[Closed] BS of the week

bloody hell, the footy's on later, almost forgot

better get finished up here, and offski - a cold one awaits

😉

I said weight, you said mass.

I think some people don't comprehend the difference - physics 101 lads

I've got a MPhys degree in Physics :p

I've got a MPhys degree in Physics :p

Have you? 😯
🙂

Which is why I am about five steps ahead of you lot. Likewise with the conveyor belt plane thing. So far ahead in fact that you don't understand and think I am talking rubbish.

so if we attach a helium balloon to the pigeon?

for a start air could get out of "the system", displaced by wing flappage etc

Which is why I am about five steps ahead of you lot. Likewise with the conveyor belt plane thing. So far ahead in fact that you don't understand and think I am talking rubbish.

I love molgrips. 🙂

But surely as the birds wings flapping creates lift using the bernoulli effect,to produce an area of low pressure beneath the wing, as opposed to a down force from the wings to provide lift/bouyancy, the overall weight would remain constant,the flying pigeons are stil being supported partly by the column of air beneath them.

That's how it looks to me anyway 🙂

and [i][b]think[/b][/i] I am talking rubbish.

Ahhhh...I see now... 🙂

High 5 BigDummy 😉

jahwomble.. low pressure ABOVE the wing not below. This means that the lift is produced by the high pressure area under the wing trying to equalise with the low pressure above it. Has no interaction with the truck. Well, apart from a very small pressure wave travelling downwards.. but that'd be very tiny in relation to the force required to keep the bird aloft.

Sorry, yeah pressure the wrong way round;)that was my point, if the bird flying has no interaction with the truck itself,assuming the volume of air within the truck stays constant.... how can the truck weigh less?

Right.. so if the pigeons are standing on the floor initially, then the weight reading on the scales is truck + birds. Once the birds take off and are gliding, the reading on the scales is just truck.

ah suddenly became clear, sorry was having a blond day 🙂 ta very much

Synopsis?

Either way I'm with Molgrips.

Woo! 🙂

Right I'm off for a quick ride before the footie.

I nominate molgrips for BS of the week on friday. This is the best bit.

This means that the lift is produced by the high pressure area under the wing trying to equalise with the low pressure above it. Has no interaction with the truck. Well, apart from a very small pressure wave travelling downwards.. but that'd be very tiny in relation to the force required to keep the bird aloft.

You can't just SAY it's bs - you have to tell me why...

Nope - I checked back to the first post, and no mention of having to justify why something is BS.

What I'm really loving about this argument is that your conclusions are right, molgrips, it's just that a lot of your working is very dodgy!

Tell me why tho!

I thought you were off cycling?
I don't have to explain but I'm going to have fun trying.
So I can only explain it by getting you to answer some questions.

1) The lift force that is holding this bird up, what is it reacting against?

People only respond to questions with questions when they are unsure of their position..

The force that keeps the bird up is reacting against the air on all sides.. but it's reacting less against the air just above the wing, hence the upward force on the wing and the bird flies.

molgrips - Member

People only respond to questions with questions when they are unsure of their position..

The force that keeps the bird up is reacting against the air on all sides.. but it's reacting less against the air just above the wing, hence the upward force on the wing and the bird flies.

Ok I should have been more specific - what is the resultant force acting agaisnt. The mass of the bird acts down under the influence of gravity, its being opposed by the upward lift force which is a resultant of the pressure differential. What is this resultant reacting against?

I just told you, did you not understand?

The pressure of the high pressure area under the wing is what pushes the wing up. So the reaction you are asking about is against the air under the wing.

against the air under the wing.

And you agree that this reaction force is equal to the downward force due to the mxg of the bird?

Yes, since the bird stays airborne.

Why don't you just tell me what your model is rather than going through all this patronising school-teacher crap. Unless you're trying to annoy me...? It's clear that I know enough about physics to understand your model if you present it.

I'm not trying to annoy you or be patronising, I just want to be clear.

So what is supporting the reacting air which is directly under the wing?

Any other engineers/physicists on here fancy one of my biccies? A nice drop of red?

I'm still at fechin work.

The air under the truck would be enough to support the air in the truck, but the truck bed is there too... If the truck is sealed then there is a contribution from the air around the truck thereby cancelling out the weight of the air.

I'm not at work, just got back in from an interval session in the gym 🙂

molgrips - Member
People only respond to questions with questions when they are unsure of their position..

Or perhaps they did a liberal arts degree?

What's a liberal arts degree?

The air under the truck would be enough to support the air in the truck, but the truck bed is there too... If the truck is sealed then there is a contribution from the air around the truck thereby cancelling out the weight of the air.

And this air what is supporting it?

Newtons Third Law:

The mutual forces of action and reaction between two bodies are equal, opposite and collinear. This means that whenever a first body exerts a force F on a second body, the second body exerts a force ?F on the first body. F and ?F are equal in magnitude and opposite in direction. This law is sometimes referred to as the action-reaction law, with F called the "action" and ?F the "reaction".

In the example being discussed there are two important factors

1. Are the birds in cages and are they mesh or solid?
2. Is the floor of the truck mesh or solid?

If the downward air from the birds flapping is stopped by the solid floor (and re-directed horizontally), the floor should exert a similar force upwards and it feels the same force downwards. This is the reason why, if there is a floor, the birds hovering do not lighten the truck.
But if the downward air can continue until the road, through mesh cages and a mesh platform truck, then the downward force is exerted on the road and not on the truck.

Either you suppose that the air is completely stopped or that the air goes through the mesh freely.

What if the cargo area of the truck was a vacuum and the truck had just driven off a cliff and was falling vertically through space?

Any unrestrained object within a falling container will surely become 'weightless'?
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Right.. so if the pigeons are standing on the floor initially, then the weight reading on the scales is truck + birds. Once the birds take off and are gliding, the reading on the scales is just truck.

LOL - which polytechnic was that degree from ?

some good craic though - so it will make a diff whether the birds are gliding or flapping ? they don't fly like airplanes fly you know ...

don't forget I'm assuming the same mass of air stays in the lorry at all times, otherwise we are in BS territory ...

btw, the footy was OK, and the beer was cold - looking forward to tonight's !

And this air what is supporting it?

The earth.

LHS - what you say is true and was covered much earlier in the thread. However birds do not always fly by flapping and pushing air downwards to gain a reaction. If a bird is gliding it's being supported by the aerofoil effect i.e. Bernoulli's principle (partly).

Greyman - in this model they do. This is about if, in theory, the birds become partially buoyant, does it have an effect on the weight reading of the scales. Not about how birds actually fly.

My degree is from Cardiff University btw, and we did not cover trucks full of birds, nor did we spent much time talking about Newtonian Physics.

What's the principle discussed recently on STW where people lower down the intelligence scale don't realise how stupid they are? 😉

LOL - touchee monsieur

Dunning Kruger I think - stating the bleedin' obvious really, although of course inherently ironic don't cha think ?

🙂

Have you lot not seen a pigeon carrying truck? I know the question is hypothetical, (maybe assuming that the cages are airtight) in which case the weight is constant, but they are pigeons! so it doesn't really work very well as a model, if you bring common sense into it the pressure created by the flapping wings (read as weight on the scales once birds are a'flappin) which would have to be equal or greater the the weight of the bird only effects an overall weight if all pressure is directed downwards, which of course it isn't, as the pressure outside the lorry (through the cage type thing) is less than the pressure created by the downward pressure of the wings innit. The argument here is hypothetical v's reality. Buy of of these, fill it with pigeons and put it on a weigh bridge and see if i'm wrong. You could do it on a smaller scale with a budgie in a cage and some kitchen scales - anyone game? 😛

[img] [/img]

Shoefiti - posting a real pigeon truck.. now you're just being silly!

Dunning Kruger - that's the chap. And yes, ironic 🙂

If the front wall/bulkhead of the cage were also mesh and the truck were travelling at speed x (where x is is fast enough to sustain gliding but not so fast that the birds couldn't glide), then the birds could glide, thus not creating any downdraft on the floor of the cargo area, not placing any weight directly on the cargo area and not beating their wings.

Then what?

kona, I am suggesting that if they are gliding then there's hardly any downdraught anwyay. That's what I am talking about - the scales only read the truck not the birds.

Then what?

liking it - you do realise in that scenario, if the pigeons spread their wings and held on to the perches - the truck would fly !

no conveyor belt required.

outstanding. 😉