Question

Hey my student asked me an interesteing question about weight.. What happens to the weight of the bird as it keeps flapping its wings..

Well said.. "hmmmm very interesting.. ok.. so when the bird flaps its wings DOWN, the bird accelerates upwards .. you can actually see that... so the birds weight must increase, and then when the wings do a return stroke, the bird is momentarily in free fall , so its weight becomes zero.. until it flaps its wings again and its weight shoots up. .. but this is from the top of my head.. i really want to think about this more and i ll get back to you"..

Answer 1

whatya guys think?

Answer 2

I don't think the weight of the bird ever changes, since that's a specific force and description. However the forces acting on the bird are definitely changing. The net force goes between being up and down at the same time as it flaps.

You could describe this downwards force with Pressure = Force / Area so that means when the bird flaps down it is applying enough pressure over the area of its wingspan to cause a force greater than that of gravity temporarily. You could then say how often it would need to apply a certain pressure in order to maintain a stable course.

Answer 3

When you are talking about weight, you have to specify according to which frame of reference.
In the Earth's reference frame, the weight of the bird is actually the gravitation force acting on him. So it never changes.

Now, for the bird as a living system (whose bones you can take as a reference frame), feeling in its muscles the reaction to its accelerations and gravitation combined, then you can say in first approximation that it will feel heavier on the down-stroke and lighter (if not weightless) on the up-stroke.

This is roughly what you feel when you jump up and down a trampoline.
Since the bird still glides on the up-stroke, it is not completely in free fall, and therefore does not feel weightless, only lighter. When a bird dives, though, its motion is like that of a fighter plane and it can certainly feel weightless.

Answer 4

@Vincent-Lyon.Fr

yes when i say weight, i mean from the reference point of the bones or muscles.. the one we usually use in mechanics

during the up stroke.. i assumed, that the bird almost puts zero force on the air, and thus air puts zero force on the bird, and the bird is in free fall

but like you are saying, if the bird actually puts an upward force in the air, then the air puts a downward force on the bird, and the bird would get a negative upward weight right?

(isn't it just like, if you take a glass of water, intvert it and thrust it downwards with an accelration higher than g, the water will get a negative weight?)

Answer 5

Yes, this is correct!

Answer 6

But I doubt the bird is ever weightless in normal flight.

Answer 7

ohww.. its weight shifts from downward to upward.. :D? . wow amazing

Answer 8

Actually, it is not so surprising or uncommon.
If you stand on your hands upside down, then your weight, as we defined it here, will point to your head and not to your feet.
I am sure there must be at least one silly picture of you doing this on your fb page.
If not, you should take one and post is for the sake of science ;-)