# Birds In A Truck Riddle

1. Dec 6, 2013

I'm doing battle with someone in a debate and I need some help here as, perhaps, I'm not articulating my side well or he's just so stubborn that he doesn't want to learn the error of his ways. :)

It's the stupid old riddle where if you have 1000 birds in a plane or a truck (or any enclosed vehicle) that each weigh 1 pound and suddenly they all start flying would the vehicle weigh less than when they were perched.

My belief of physics on this and from everything I have read, the answer is no. The air they displace to stay in the air in the closed environment is equal to their mass so the vehicle, as a whole, would weigh the same.

Someone posted this question on Facebook and there were a few people who responded yes and some no but one person in question was quite emphatic about his belief that the vehicle would weigh less. His background is that he has spent many years as a pilot and his background in avionics tells him that because the birds are creating lift (low air pressure under the wings, etc.). I've tried to explain, in every way that I can, that to keep something in the air you need thrust, and that thrust in an enclosed environment is equal to the mass of the object to keep it in the air so the weight does not change of the vehicle whether or not the bird is flying inside.

The other individual keeps insulting me saying that I'm wrong because this all has to do with lift. I've cited numerous forums (this one included, including this link here):

I even went so far as to show him the link to the Mythbusters episode where they dealt with this specific question. No go. He cites how even encyclopedias can be wrong (guilt by association fallacy), how Mythbusters is a poor citation, and how people in science forums are just giving opinions. I tried to even explain Newtons 3 Laws of Motion and that was to no avail. Here is one of his rebuttals:

So, in my frustration, I came here. Is there any better way I can articulate my argument? Are there any people in here with some letters behind their name who can help me out here. :) Here is the last statement I made in the argument after explaining numerous times that lift and aerodynamics have nothing to do with this riddle at all, it just has to do with basic physics:

The one factor [He] just cannot wrap his head around is the factor of thrust. The four elements to an airfoil are Lift, Weight, Drag, and Thrust. The reason that a plane does not fly when just sitting on the ground is that there is no thrust to push it into the air for lift to occur. No thrust, no lift. So when you exert thrust as a means of propulsion, you can get lift with a wing which creates a pressure flow which creates lift. The lift compensates for the weight and is counteracted by drag. The same thing occurs with a bird. But instead of an engine to create the thrust, the thrust is created by flapping a wing. In the motion of the flapping, thrust is occurring to then create the lift etc. But in the manufacturing of the thrust, you are pushing down against the inside of the airplane or the truck or the box or whatever enclosed environment the bird is in. That thrust is the same amount of force as the mass of the bird so it balances itself out. Just because you can create lift doesn't mean that you have an anti-gravity device, the lift has to be made by creating thrust. Again, Physics 101.

So, can anyone help me with this? Thanks in advance!

2. Dec 6, 2013

### Staff: Mentor

This is an interesting question, so if I place a bucket on a scale with a little remote helicopter sitting on the bottom the weight would be the sum of the two but if the helicopter takes off and floats in the bucket what would the weight be? then as it flew out of the bucket higher and higher. It seems the weight would be only the bucket.

Next consider a bucket full of water with a submarine toy at the bottom the weight would clearly be the sum of the two. If the submarine begins to float to the surface the bucket+ submarine would still weigh the same right?

In these two cases the only difference is the air embedded in air versus water embedded in air environment.

Anyway this was discussed in PF some years ago:

3. Dec 6, 2013

### atyy

That's the "Bernoulli" explanation of lift. You are giving the "Newton" explanation of lift. Maybe by explaining to him that these are equivalent, he can also accept the "Newton" explanation.

http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html
("from a pilot's perspective")
http://www.planeandpilotmag.com/component/zine/article/289.html

Last edited by a moderator: Sep 25, 2014
4. Dec 6, 2013

### rcgldr

For this riddle to make sense, it needs to be a closed system. Assume there are "scrubbers" in the sealed compartment that consume the carbon dioxide and produce oxygen for the birds in order for it to be a closed system.

Next for the riddle to make sense, you have to assume that some steady state is reached where there is no vertical component of acceleration of the center of mass of the system.

Now you have a closed system with no vertical component of acceleration. The weight of that system is constant regardless of what occurs within that system as long as there is no vertical component of acceleration.

Perhaps it would help to remove the birds and note that the weight of the closed system includes the weight of the air. The air exerts it's weight by creating a pressure gradient that decreases with height, less pressure at the top, more pressure at the bottom, and the net downwards force exerted by the pressure gradient is exactly equal to the weight of the air.

Now put the birds back inside and have them flying so that there is no vertical component of acceleration of the center of mass of the birds. The result of producing lift by the birds is an increase in the pressure gradient within the container, so that the net downforce of the increased pressure gradient is exactly equal to the weight of the air and the flying birds.

The same principle would hold for a ballasted helium balloon hovering within the container. The pressure gradient downforce equals the weight of the air and the hovering balloon.

Generating lift produces downwash, or if in ground or ceiling effect, produces a pressure gradient, lower pressure above, higher pressure below. Once the downwash impacts with the bottom of the container, the momentum transfer results in a force equal to the component of weight lifted by the downwash.

5. Dec 7, 2013

### Staff: Mentor

This is an easy problem to analyze. In the initial state, the birds are standing on the floor of the truck. In the second state, the birds are soaring at constant height within the truck. I don't want to address the intermediate situation where the birds take off and accelerate upward (with flapping wings), since this is not what is asked for.

