Why is water heavy?

[Lockjaw]

 

Notwithstanding that weight is a force, if you ask somebody how much they weigh, how many do you think will say 'I weigh x kilos' or 'I weigh x stone y lb' ?

 

How would that compare with the number who would say 'I exert a force of x kilos'?

 

I don't know. They're all incorrect, though.

 

 

I seem to remember there were problems in the UK a few years ago concerning a grocer who was selling bananas 'by the pound'. He was told he was obliged to use metric units.

 

Are items still sold 'by weight' in the UK?

 

Is that weight expressed in Newtons?

 

That was an argument about two different units of mass.

 

The issue arises, as do many, because of an incorrect use of language. In this case it's scientific language.

 

When you buy some bananas you, I assume, don't give a rat's arris about the force they're experiencing due to gravity; you want to know how much banana you're getting - the mass.

 

There is no easy method of measuring mass directly so we've developed an alternative method because we know that everything with the same mass, on the same planet, feels the same force due to gravity.

 

We could have adopted a "by volume" method but that would have resulted in a lot of wet fruit and peeing "jokes".

 

Anyhow, since the scales used to "weigh" the bananas have been calibrated by someone who knows that all kilograms (or pounds) experience the same pull, they are able to indicate on the reading, not Newtons but kilograms (or pounds) so, what the scales are effectively doing is finding the weight, doing the f=ma calculation (actually m = f/a but, same thing) and presenting you with the answer. A mass.

 

Since most people have no reason to care about this detail it has become incorrectly accepted that the weight of something is measured in kilograms (or pounds). Obviously, since the world is not falling apart because of this it generally doesn't matter unless you are studying GCSE or any higher level physics or chemistry or, perhaps more importantly, using that science.

[Rupert_Baehr]

I don't know. They're all incorrect, though.

 

 

 

 

That was an argument about two different units of mass...

 

 

When you buy some bananas you, I assume, don't give a rat's arris about the force they're experiencing due to gravity; you want to know how much banana you're getting - the mass.

 

There is no easy method of measuring mass directly so we've developed an alternative method because we know that everything with the same mass, on the same planet, feels the same force due to gravity...

 

 

Since most people have no reason to care about this detail it has become incorrectly accepted that the weight of something is measured in kilograms (or pounds). Obviously, since the world is not falling apart because of this it generally doesn't matter unless you are studying GCSE or any higher level physics or chemistry or, perhaps more inportantly, using that science.

 

"Since most people have no reason to care about this detail it has become incorrectly accepted that the weight of something is measured in kilograms (or pounds)."

 

Indeed.

 

So why were you making such a fuss when I said 'A litre of water weighs less at the poles or at the equator?'

[Lockjaw]

"Since most people have no reason to care about this detail it has become incorrectly accepted that the weight of something is measured in kilograms (or pounds)."

 

Indeed.

 

So why were you making such a fuss when I said 'A litre of water weighs less at the poles or at the equator?'

 

Now then...

 

Firstly, that statement (in bold) is perfectly correct but isn't what you said. You gave values for weight with kg and g as the unit, rendering it incorrect.

 

Secondly, given that this is a thread about a scientific question, one would expect posters offering information to be scientifically correct.

 

Thirdly, I didn't make a fuss about it. That was another poster. I simply clarified her point when you appeared not to understand what she was getting at. If you read my clarification you'll see that I was backing up your science at that point.

 

Fourthly (for completeness), I only took issue with your input when you claimed pressure was irrelevent and subsequently suggested that "water" only meant the liquid state of the substance.

[smithy266]

You can just imagine then, how heavy air is, when you see a hot air balloon, and the total weight it is carrying.

[Tony]

Thank you Mr Strix

 

If water is made up of hydrogen, which is a gas...

and oxygen, which is a gas...

... why doesn't it weigh nothing at all?

 

 

 

What do you mean by "heavy" ?

[Strix]

Actually, the whole thread came into being after watching 'something for the weekend' where their Scouse cook defined the difference between an apple and a pear as pears sink in water, but apples float. they attempted to prove this with the pear they were cooking at the time, and an apple from the display at the back... which was plastic

 

The question was deliberately ambiguous Tony

 

It's nice to see SF can still get it's teeth into a pseudo science thread with gusto

[Highfields]

Why is water heavy?

Because its specific gravity says it is.

[Highfields]

Are items still sold 'by weight' in the UK?

 

Is that weight expressed in Newtons?

Is a 'Newton' the same weight as an 'Oldton'?

[Tony]

Actually, the whole thread came into being after watching 'something for the weekend' where their Scouse cook defined the difference between an apple and a pear as pears sink in water, but apples float. they attempted to prove this with the pear they were cooking at the time, and an apple from the display at the back... which was plastic

 

The question was deliberately ambiguous Tony

 

It's nice to see SF can still get it's teeth into a pseudo science thread with gusto

 

You should try my "what's gravity" thread somewhere else.

[cgksheff]

The "plane taking off from the conveyor belt" thread?