[Closed] BS of the week

Most people get there before me 🙁

So - put a balloon full of helium on one side of a balance ( you have tie it to the balance) and a empty balloon on the other side - which is heavier? Which goes up and which side goes down?

TJ - stop confusing weight and buoyancy you ****in eejit 🙂

I was hoping this would run as far as the plane on a conveyor belt 🙂

Ah now, If you take my theoretical scales and weigh a flying aircraft, that has left the conveyoer belt............

Ah

found this dagnamit

Such measurements are impractical, and therefore to correct for the buoyancy of air, the apparent weight of objects weighed by a spring-scale in air must have an additional calculated measure added, using the product of the density of air and the object's volume, as described in Archimedes' principle. However, the true weight of the object in such circumstances is unchanged, just as in the other "unmeasured support" examples.

Damn and blast

If a lorry is carrying a load of pigeons on perches and the pigeons all take off and start flying around inside the lorry, will it weigh less ?

MilitantGraham - Member

If a lorry is carrying a load of pigeons on perches and the pigeons all take off and start flying around inside the lorry, will it weigh less ?

The lorry will weigh the same, of course! the total weight (lorry and cargo, with pigeon being the cargo) will be less, what are you, fick?

cynic-al - can't help thinking you've made a rod for your own back by creating this thread. Expect every post you make to be very attentively read by people just waiting to jump on any inaccuracy!

Not at all oknabunny, I am hoping to get referenced onthis thread, or perhaps the impending "pwned of teh week" or "willy wave of the week".

How come road riders don't use triathlon style steep seat tubes if it's that much of an advantage?

I didn't say how much of an advantage it was, did I?

Comfort is the reason. Otherwise we'd all be riding around like Graham Obree.

Helium.

If I tied helium balloons all over my bike would I be quicker uphill?

Are pigeons made of helium then ?

molgrips please stop!

Re the pigeons - if you put the lorry on scales and the pigeons took off inside it, the scales would read the same, because each pigeon is supported by a downdraught from its wings, which would in turn result in an air current pushing down on the bed of the lorry.

UNLESS the lorry is big enough for the pigeons to glide, in which case each bird would be supported by the buoyancy of a low pressure area above its wings.. in which case the lorry would indeed weigh less.

I think the issue is that we don't have a concrete definition for weight. We were taught that weight was the downward force resulting from gravity. No downward force = no weight.. but that doesn't really matter since you consider all the forces acting on a body when doing a problem.. doesn't really matter what the forces are called. I mean a downward arrow on your picture labelled 'weight' and an upward one labelled 'buoyancy' will cancel out (partially or totally) anyway, so it doesn't matter.

Incidentally, a spaceship in orbit does have weight - it's what keeps it in orbit, and not flying off into space. Its occupants don't FEEL weight in their frame of reference (ie the ship).

[img] [/img]

i'm no physicist but that guy is clearly not going to go faster due to the balloons although i do want one.

if you put the lorry on scales and the pigeons took off inside it, the scales would read the same, because each pigeon is supported by a downdraught from its wings, which would in turn result in an air current pushing down on the bed of the lorry.

What about if the cargo containing space was a cage made of thin wire? Then the effect of the downdraft would be mitigated, right?

Quite possibly, yes 🙂

can't leave it on 99

assuming it's an airtight lorry, it still contains the same amount of air, the same amount of perches, and the same amount of pigeons. They can fly about if they want, makes no difference.

I think the BS of the week thread should run forever !
😉

assuming it's an airtight lorry, it still contains the same amount of air, the same amount of perches, and the same amount of pigeons. They can fly about if they want, makes no difference.

Certainly wouldn't, they'd all be dead.

They can fly about if they want, makes no difference.

It does, for the reasons outlined above. A bit more complicated than you think. But yes there won't be a long-term difference unless the birds can glide. A gliding bird (or plane) is actually a bit buoyant, so it'd be like filling the lorry with helium balloons. The indicated weight on the scales would go down if you did that.

Certainly wouldn't, they'd all be dead.

When does this thread start again? What day of the week do we decide on the BS of the previous week? Will it be Friday and if so, will it be from the previous Friday to the Thursday just before or will it be inclusive of the day on which it is started? In which case, shouldn't it be titled "BS of the week ending dd/mm/yy"? And only be allowed to last for one day?

