Weighty Problem

[Pangolin]

A lorry is carrying a large cage full of birds which are all standing on the floor of the cage. If the birds decide to fly about in the cage will the weight of the lorry and its load change?

Answer please and explanation if possible, thank you.

[joggerjayne]

No,because the downward force of air exerted by the birds' wings will be equal to the weight of the birds.

Disclaimer: This is a total guess, and may be complete poop.

[Old_Geezer]

No, I do not believe so. The pressure of the air forced down by the flapping equals the pressure the birds previously made with their weight when standing. In a closed system there is no change.

[joggerjayne]

YES !! ... Wooo Hooooooo !!!!!!

[Maydup]

Congratulations JJ, not poop at all. I'll second that.

[joggerjayne]

Dear Ed,

I enclose my application to upgrade my AB status ...

... from "Frivolous Airhead" to "Science Heavyweight"

Love (ooh, sorry, I mean "Yours sincerely")

JJ

[sunny-dave]

Daer JJ

Sorry - you is still a smelly biker

love habdobs

[Old_Geezer]

Ed will meet you half way.

Would you prefer "Frivolous Heavyweight" or "Science Airhead" ?

[factor-fiction]

Is it an enclosed lorry or is the cage on an open trailer?

[joggerjayne]

Aww, poop ... okay, I'll settle for "Science Airhead".

I'm not smelly Dave. I haven't been to London for days.

[DTCwordfan]

I thought that Croydon was technically in Greater London, Jayne....a trip to Costco perhaps, on the wing of a (caged) bird, perhaps...?

[ChuckFickens]

http://www.yourdiscov...-10-birds-in-a-truck/

It's been tested too.

[joggerjayne]

I've been to Marylebone more recently than I've been to Costco.

I'm just worried that F30 will come up with some clever answer, and we will turn out to be wrong.

[Old_Geezer]

Unless the birds can fly off and press down on the air outside of the lorry, then it should make little difference.

[Kayless]

well known thought experiment, the weight does not change.

[RATTER15]

If all the birds went into free fall at the same time would the weight change, because you would still have a down force of air.
I think it would be lighter then.

[vascop]

if an aeroplane is say 10 miles away from an aircraft carrier and it flies towards the aircraft carrier at a height of say 50 feet above the carrier and then flies the full length of the carrier but does not land on the carrier, but keeps going, does that mean that while the plane is travelling above the carrier the carrier's weight increases by an amount equal to the weight of the aeroplane?

[Old_Geezer]

If all the birds were in synch then the weight momentarily varies up and down, but the average weight over time stays the same.

However the interesting question is whether the floor is not solid but say, cage-like. When landed the birds weight transfers in total to the vehicles frame, but in the air much of the downforce goes through to press on the road below. So what about the total weight then ?

Unsure about that one vascop. Planes stay aloft due to the speed of airflow over and under the wing, rather than flapping. My suspicion is that the weight doesn't change for either vehicle.

[factor-fiction]

Yes, the variation I've never been sure about is as follows.

Suppose a parrot weighing 0.2 kg is sitting on its perch in a 0.8kg cage and the cage is on a set of scales showing a total weight of 1.0kg. The parrot then flies off his perch and hovers in the air. Presumably the weight recorded on the scales is still 1.0kg.

Then a small bird weighing 0.1kg flies through the cage's open door and flies out of another open door a few seconds later. Does the recorded weight increase to 1.1kg for those few seconds?

[Old_Geezer]

There is the thing. It surely depends on the floor of the cage ? If, when the bird hovers over the perch, not all pressure from the flapping is hitting the perch/cage floor then some will get dispersed outside of the system.

Same applies to any small bird flapping through. It will add some downwards pressure, but depending on circumstances it's possible not all of its weight. Surely it depends on whether all the air pressure presses down on the perch/cage ?

[vascop]

I was trying to make the same point as factor in my post, but using an example where the "bird" is very heavy. Obviously the downward sound wave from the birds wings can only have an appreciable effect if the bird is only a short distance from the scales. A bird flying at about 50 feet or more over some scales will not have any appreciable effect surely.