If you can't be arsed then neither can I. This does sound like an attempt to back pedal though.. (baits trap and retires to safe distance) also someone has tagged you with this "molgrips flounders and blusters yet again." How terribly mean.

Truck weight stays the same. Molgrips you're wrong. Stop now.

No, come on, this is meant to be a discussion of the physics.. I'm just discussing it out loud.

No bluster, no angry denouncements.. 🙁 I just don't see your argument. What part of the wiki article disagrees with me? I meant I haven't got time because I am meant to be workign, and I'm already STWing and lsitening to the TdF on eurosport 🙂

When I said lift and buoyancy were different, I meant aerofoil lift the way planes and birds do it.

F=Ma innit
unless you change M or a you ain't affecting F

BS reigns supreme though …

Greyman.. there are many Fs in this problem coming from all sorts of things. You're not contributing 🙂

I think I might agree with molgrips (at least to some extent) – now I've not followed the whole argument, but are we considering here a real lorry filled with real pigeons? ie it's not a sealed system (as pigeon fanciers tend to like their birds to survive trips in lorries). The thing is, as it's not a sealed system, some of the downwards force on the air will inevitably be dissipated through the vents. The thing is, the lorry would be just as functional with a mesh floor, in which case it's surely obvious that the pigeon taking off will decrease the weight of the lorry+contents?

there are many Fs in this problem

no there aren't

mass (M) is constant, gravity (a) is constant, so therefore is the "weight" (F)
😉

well, if you're going to bring in mesh lorries etc, then you are changing the mass in the system – stop changing the rules !

mass (M) is constant, gravity (a) is constant, so therefore is the "weight" (F)

Weight of what? Birds? What about the accleration of each bird as it flaps? What about buoyancy caused by aerofoil lift?

Sounds like you're not understanding the problem fully.

How am I changing the mass in the system by having a mesh floor lorry? We're still considering the same lorry with the birds on perches and then flying around? Or if you like I'll add some lead to the mesh floor lorry so it weighs the same as the original one.

How am I changing the mass in the system by having a mesh floor lorry?

I think he means that the mesh would weigh less than a standard wooden lorry bed. I think he was trying to be funny…

What about if the lorry is on a conveyor belt

*runs away*

How am I changing the mass in the system by having a mesh floor lorry?

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

I said mass, not weight

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

a bit tongue in cheek though I'll admit 😉

just keeping it going really

Weight of what?

mass – of all the little atoms making up birds, lorry, perches et al

What about if the submarine model was in a mesh beaker?

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

Oh, so you were assuming a theoretical completely closed system – which solves the problem quite neatly as the pigeons can't fly around when they've suffocated.

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 think 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.. Not a bad try but I like to use questions to get you to understand.

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.