Straight Dope Message Board > Main Sounds like a physics problem to me...
 Register FAQ Calendar Mark Forums Read

#1
12-29-2003, 01:42 PM
 CrAcKmOo Guest Join Date: Sep 2001
Sounds like a physics problem to me...

Ok so here is the situation: You pack an airplane (or any other container for that matter) with hundreds of pounds of flies, or birds, or anything that can suspend itself off the ground for a period of time. Then you weigh the container. The weight should equal the container plus the flies or birds. Now you get the flies or birds to take off and fly at the same time, staying in the air without landing. Obviously the mass of the container would have to stay the same, but would the apparent weight be different with the objects in flight, or would the weight remain the same because of the air pushing down or some other factor my brain isn't taking into account? Help me out, I don't know what to think.
#2
12-29-2003, 01:58 PM
 Crafter_Man Guest Join Date: Apr 1999
Re: Sounds like a physics problem to me...

Quote:
 Originally posted by CrAcKmOo would the apparent weight be different with the objects in flight
No.

While the flapping of wings could cause short-term fluctuations, the mean weight will remain constant. (Well, fuel is being used up, so the weight of the plane will obviously decrease over time. But I assume we’re ignoring other factors such as this.)
#3
12-29-2003, 02:11 PM
 Micro Furry Guest Join Date: Nov 1999
Why would the weight remain the same? This isn't a mass check but a weight check. A scale would seem to weigh only that which is pressing down on it, and all these birds, flies, etc aren't being suspended by wires from the crate they are in. If they are actually in the air, not touching the sides of the crate, the weight of the crate as measured by a scale would really be the same as when they are all sitting on the "floor" of the crate?
#4
12-29-2003, 02:16 PM
 Podkayne Guest Join Date: Aug 1999
How do the birds stay in the air? Because of lift from the air. But if the air is exerting a force on the birds, the birds also exert a force on the air. And the air exerts a force on the plane, so the weight of the plane doesn't change.

You might as well ask, if I put a bird on a chair, does the weight of the plane change? No, because the chair exerts a force on the bird, but the bird exerts an equal and opposite force on the chair, and thus the chair exerts the bird's weight on the floor of the plane.
#5
12-29-2003, 02:49 PM
 Quint Essence BANNED Join Date: Dec 2003 Posts: 281
think of it this way.
Is a fish bowl lighter if the fish are swimming?

....ummm the answer is no in case you are stumped.....
#6
12-29-2003, 03:21 PM
 Tyrrell McAllister Guest Join Date: Feb 2001
But for that situation to be analogous to the OP, we must imagine imagine that the fish bowl is itself submerged under water, along with the scale it rests on. Now imagine that a fish swims from the ambient water into the bowl and then swims out again. Does the reading on the scale increase while the fish is in the bowl and then decrease when the fish swims out of the bowl?
#7
12-29-2003, 03:29 PM
 Tyrrell McAllister Guest Join Date: Feb 2001
To add to my previous post, I think that the scale will not change depending on whether the flies are flying or not. The mass of the cylinder and its contents will be the same, and therefore the force pushing it towards the center of the Earth will be the same. Hence the scale will have to apply an equal opposite force to keep the cylinder stationary.

I just don't think that thinking of fish in fishbowls makes this any more intuitively obvious.
#8
12-29-2003, 04:02 PM
 Exapno Mapcase Charter Member Join Date: Mar 2002 Location: NY but not NYC Posts: 20,918
Juat want to note that this is indeed a very old, thorougly discussed physics problem, and the answer is, as everyone has said, that the scale will not show a change in average weight no matter what the birds are doing.
#9
12-29-2003, 07:51 PM
 Quint Essence BANNED Join Date: Dec 2003 Posts: 281
As long as the creatures travel WITH the vessel their mass is part of the vessel.
Only if the creatures leave the closed system will a
change in mass/weight occur.
#10
12-29-2003, 08:07 PM
 Squink Guest Join Date: Oct 2000
So, if I place a 1 liter graduated cylinder on a scale, measure the weight, climb onto a chair, and drop a marble into the cylinder, the measured weight will increase at the moment the marble passes the lip of the cylinder, rather than when the marble hits bottom?
What if, instead of using an actual cylinder, I merely consider a cylindrical air column extending from the pan of the balance to the edge of the atmosphere. Will an airplane passing through that cylinder show up on my balance readout?
#11
12-29-2003, 08:50 PM
 viking Guest Join Date: Dec 2002
But the marble is not travelling with the graduated cylinder. It is accelerating. If all the birds in the plane die in mid-flap, then the weight of the plane will in fact be reduced while they are free-falling.
#12
12-29-2003, 08:54 PM
 friedo Charter Member Join Date: May 2000 Location: Brooklyn Posts: 19,248
Squink, the marble will affect the weight of the cylinder as soon as it starts exerting force on it. If the cylinder is full of air, it probably doesn't exert enough force to be measured accurately until it hits the bottom. And since it's not a closed system, like an airplane, a lot of its kinetic energy is used expelling the air it displaces out of the cylinder.

