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Discussion Starter #1
This has nothing to do with ships, only a musing that I can't quite get to grips with. I take a car wheel and inflate the tyre to, say 20 psi, then I put it on the car and lower the jack. Does the tyre pressure stay the same due to tyre deformation or does it increase due to loading? If it does increase, would it still increase if I inflated the tyre hard? i.e. no "footprint" on the ground.
 

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My thoughts are that the air is inside the tyre stays contant whereas the pressure/weight of the car is distribited on all four tyres out side.
John
 

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Yes, the pressure does increase when you deform the tyre, but you would not be able to measure it with normal equipment. Also it depends on the tyre construction, a cross ply tyre would not deform as much as a radial ply tyre. The number of plies, type of rubber, tread pattern etc would aslo have an effect. The effect of higher pressure would be to alter the differential, that is the weight of the vehicle is the same but is acting upon a harder object, the pressure would still change but the difference would be even less measurable.
If however the weight of the car changes and you intend to do a long run you really ought to adjust the pressures to suit. most car makers nowadays give you a chart showing the appropriate tyre pressures for vehicle loading and tyre type.

The thing that you can measure is the effect of temperature. Pump up your tyres on a sunny day when one side is in shade, turn the car around and leave it for an hour or so, check the pressures again and you should be able to see the effect.
The temperature effect comes into play after a journey, the tyres will get hot and the pressure will increase, so always check them cold if possible.

Oh, and by the way I should perhaps mention that I once worked for Dunlop....


MIG (Thumb)
 

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This is O level physics stuff - or is it?

Boyle worked out that p*v*t is a constant where p=pressure, v=volume and t=temperature. So given that the same amount of air remains in the tyre before and after it is fitted, the pressure is dependent on the volume of air in the tyre. To know whether this will change we need to know if the volume of the tyre changes when it is put under load. If the volume decreases as a result of the weight of the car being put onto the tyre the pressure would increase.

Intuitively I would expect that this would happen but I think an experiment is called for. All you have to do is to check the pressure of a car tyre with it on the road, then jack the wheel up and see if it changes.

Regards,

Brian
 

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Ah yes Benjidog but that little puff of air that escapes as you put the gauge on the tyre might give your experiment a wrong reading
 

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Sorry to be a BOF but:
Regarding Boyles Law I thought that he only mentioned Pressure & Volume, which was the original question.
Wasn't it Jaques Charles who brought question of Temperature into things. hence Charles law, don't ask me to quote it - too long ago..
And the term an 'Ideal Gas' springs to mind which I don't think includes a rather polluted mix of nitrogen and oxygen.

MIG.
 

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Oh bugger - old age is catching up with me!

Boyle's law is the p*v=a constant and does not mention temperature so is the one relevant here.

Charles law is about p1*t1=p2*t2 so about the relationship between pressure and temperature.

The combined one is I think actually called something like the "ideal gas equation".

Thank the Lord I don't have to worry about things like that any more!

Brian
 

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Peter4447 said:
That reminds me........

Did you hear the one about the queer wizard - he went out with a puff!

Peter4447 (Night)
Must have been a happy old fart then (LOL) (LOL)
 

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Here is one in a more nautical trend.

A man is on a pond rowing a boat full of scrap iron.
He gets to the middle of the pond and dumps the scrap iron.
What happens to the level of the pond?????
 

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Lakercapt

Interesting, verrrrry interesting, Colonel Klink!

Theoretically nothing but I'm sure I've missed something!

The level of the pond would have risen as he loaded his craft and since he's dumped it in the pond (naughty boy) the level will not go down when he leaves apart from the amount he weighs translated as displacement/weight of water.

So what happens to the level of the pond when all this scrap iron is reduced to sludge and has given off noxious gasses. (EEK)
 

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Discussion Starter #14
Good old Archimedes! The level in the pond should stay the same as the extra displacement in the boat due to the load has been transferred from the said boat to the water of the pond, I think!
 

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I've just had a thought about that one.
The displacement is for a floating object - ship etc, but the iron is an immersed object and the rules are, I think, different because the iron is denser than water.
I seem to recall that we used to hang the immersed object from a spring balance in & out of the water.
So therefore the iron when it is in the boat is effectively floating and displaces its weight. But, when it goes over the side it displaces less water so the water level would go down.

Hope that makes sense,

MIG.
 

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I suppose the iron displaces it's weight of water but when dumped in the pond displaces it's volume of water.
Unless the two happen to be the same (in which case the first answer is correct) or we know the two figures we can't judge.
A wild stab would be that iron is heavier than water so the level of the pond would go down because the volume of the displaced water is greater than the volume of the iron.
It must bear some relation to the formula
B4I (root U) RU
16

Or would it be the other way round..... I've got a headache. can I pleased be excused sir. I want to go hide in ttthhhe wwwaaardrobe (Ouch)
 

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Right on Marconi man.
When the iron is in the boat it displaces its own weight of water.
When in the water it only displaces its own volume.
The latter is less.
Consequently the level in the pond will go DOWN.
 

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Does that mean if you fill the pond with scrap iron, the water disappears completely ? If so, where does the water go ?

fred

" any old iron, any old iron, any any any old iron ?
 

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An interesting thought that Fred!

There was a community effort to put this theory to the test using the Grand Union Canal near where I lived in North-West London as a kid. Anyone with access to a vehicle would dump their old crap down a county lane; the rest chucked it into the cut. The water didn't disappear though - it just got shallower and shallower and the barges used to scrape things as they went past.

Eventually it got so blocked up the Waterways people had to drain it and clear the bottom out. I remember watching them - in one short stretch they pulled out three safes that had been broken open and the remains of enough bikes to supply the Tour de France. There were also numerous suitcases, mangles and the like plus if I recall clearly some unexploded bombs dropped by the Germans. I have never really understood how some of those things actually got into the middle of the canal (apart from the bombs) as they were really heavy. Maybe the dumpers walked across the previous layers of crap to get to the middle?

Brian
 
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