This thread started three days ago. Has cynic-al lost control of it now? Does he even care anymore?

molgrips - Member

They can fly about if they want, makes no difference.

It does, for the reasons outlined above. A bit more complicated than you think. But yes there won't be a long-term difference unless the birds can glide. A gliding bird (or plane) is actually a bit buoyant, so it'd be like filling the lorry with helium balloons. The indicated weight on the scales would go down if you did that.

I think yer wrong here molgrips - if you had a beaker full of water with a model sub on the floor of the beaker, then the beaker would weigh the sum of the water and the sub. The sub starts to circulate and creates lift on its wings, it rises up. Does the beaker weight the same? Yes. Then the sub stops driving and glides, does the beaker weigh the same? Yes.

The situation you describe is the same as the one I described, toys - don't understand what you are saying. We seem to agree.

Yeah apart from the gliding bit, you said if they glide it makes it lighter. Not true.

I love this thread...

How about if the truck was full of helium and the birds started to fly?

@tandemjeremy

High pitched cooing perhaps?

I wish I could fly, right up to the sky, but I can't...
[img] http://t0.gstatic.com/images?q=tbn:qMI0MTzGN1-WaM: [/img]

Yeah apart from the gliding bit, you said if they glide it makes it lighter. Not true.

If the bird is standing on the bed of the truck, its weight is couteracted by the upward force of the truck bed, which is in turn supported by the wheels and hence the weight of the bird will read on the scales.

However, if it glides, then the weight of the bird is partially supported by buoyancy ie the imbalance of atmospheric pressure on all sides of the bird. So how does that affect the reading on the scales?

If you added a balloon filled with helium and air such that it was neutrally buoyant and floated mid-air - that would also not affect the weight on the scales. (Except it would in that case, it woudl make the weight less since you'd be displacing some heavier air for lighter helium)..

However, if it glides, then the weight of the bird is partially supported by buoyancy ie the imbalance of atmospheric pressure on all sides of the bird. So how does that affect the reading on the scales?

So what's the air sitting on, then?

So you agree with my sub model?

if you had a beaker full of water with a model sub on the floor of the beaker, then the beaker would weigh the sum of the water and the sub. The sub starts to circulate and creates lift on its wings, it rises up. Does the beaker weigh the same? Yes. Then the sub stops driving and glides, does the beaker weigh the same? Yes.

Buoyancy is just another word to describe a lift force, it's still a force acting against gravity. That force (in this sealed system as defined previously) will be transferred through the air to create a downward reaction which will register the mass of the bird on a scale. The birds mass cannot disappear..

Sounds like a pretty cool sub model, if it can glide around in a beaker. 🙂

How about if the truck was full of helium and the birds started to fly?

Tj. I tihnk you are forgetting a certain conveyor belt.

BigDummy - Member

Sounds like a pretty cool sub model, if it can glide around in a beaker.

It's an awesome sub model, with torpedoes, two periscopes and silent running.

That force (in this sealed system as defined previously) will be transferred through the air to create a downward reaction which will register the mass of the bird on a scale

Not necessarily. The forces involved in air buoyancy come from the weight of the atmosphere. Nothing to do with the truck.

There is only a reaction if the bird is flapping its wings (or its wing angle of attack is greater than 0.. which it will be... but that's fine. This is about buoyancy)

Btw I disagree that buoyancy and lift are the same. If a helicopter hovers above you, you feel the pressure of the downdraught on your head. If a balloon hovers above you, you feel nothing.

Not necessarily. The forces involved in air buoyancy come from the weight of the atmosphere. Nothing to do with the truck.

So in a sealed system the pressure in the air is from the atmosphere outside?

There is only a reaction if the bird is flapping its wings (or its wing angle of attack is greater than 0.. which it will be... but that's fine. This is about buoyancy)
Btw I disagree that buoyancy and lift are the same. If a helicopter hovers above you, you feel the pressure of the downdraught on your head. If a balloon hovers above you, you feel nothing.

You can disagree but you are wrong! 😀

[url= http://en.wikipedia.org/wiki/Buoyancy ]Buoyancy[/url]

So in a sealed system the pressure in the air is from the atmosphere outside?

Well in a sealed system the pressure is what it is - what's your point?

I haven't got time to read the wiki article - on which point am I wrong?

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

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.