An airplane flying overhead exerts force on the air around it. That force gets diminished very rapidly as it spreads out further and further from the plane. It is very unlikely that you would be able to measure the effects from the ground without some super-duper sensitive equipment. Even then, there is probably far too much "force noise" caused by wind, cars, people, pigeons and whatnot to accurately observe an airplane in such a way.
#13
12-29-2003, 09:18 PM
 Tyrrell McAllister Guest Join Date: Feb 2001
Quote:
 So, if I place a 1 liter graduated cylinder on a scale, measure the weight, climb onto a chair, and drop a marble into the cylinder, the measured weight will increase at the moment the marble passes the lip of the cylinder, rather than when the marble hits bottom?
So long as nothing is opposing the velocity of the marble, the scale will not be affected by it. So, of course, if you perform this experiment in a vacuum, the weight will not increase until the moment the marble hits the cylinder. If you perform this experiment in an atmosphere, then the falling marble will add minutely to the weight of the cylinder. The amount of additional weight will correlate to the degree to which the air that is resisting the marble's fall is itself supported by the scale.

So, when the marble is high up, the force that the falling marble transmits to the air is distributed all over the place, with only an infinitesimal portion of it eventually impinging on the scale.

When the marble has passed the lip of the cylinder, but still hasn't reached the bottom, a greater proportion of the force applied by the marble to the air reaches the scale. But since the marble is not being entirely stopped by the air, there will still be only a small additional increase in the weight registered by the scale.

Finally, when the marble reaches the cylinder's bottom, or reaches some contents of the cylinder capable of bringing it to a halt, the entire force of the marble's weight will register on the scale.
#14
12-29-2003, 09:44 PM
 Squink Guest Join Date: Oct 2000
So let's go back to the OP's airplane for a moment. What if, instead of a hermetically sealed airshell, the airplane has a window which the birds, flies or whatever, can open and close by turning a tiny crank.
The airplane is weighed, a bunch of flies take off, leaving the weight unchanged. Then an industrious fly opens the window. Suddenly, instead of a closed system, we have an open one which, depending on the shape of the window, resembles the grad cylinder I brought up earlier. What happens to the weight as the fly opens the window?
-BTW, I'm not yanking anyone's chain here, there's a transition in the result between the closed system and an open one, and undergrad physics never really did say what happens as you move between the two cases.
#15
12-29-2003, 10:42 PM
 Xema Guest Join Date: Mar 2002
Quote:
 Originally posted by Squink What if, instead of a hermetically sealed airshell, the airplane has a window which the birds, flies or whatever, can open and close by turning a tiny crank.
Cracking open a window doesn't really make an open system. It remains true that the birds fly by deflecting air downwards. As long as the aircraft they are in is affected by this deflected air, the aircraft continues to support their weight.

In the case of a graduated cylinder on a sensitive scale, much the same effect would be seen. A fly hovering over the cylinder would cause the indicated weight to increase. Blowing gently down onto the cylinder would have the same effect - that's pretty much what the fly's wings are doing.

To illustrate this more vividly, imagine standing in an open field on a bathroom scale. Now suppose a helicopter appears and hovers just over your head. It should be plausible that the weight reported on the scale increases.
#16
12-30-2003, 01:17 AM
 Tyrrell McAllister Guest Join Date: Feb 2001
The distinction between closed and open systems is not so important in this case. For the case with the flies in the plane, all that matters is: how much of the air supporting the flies is ultimately supported by the scale, and how much is supported by portions of the ground beyond the scale? Opening a window will matter only if it changes these quantities.
#17
12-30-2003, 01:46 AM
 Tangent Guest Join Date: Sep 2002
Quote:
 Originally posted by Squink So, if I place a 1 liter graduated cylinder on a scale, measure the weight, climb onto a chair, and drop a marble into the cylinder, the measured weight will increase at the moment the marble passes the lip of the cylinder, rather than when the marble hits bottom?
Don't forget that when the marble does hit the bottom of the cylinder, it will exert a downward force greater than its weight while it is being brought to rest. So even though the marble may not add any appreciable weight to the cylinder while falling, the "impact force" will make up for this when you average it out.
#18
12-30-2003, 02:38 AM
 Desmostylus BANNED Join Date: Nov 2002 Location: Sydney Posts: 5,539
Windows in the side might not make much difference. Holes in the top and bottom would.

When the flies reach some equilibrium motion in the plane, there'll be a slight increase in the pressure differential between the top of the container and the bottom, compared to what it was before they took off. The flies wings are acting as fans, causing this pressure increase. The slight extra pressure at the bottom pushes on the bottom of the container, and thus onto scale, and exactly matches the weight of the flies.

If you start punching holes in the top and bottom, air will start to get sucked in the top and blown out the bottom. The weight shown by the scale will start to drop. Once the holes are large enough to eliminate the pressure increase caused by the flies, the scale will ony register the weight of the container, and not the flies.
#19
12-30-2003, 03:09 AM
 Eleusis Guest Join Date: Oct 2002
Wow, 17 replies, and nobody mentioned Cecil's take.

Quote:
 Now suppose somebody opens a window. Does the weight of the plane change? Well, we do have the problem that everybody in the cabin will be sucked out and killed. But that's not the point I was trying to make. Let me think. Oh, yes. Antecedent to the loss of payload, the plane still weighs significantly less, if not necessarily a whole pound less, because it's no longer a closed system. Some of the downward force of the air being beat down by the pigeon's wings is dissipated to the exterior darkness. This may not seem like a big deal, and perhaps it isn't, but I have always thought an appreciation of the thermodynamic realities enhances one's quality of life.

 Bookmarks

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is Off HTML code is Off
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Main     About This Message Board     Comments on Cecil's Columns/Staff Reports     Straight Dope Chicago     General Questions     Great Debates     Elections     Cafe Society     The Game Room     In My Humble Opinion (IMHO)     Mundane Pointless Stuff I Must Share (MPSIMS)     Marketplace     The BBQ Pit Side Conversations     The Barn House

All times are GMT -5. The time now is 02:55 AM.