Let's take as a free body one of the soaring birds. There are two forces acting on this bird: its weight and the upward lift force of the air. Since the soaring bird is not accelerating vertically, these forces must be equal. So the force that the air exerts on each bird must be its weight. The total upward force that the air exerts on all the birds is their total weight. The details of how the lift is established is not important, but the (upward) pressure force on the bottom of each bird is higher than the (downward) pressure force on the top of each bird. This is over and above the hydrostatic pressure variation that would exist if the birds were not soaring. Now lets take the air as a free body. Aside from the gravitational force on the air (which doesn't change), the air is exerting a net upward force on the birds, and, by Newton's third low, the soaring birds are exerting and equal and opposite net downward force on the air. The only other force on the air are the forces exerted at the floor and roof of the truck. The air is not accelerating upward or downward (on average), so the net upward force exerted by the truck on the air must be equal to the net downward force exerted by the soaring birds on the air. This is equal to the weight of the birds. Now, let's do a force balance on the truck. The additional net downward force by the air on the truck is now equal to the weight of the birds (since the truck is not accelerating in the vertical). So, if there's a scale underneath the truck, the reading on the scale is the same when the birds are soaring as when the were standing in their initial state.

6. Dec 7, 2013

### Staff: Mentor

The pilot is correct that a birds wings do create lift. Lift is the upwards force that the air exerts on the wings. However, the wings also exert a downwards force on the air, and this force cannot be neglected. The fact that there is a downwards force on the air should be obvious to a pilot who has undoubtedly experienced the downwash from helicopters rotors or downdraft from an airplanes wings. This downwards force on the air is equal and opposite to the lift.

7. Dec 7, 2013

### 256bits

I would much rather go with your explanation of increased pressure produced by the birds, since it is the simpliest

If you go with the birds acting as a pump to circulate the air at a velocity v within the enclosed space, then one has to consider the exchange of momentum of the air at the box bottom face where the air experiences a change in momentum of m2v(bottom), and also, at the top of the container, where the change in momentum is m2v(top).

Shear along the box wall as the air travels upwards will attempt to lift the box upwards at the expense of lowering the pressure from bottom to top. Turbulance, and the wall shear, would increase the temperature of the air within the box.

8. Dec 7, 2013

### sophiecentaur

We had a PF thread, not long ago, about this and people who would, under most circumstances, be quite rational with Physics problems, were convinced that Bernouli was somehow opposing Newton in this. The argument went on and on and all because flying instructors know less about Physics than about the practicalities of flying.
The Bernouli protagonists seemed to ignore the fact that their view relies, in the end, on sky hooks. But any 'downward force' on the Earth or Van, due to the flying effort must be exactly the same as the weight of all the birds. This downward force is, of course, spread all over the floor of the van (or the ground), producing a minuscule pressure, compared with the pressure from the bird's feet , when standing on the ground. It's a bit like the idea being run over by a hovercraft or a tractor with bungee tyres, for going over marshes; no appreciable force on any one spot on the ground. Somehow, the Pro-Berlouliists seem to think that, below a certain value, the force just doesn't exist. The same people, no doubt, believe that Independent Television programmes cost them nothing.

9. Dec 13, 2015

### Vibhor

Could you please give your views in the case where a bird in a cage (air can move in and out) hovers somewhere in the middle ? Does the reading on the scale change ? Is there a definite answer (as in the case of bird inside a closed container ) in this situation or it depends on other factors ?

I think the reading decreases as some of the air is moved out of the cage due to the movement of bird's wings . Not sure .

Thanks

10. Dec 13, 2015

### sophiecentaur

Take it to an extreme - where the mesh of the container is more and more open and the weight force of the hovering bird, acting on the van could approach zero.

11. Dec 13, 2015

### Staff: Mentor

This is a much more complicated situation, in my judgment. I would like to take some more time to think about it for the case of an open cage.

Chet

12. Dec 13, 2015

### sophiecentaur

You are the angel, fearing to tread. Whilst I am . . . .

13. Dec 14, 2015

### CWatters

I'm with Chet. I think it's more complicated.

In order to fly the birds/helicopter have to accelerate air downwards. Being a closed system the air accelerated downwards has to circulate back to the top of the truck. Can the birds/helicopter keep accelerating the same air faster and faster? Yes? No?

The problem is known to helicopter pilots as a "Vortex Ring State" or "settling with power".

https://en.wikipedia.org/wiki/Vortex_ring_state

Usually occurs when descending too fast however it can also occur on just one side of a helicopter when it flies too close to a hanger or building. The helicopter can tilt and be "sucked" towards the building.

14. Dec 14, 2015

### CWatters

Ok thought about it some more. The helicopter might not be able to fly in the truck. The birds might be able to as friction and turbulence will slow the air down as it circulates so the birds can keep accelerating (the air will warm up).

In both cases the weight of the truck stays the same.

15. Dec 14, 2015

### Staff: Mentor

In my judgment, your assessment is correct. Just go to the limit where the top of the cage is 1 mile high, the bottom of the cage is on the ground (sitting on the scale), and the bird is 1/2 mile up. So, you have a very high cage (compared to its lateral dimensions). In that case, air could flow in and out of the cage very easily at the elevation of the bird, and the presence of the bird would have no significant effect on the scale reading.

16. Dec 14, 2015

### sophiecentaur

I am sure the OP was aimed at a simple case and having settled the basics, the thread has suffered mission creep. The problem, now, is where to stop in the added complications.

17. Dec 14, 2015

### Staff: Mentor

Yes. I'm closing this thread, considering we have never heard back from the OP, and there have no responses to the thread in over two years until 2 days ago.

Chet