Things I want to know

[TimberWolf]

So....

If the back doors were open on a large artic, on a very sensitive weigh bridge and I threw a radio controlled glider in - would it register as increased weight before it hit the 'cab' partition at the back?


Bing!

Yes, because the glider pushes on air which pushes on the ground, which is what keeps the glider up. Unless you're playing word games with 'open', such that some of the weight leaks out.

[DimPrawn]

An interesting question. Prawn's answer of earth and neutral being the same isn't acutally that daft, although strictly speaking it is wrong.

The substation will have tap offs from the transformer, each one being a phase. As a home owner you only consume one phase, so your neutral is acutally bolted down to the return path of this phase.

Where Prawn is verging on correct is when the supply head is of TN-CS type (terra/netural combined) where the earth and neutral are both earth at the supply.

If you've ever got a pisser from a florrie circuit where some little darling has borrowed a neutral you will understand that neutral should be considered logically seperate from the earth (actually now called CPC - circuit protection conductor).

It is quite feasible to provide your own earthing conductors and use this instead of the company's one, and in section 6 of the regs (Building sites, caravans and marinas IIRC) you have to. The maximum disconnection time is also halved from 0.4 to 0.2 seconds I think.

So in short (pun intended) it is no less efficient to provide your own earthing arrangements, but neutral and earth are not the same.

Do you get invited to many parties?

[Spacecadet]

Yes, because the glider pushes on air which pushes on the ground, which is what keeps the glider up. Unless you're playing word games with 'open', such that some of the weight leaks out.

Assuming it is gliding using Bernoulli's principle the air pushes the glider

[Sands of Time]

Assuming it is gliding using Bernoulli's principle the air pushes the glider

What happens if the glider flies into a open topped truck - below the sides?

Is there a difference if it was an inch above the surface of the truck?

Stephen Fry said something about 'being part of the same system'?



Bing!

[DaveB]

There was one thing on QI the other week (Dave / Gold or whatever)

Birds in a lorry - weigh X, birds takeoff and fly in the lorry same weight.

The lorry only weighs less when they fully exit out of the lorry - why?


Bing!

This only works when the conatainer with the birds in it is a sealed system. If it is then the force exerted by the birds flapping will be transmitted through the air to the truck, meaning no loss of weight. If it is not a sealed system then the force of the air will be lost to the outside and there will be a reduction in weight.

As an aside, in a sealed system the birds would suffocate so wont be able to fly anyway.

So....

If the back doors were open on a large artic, on a very sensitive weigh bridge and I threw a radio controlled glider in - would it register as increased weight before it hit the 'cab' partition at the back?
Bing!

No, as it is not a sealed system there is no distinction between the air inside or outside of the lorry so no change in weight would occur.

[TimberWolf]

Assuming it is gliding using Bernoulli's principle the air pushes the glider

Newtons law is best. If the glider is flying horizontally, the air will be pushing it upward with a force equal to its weight (mg). If the glider is on a downward path, the upward force (and weight felt by the truck) is slightly less. Naturally the gliders entire weight won't immediately be felt at the truck floor because of propagation delays and various air currents going up and down and swirling and whatever, so the weight of the truck will sometimes by higher and sometimes lower if using sensitive equipment, but averaged over time the weight of the truck floor will be the weight of the air plus the weight of the glider from time the plane entered the truck, before and after it hit the opposite wall.

[Spacecadet]

Newtons law is best. If the glider is flying horizontally, the air will be pushing it upward with a force equal to its weight (mg). If the glider is on a downward path, the upward force (and weight felt by the truck) is slightly less. Naturally the gliders entire weight won't immediately be felt at the truck floor because of propagation delays and various air currents going up and down and swirling and whatever, so the weight of the truck will sometimes by higher and sometimes lower if using sensitive equipment, but averaged over time the weight of the truck floor will be the weight of the air plus the weight of the glider from time the plane entered the truck, before and after it hit the opposite wall.

The glider is held up by the inherent air pressure - there would be no extra downward force

To put it another way - the air is always pushing in all directions mainly against itself but also against the floor, walls and roof of the truck. The glider separated the air and creates a localised upper layer which is pushing less in the downward direction. The lower band of air does not have to exert any extra force.

[DaveB]

Assuming it is gliding using Bernoulli's principle the air pushes the glider

Benoulli's principle only partially explains lift. The majority of lift arise as a result of Newton's Third Law (For every action there is an equal and opposite reaction).

The result of the wing moving through the air is that the airflow is deflected downwards at the trailing edge of the wing. In order to deflect the air force must be exerted on it, this results in an equal upwards force on the wing. As velocity increases a greater mass of air is deflected resulting in a greater upwards force which produces more "lift".

[TimberWolf]

The glider is held up by the inherent air pressure - there would be no extra downward force

To put it another way - the air is always pushing in all directions mainly against itself but also against the floor, walls and roof of the truck. The glider separated the air and creates a localised upper layer which is pushing less in the downward direction. The lower band of air does not have to exert any extra force.

Yes, there air pressure is lower above the wing than below, but it's easier to think in terms of Newton. It's just lots of collisions with air molecules. As the glider moves along it pushes air molecules down and the change in momentum of those molecules results in an equal and opposite force that pushes the glider up.

You can see this perhaps more clearly with a helicopter, which is basically lots of wings going around in a circle. I think you can see here that the force (equal to the helicopters weight) would be felt on the floor.

[thunderlizard]

All this is as chicken feed compared to the real Ultimate Question:

How do they print the white bits in the Yellow Pages?