If you have a sealed container with 500 perched birds in it would it be heavier than if the birds were flying?

Question

This is a serious question not a catch. I have quoted a sealed container to bring into the equation the fact of air pressure fom the wings whilst they are beating to stay flying. It would be the same container with the same perches in each scenario. Nothing added and nothing taken away.

Answer 1

The container would be heavier if the birds were accelerating in the upward direction.

When all the birds are perched in the container, the container will have some weight....call it weight W.
If those birds decided (individually or all together, it doesn't matter) to all of a sudden jump into flight, the container's weight would increase. In order to accelerate upward, the birds must exert a net force in the upward direction (which is countered by an equal force pushing downward). The downward pushing force adds to the container's weight (making it feel heavier), while the upward force goes into accelerating the birds into flight.

Once the birds stop accelerating (notice I didn't say stop flying/moving), the weight will return to normal. Even if the birds are still flying upward with a constant velocity (no acceleration), the container will weigh its normal weight W. But if the birds accelerate upwards, the weight of the container will be > W.

If the birds were to accelerate in the horizontal direction, this would have no effect on the container's weight since the net force is acting perpendicular to the force of gravity. The container's weight only increases when the birds either accelerate up (increasing the weight) or down (decreasing the weight).

The weight of the container will only change when the birds change their vertical component of their velocity (i.e. when they accelerate upward or downward), when this is not true, the container's weight will be the same.



EDIT:

To “Bob G” (et al),
I am not immune to mistakes and it is always a good idea to inspect others’ logic before accepting it as one’s own (or rejecting it).

Here is my reasoning…..
There are essentially 3 different states which we need to consider.

State 1: Container is stationary and the birds are not moving (everything is at rest).
In this case, the overall system’s weight is obviously the sum of the container’s weight and the weight of the birds, all of their weights act downward and there are no other forces to consider. The total weight is the total weight.

State 2: Birds are moving at a constant velocity (upward or downward, it doesn’t matter, but their velocity is constant and there is no acceleration).
Since there is no acceleration, there is no net force acting on the birds. Obviously the birds still have a weight acting downward, so for there to be no net force then there must be something else acting upward on them. I suggest two possibilities of the source of this extra, “supporting force”. Possibility 1: The birds are falling at their terminal velocity in which case the upward force is provided by the drag force caused by air resistance. Possibility 2: The birds are flapping their wing just enough not to fall out of the sky, but not enough to accelerate upward. With both these possibilities the birds are pushing against the air (either because they are falling really fast or because of the birds’ wings flapping). When the birds push against the air, the air pushes back. The weights of the birds are being supported by the air, which in turn is being supported by the container’s bottom (as was the case in State 1). Here, too, the total weight is the total weight.

State 3: The birds are accelerating (upward or downward will only affect the direction the net force is acting, and thus the direction the weight changes).
As always, the birds’ weight is acting downward. But here we will consider the possibility of the birds accelerating upward. Since they are accelerating upward, there must be a net force acting upward. For the bird to exert this upward force it must flap its wings and push against the air in the container. Like in State 2, the force the bird exerts on the air is transferred though the air into the container. Essentially the birds are pushing against the container bottom. This extra downward push which the birds exert on the air/container acts in the same direction as the container’s weight so the forces add. The force the birds exert must be greater than their own weight (lest they move at a constant velocity), so the sum of the exerted force plus the container’s weight will be greater than the overall total weight of the mass within the container.
If, instead, the birds were to accelerate downward then they would be pushing against the top of their container and this force would subtract from the container’s weight, making it seem lighter than before.

Consider this comparison,
If one stands still on a scale and looks at one’s weight, it will read some constant value. If one suddenly goes limp and collapses, as you fall, the scale will read as if you had zero weight. If you were to jump up on the scale, the scale would read a much higher weight than the true value since you must push off against the scale in order to jump.



EDIT, again,

I performed an experiment.

I carefully placed a tall, narrow plastic bottle inside a glass jar (with a lid). On top of the plastic bottle, inside the jar, I placed two coins (a quarter and a penny). I closed the jar and placed it on an electronic scale. I then carefully applied a horizontal jarring force to the jar on the scale. This jolt I applied acted to knock the plastic bottle over inside the jar and allowed the coins to fall off the bottle to the bottom of the jar (they accelerated downward due to gravity). As I did this, I watched the read-out on the electronic scale. As the coins fell, the jar momentarily became lighter. After all was settled inside the jar, the weight was back to its original value.

This experiment corresponds to this question by testing what would happen if the birds were to fly / accelerate downward inside their closed container. While the coins (which represented the birds) fell, the overall weight of the container seemed to lessen only to return to its original value when the coins (birds) stopped accelerating/hit the bottom.

This experiment is simple enough to perform on one’s own assuming one has a scale capable of such precision (most ordinary kitchen/bathroom scales will not work).

Answer 2

Interesting question and my answer is as follows: Yes, unless you can train all 500 birds to sit nice and patiently until the container is sealed. Imagine this. A bird is hovering in place and you put a box on a scale underneath it. Does the box weigh heavier? No. Now gently close the top of the box over this bird as it continues to hover in place. Does the box weigh more now? No. So going back to my original hypothesis. . . unless you can keep all 500 birds from flying before the container is sealed, the 500 perched birds will be heavier.

Answer 3

Wow, I'm shocked! I've never seen mrjeffy321 give a wrong answer before (in fact, it makes we wonder if my answer is right).

There's two forces that can accelerate the birds. Their wings and gravity.

Gravity is acting on them no matter what, so there's always a downward component to the bird's acceleration whether they're flying, sitting on a perch, or falling. Because it's a sealed box, the weight won't change because of the gravitational component regardless of the birds' motion (as they fall, they would force air towards the bottom of the box, cancelling out the bird's weight).

The birds can supply an additional component to acceleration by beating their wings, forcing air to hit against the sides of the box. If the birds flap their wings in a downward motion, air will hit the bottom of the box and either supply more downward force on the box or force the air that was originally near the bottom of the box to moved around the sides towards the top to fill the less dense air above the bird's wings. All in all, I think the pressures have to cancel out giving no net force. If, for some reason, the birds turned upside down and propelled the air towards the top of the box, the only way the box might conceivably get lighter, I think the same principle of the air distributing itself within the box to balance the pressures would still prevail.

If there is at least a minor fluctuation in the weight of the box because of the bird wings' downward motion, I think it would be almost immediately be countered by a slightly lighter weight as the high pressure area near the bottom of the box forced the air towards the top of the box. One way or the other, everything has to cancel out to no net force over the long term.

In fact, the more I think about it, the more convinced I am that there can be no net change in the weight. Substitute all of the birds with a big fan set against the top of the container blowing air towards the bottom. The fan will push against the top of the container because of the air it's blowing towards the bottom. That means the air hitting the bottom of the container has to have the same force that the fan is exerting against the top. Everything always cancels out because the air has no place to escape to.

Answer 4

What a great question.
After pondering it for awhile, I've decided that the birds cannot simply defy gravity and suspend themselves in the container - the force holding them against gravity must be equal and opposite in the container - hence, the container will always weigh the same. The weight may seem to fluctuate - like when the bird pushes of his perch or accelerates upward or glides down - but the overall mean weight of the container must remain the same.
The mass of the container is not changing - only the forces within the container.

Answer 5

No, the weight will not change!

In fact, what happens it that when the birds fly in the sealed container, their wings push the air under them so that they can lift.
Now the 'weight' (or more accurately, the momentum) of air displaced under them per unit time will be same as the weight of the birds. So there is a compensating effect, the balance will now not measure the weight of the birds but it will measure the force that the air exert on it (which is same as the weight).

So, no, there will be no change.

Answer 6

the weight of the container would fluctuate ,
assuming the birds, were to fly occasionally.

if they were all airborne at the same time ,
the container would be substantially lighter.

Than if they were all perched .

I do not think that the beating of the wings ,would add ,
or subtract from the weight of the container .

Now, be a good scientist and let those birds , fly free!!!

Have a Happy Christmas
>^,,^<

Answer 7

If the birds where flying they would be supporting their own weight on their wings, not on the container. However, if the container was sealed the energy from their wings would be transferred to the walls of the container, and therefore the weight wouldn't change.

Answer 8

It is a closed container.

The masses of all that inside are added up to give a net mass.

The weight is mass times the acceleration due to gravity.

Again since the container is a closed one, we need not bother about the motion of the objects inside it.

Whether the objects are moving or at rest, the weight will be the same.

The molecules of air inside the container are not at rest; they are in random motion inside it. Same with the birds.

Whether the birds, or molecules or electrons inside the container are MOVING or AT REST, BY THE LAW OF CONSERVATION of momentum THE NET MOMENTUM of the container is ZERO.

Therefore the change in the momentum of the birds due to their flight will be compensated by the change in momentum of the container wall. And the weight of the container with birds will remain the same.


If the container is not a closed one, then the net momentum will change and there will be a force in the direction of change in momentum which will affect the net force acting on the container.

Answer 9

If you have the scales underneath then it would be heavier if they WEREN'T flying. If you're asking if the mass would change then no, it wouldnt.

If you worked out how much energy the birds were using it would equal the energy required to make the container lighter by that amount. You use the same energy climbing a flight of stairs as a lift has to in lifting you up the same amount.

Answer 10

No. If cage is air tight it will not weigh heavy because by the concepts of Fluid mechanics flying or floating means to displace fluid equal to your weight. Hence though birds are flying the displaced fluid will fulfill their weight hence same weight of that cage.