectory of the plane before and after the death of the fly, you would measure it very, very slightly higher (though still traveling horizontally). You would also note, contrarily, that the centre of mass of the airplane-AND-fly-system would NOT have changed in altitude.

BTW, this actually happens when bombers drop bombs or firefighters drop water; the plane will shoot skyward and the pilot has to fight to keep it level. It doesn't matter whether the cargo is internal or external.

#### Attachments

• PF070215fly-in-a-plane.gif
9.4 KB · Views: 408
Last edited: