Bird inside a box conundrum
- Thread starter Yoga Face
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If the box is filled with helium it will rise of course but what if a ballon filled with the helium is tied to the floor of the box would that make the box rise
You must weigh it in a vacuum to get the true weight.
Give it a few minutes and you will have the weight of the bird included.
Give it a few minutes and you will have the weight of the bird included.
I see a woman taking her t-shirt off getting ready for "doggie"?
What's wrong with that? I happen to like that position for a woman!
Maybe if the bird went to the moon his weight would change, but on earth, his weight will be constant since w=mg, where m is his mass (1 kg) and g is the acceleration due to gravity, which on Earth is 9.81 m/s2.
The bird's weight will be 1 kg x 9.1 m/s2 = 9.81 Newtons.
His mass is 1 kg.
... but what does the box weigh (with a bird, a balloon filled with helium and a pound of coke inside of it)?
There are no absolute answers.
In a closed space (which isn't much larger than the bird), it doesn't matter much if the bird is sitting or flying - the air pressure exerted by the bird in flight replaces the weight of the bird itself.
If the top is replaced with a cloth, the air pressure effect will be diminished and the apparent weight will decline for a bird in flight. The effect continues to diminish if the top (or sides, or bottom) are replaced with a mesh, or are open. Similarly, if the space is much larger than the bird (which, really, it needs to be for the bird to achieve proper flight) then the air pressure effects are similarly dissipated and the combined weight with the bird in flight is reduced compared to a bird sitting on the floor.
Let's consider another example: you're in a stationary elevator, standing still. The elevator has to hold your weight. (Maybe you're standing on a scale in the elevator.) You decide to jump off the floor: how much weight is the elevator experiencing as your feet leave the floor? What if the elevator were rising at the time? Falling? Falling, but decelerating? How much weight is experienced as you land again?
Now let's go back to the bird-in-a-box: in order to achieve flight, how much pressure must the bird exert against the floor of the container? What happens when it flaps its wings?
The changes in force are very short-lived, and very difficult to measure accurately, in part because spring-based scales have a significant "bounce" to them (according to the "k" of the spring) and are not good for dynamic loads.
In a closed space (which isn't much larger than the bird), it doesn't matter much if the bird is sitting or flying - the air pressure exerted by the bird in flight replaces the weight of the bird itself.
If the top is replaced with a cloth, the air pressure effect will be diminished and the apparent weight will decline for a bird in flight. The effect continues to diminish if the top (or sides, or bottom) are replaced with a mesh, or are open. Similarly, if the space is much larger than the bird (which, really, it needs to be for the bird to achieve proper flight) then the air pressure effects are similarly dissipated and the combined weight with the bird in flight is reduced compared to a bird sitting on the floor.
Let's consider another example: you're in a stationary elevator, standing still. The elevator has to hold your weight. (Maybe you're standing on a scale in the elevator.) You decide to jump off the floor: how much weight is the elevator experiencing as your feet leave the floor? What if the elevator were rising at the time? Falling? Falling, but decelerating? How much weight is experienced as you land again?
Now let's go back to the bird-in-a-box: in order to achieve flight, how much pressure must the bird exert against the floor of the container? What happens when it flaps its wings?
The changes in force are very short-lived, and very difficult to measure accurately, in part because spring-based scales have a significant "bounce" to them (according to the "k" of the spring) and are not good for dynamic loads.
Yoga Face, have you swallowed a book of "oddities of science" or something?
When it comes to birds registering their weights, you can't get away from Newton's 3rd law of motion - for every action, there is an equal and opposite reaction. The law applies to air, just as it does to all physical things. If a bird's wing drives air downwards with a force F, the air pushes back on the wing with, yes, force F.
Given that the bird's weight is acting downwards, if the bird is flying in air, the air must be pushing upwards on the bird with a force equal to the weight of the bird.
One way of regarding it is that the bird has developed a way to make the the air under the bird slightly denser (i.e at a higher pressure) than the surrounding ambient air, and of making the air above the bird slightly less dense. It does this by driving air downwards with its wings. There are more molecules of air per litre in the air below the bird than in the air above the bird.
The higher air pressure below the bird acts downwards on the floor of the box, and so raises the reading of the weighscale. Equally, the lower pressure above the bird sucks the top of the box downwards, and likewise also raises the reading of the weighscale. The two effects added together mean that the weighscale registers the weight of the bird plus the weight of the box.
The situation is quite different if the box is replaced by a cage. Now, the air pressure and density differences needed to suppor the weight of the bird will not reach the top and bottom of the cage - the differences are just dissipated, as air moves in and out through the sides of the cage.
So, the weight of a bird flying round in a cage would not register on the weighscale, but the weight of a bird flying round in an enclosed box would register.
If the bird clags on the floor, now the weight of the bird has gone down, so the difference in pressure or density, above vs below, needed to support the weight of the bird, is smaller. However, there is now a turd resting on the floor, and so the overall weight does not change.
When it comes to birds registering their weights, you can't get away from Newton's 3rd law of motion - for every action, there is an equal and opposite reaction. The law applies to air, just as it does to all physical things. If a bird's wing drives air downwards with a force F, the air pushes back on the wing with, yes, force F.
Given that the bird's weight is acting downwards, if the bird is flying in air, the air must be pushing upwards on the bird with a force equal to the weight of the bird.
One way of regarding it is that the bird has developed a way to make the the air under the bird slightly denser (i.e at a higher pressure) than the surrounding ambient air, and of making the air above the bird slightly less dense. It does this by driving air downwards with its wings. There are more molecules of air per litre in the air below the bird than in the air above the bird.
The higher air pressure below the bird acts downwards on the floor of the box, and so raises the reading of the weighscale. Equally, the lower pressure above the bird sucks the top of the box downwards, and likewise also raises the reading of the weighscale. The two effects added together mean that the weighscale registers the weight of the bird plus the weight of the box.
The situation is quite different if the box is replaced by a cage. Now, the air pressure and density differences needed to suppor the weight of the bird will not reach the top and bottom of the cage - the differences are just dissipated, as air moves in and out through the sides of the cage.
So, the weight of a bird flying round in a cage would not register on the weighscale, but the weight of a bird flying round in an enclosed box would register.
If the bird clags on the floor, now the weight of the bird has gone down, so the difference in pressure or density, above vs below, needed to support the weight of the bird, is smaller. However, there is now a turd resting on the floor, and so the overall weight does not change.
I doubt the size of the box matters
Apparently Myth Busters tried it out with birds inside a truck and the truck weighed the same if they flew or not
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buttercup to the rescue. That high school physics is paying off big time But as in the pendulum and the hammer , where I still think there is a transfer of weight ,I agree with you but feel I am also correct
Actually, that was one of my original thoughts exactly as to why the weight may be the same but was not sure because I thought of the following :
What happens when the wings are moving upward now the air above is denser and the air below has less molecules so the container weighs less than if it was empty
but
because the bird falls on the up stroke maybe that accounts for the weight remaining stable
but
the bird is expending energy so it also makes sense if the box would weigh less the same as if I expended energy in lifting the box or if I was in the box then jumped up. As I came down I would be pressing the molecules beneath me but not on the way up so the box would weigh less, at least on my ascent
and
what if the bird is an eagle and it glides but in this scenario I suspect because it must fall as it glides the air pressure is denser below so the third law takes over again but to what extent would it account for the gliding birds weight weight
Oh yeah, Newtons laws were rescinded by Einstein anyways
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Weight is defined as the force of gravity so unless the bird loses all of its mass, its weight will remain 9.8 Newtons (the weight of a 1 kg object near the earth's surface).
Yes, the size of the box does matter. Because air pressure is highly variable.
Imagine a very large box, with the bird on a perch high up in that box. When the bird flaps its wings, the air pressure under the wings will be increased (and decreased above the wings), but the air pressure waaay down at the bottom of the box won't be measurably altered. And as the bird flies around the air will churn but again the air pressure won't be significantly affected.
Now imagine a very small box, just larger than the bird. The bird won't be able to fly in such a confined space, but if it were then the air pressure exerted by the bird would affect the surface of the box.
As to what the Mythbusters demonstrated, well, there are many flaws. The size of the box relative to the size of the bird is one. The large number of birds is another. They would have done better to create a cage large enough for a bird to fly, out of light wire and saran wrap, suspended from a scale to measure a dynamic load.
Newton suggested that, when you jump up off the ground, what is happening is that, as you push the ground downwards, the ground pushes back, and propels you upwards. (Don't worry if that idea seems hard to grasp, if it's the first time you've thought about it -- Newton has gone down in history as a first-rate genius for first putting the idea into words.)
A bird flies by pushing air downwards at a high enough rate that the reaction of the air to being pushed downwards so violently is enough to support the weight of the bird. We can't fly, of course; what that means is that we don't have the muscles to push air downwards at a high enough rate to create enough of a pressure difference, between the air above us and the air below us, to support our weight.
Now, when a bird is flying by flapping its wings, you are correct that the bird is only pushing downwards on the air when the wing is travelling downwards. If the bird were to move as much air upwards on its upstroke as it moves downwards on its downstroke, the bird would never get off the ground. The bird must be sure to angle its wings to move as little air as possible in the wing's upstroke, and to move as much air as possible on the downstroke.
Incidentally, it's worth pondering just how big a deal it is for a bird to take off. It's interesting that different birds have evolved different techniques. Some birds climb trees or cliffs, and simply fall from a height, to become airborne. That's the easiest way.
At the other end of the scale, taking off vertically upwards from a standing start is the biggest shock to the system. But even some large birds, geese and turkeys for instance, are able to do it. They have evolved special muscles to enable them to beat their wings downwards on the air very powerfully.
However, they don't need to maintain that explosive power for very long. The breast muscles of a turkey are very effective at delivering tremendous power, but only for a short time. These muscles are anaerobic - they do not accept oxygen, nor blood flow, while they are working. Which is why they cannot work for more than a few seconds -- and, becasue there are no blood vessels in it, the breast meat is white. (Same with frogs' legs - using the "white meat" of their leg muscles, they can perform huge sudden jumps, but cannot sustain e.g running movement for more than a second or two. Also, frogs' hearts have only two chambers, not four, like mammals and birds, so they quickly get tired if they have to sustain movement -- which is why it is very cruel to torment a frog to make it run. )
Back to birds -- of course, the ability to take off vertically from a standing start is a good ability to have, to escape predators. But just look at the mechanics of the thing in a little detail. It's interesting that, when taking off vertically from a standing start, it's the second beat of the wings that is the most demanding, and needs the most energy. Not the first beat - the first beat is accompanied by pushing against the ground with the feet and legs.
So, say the bird is 20cm off the ground at the end of the first downstroke of the wings, this height having been achieved by a combination of jumping with the legs and downwards movement of the wings, using all its power. But now, after the first downstroke is complete, the wings have to move upwards in order to get the wings ready for the second downstroke -- and during the wing upstroke the bird is falling, as you rightly point out.
Let's say the bird falls 10cm before it can get its wings started on the second downstroke. But now, on the second downstroke, it can get no assistance by pushing off the ground with its legs. The bird must put enough strength into its second downstroke so that, at the end of its second upstroke, the bird is still clear of the ground, with a bit left over. The third downstroke is a little easier, then the fourth, and so on. And we can eat tasty white meat.
How can anyone not be fascinated by such stuff? All kids should be forced to study physics in school, so they can be ready, later, to start to understand the wonders of natural things, and where energy comes from, and why the earth is round, and women's beach volleyball.
- can't see what you're getting at here.
- Now, you're cooking! If the air is still, a glider must be constantly falling, in order to create enough of a pressure difference between the air above and the air below, to support its weight. Gaining height by flapping your wings is exhausting, so eagles etc (and human gliders) have to find thermals, which are columns of air rising upwards from warmer areas on the ground. Eagles, etc, fly under cumulus clouds, as do glider pilots, becasue that's usually where the rising air is to be found.and
what if the bird is an eagle and it glides but in this scenario I suspect because it must fall as it glides the air pressure is denser below so the third law takes over again but to what extent would it account for the gliding birds weight weight
Attaboy, Einstein! But Newton's still our main man.
