Why is water heavy?

[Lockjaw]

Really? - It's a long time since I did physics, but I haven't quite managed to forget everything (it's the things I did 5 minutes ago I can't remember.)

 

I said Weight = Mass x Gravity. Given that you disagree, I thought I'd not rely on my memory and so I googled it.

 

Wikipedia (I know they don't get everything right) say: In one of the more common definitions, the weight of an object, often denoted by W, is defined as being equal to the force exerted on it by gravity. This force is the product of the mass m of the object and the local gravitational acceleration g.[1] Expressed in a formula: W = mg. further down, the article says: The ISO definition

 

The ISO standard ISO 31-3 (1992) defines weight as follows:

The weight of a body in a specified reference system is that force which, when applied to the body, would give it an acceleration equal to the local acceleration of free fall in that reference system.[9]

 

This definition allows use of the formula "W = m g"

 

Wikipedia say Weight = Mass x Gravity.

 

What's the difference between Weight = Mass x Gravity (what I said) and Weight = Mass x Gravity (what Wikipedia said)?

 

According to you, that is 'cleary not correct'

 

Would you like to explain why?

 

Hi Rupert.

 

I think she was referring to the fact you stated:

 

 

A litre of water would weigh less than 1Kg at the North Pole,

 

[and]

 

(At the top of mount Everest, a litre of fresh water would weigh about 999.972g)

 

Both g and kg (small k for kilo, by the way) are units of mass, not weight. Weight is a force and is, therefore, measured in newtons, N (Capital N because it's named after a dead bloke).

 

An object can no more weigh 1 kg than it can travel a distance of 8 minutes or have an age of green.

 

Having said all that, the use of an incorrect term did not detract from the meaning, or the general scientific correctness of your post which renders the general mass / weight confusion even more annoying to one such as myself who is required to un-confuse it on a regular basis.

[Bloomdido]

See post 19 for details.

 

Why 4 degrees? Why does water become less dense at this temperature? Is this proof of a god? Intelligent design?

[HeadingNorth]

Given that one is a force; ie, in this case, a "pull" and the other is an amount of matter I'm afraid you couldn't be more wrong.

 

Actually I'm right. Most people *DO* consider the two terms to be interchangeable, and as long as you stick to the surface of the Earth, where gravity remains just about at one Earth gravity - there's no harm in doing so.

[Guest sibon]

Why 4 degrees? Why does water become less dense at this temperature? Is this proof of a god? Intelligent design?

 

I'm a bit sketchy on this, so if anyone knows better, feel free to add to my knowledge:)

 

I think 4 degrees is the temperature where the molecules start to gain the correct orientation to stick together. They are V shaped which leaves inevitable spaces between them. At 4 degrees, they have slowed down enough for those spaces to begin to matter, so the density decreases.

[Rupert_Baehr]

Hi Rupert.

 

I think she was referring to the fact you stated:

Quote:Originally Posted by Rupert_Baehr

 

A litre of water would weigh less than 1Kg at the North Pole,

 

[and]

 

(At the top of mount Everest, a litre of fresh water would weigh about 999.972g)

 

 

Both g and kg (small k for kilo, by the way) are units of mass, not weight. Weight is a force and is, therefore, measured in newtons, N (Capital N because it's named after a dead bloke).

 

An object can no more weigh 1 kg than it can travel a distance of 8 minutes or have an age of green.

 

Having said all that, the use of an incorrect term did not detract from the meaning, or the general scientific correctness of your post which renders the general mass / weight confusion even more annoying to one such as myself who is required to un-confuse it on a regular basis.

 

Time to go back to 'Slugs' (Standard Pound Under Gravity.) Newtons (and I've seen that unit written as 'New Tons') cause far too much confusion.

 

I'm well aware that weight is a force ... One of the things I hadn't forgotten is that P=Mf (My Applied teacher's version) The formula I gave (Weight = Mass x gravity is quite correct. Weight is a force, Mass is Mass and gravity is acceleration due to gravitational pull.

 

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 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?

[Guest sibon]

 

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?

 

You cant introduce grocer's to this argument. They cant even use apostrophe's, let alone correct scientific unit's.

[katy1981]

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?

 

 

brill question!

 

oxygen does weigh something

see atomic mass

http://chemistry.about.com/od/elementfacts/a/oxygen.htm

 

and so does hydrogen

see atomic mass

http://www.lenntech.com/periodic/elements/h.htm

 

its impossible for it to be weightless as both gases have a mass.

[Rupert_Baehr]

Well, I'm going to carry on weighing my strawberries in Slugs. Newton can pick his own.

 

- Until I can find a way to get rid of the bloody slugs! - The plants are crawling with them - and I grow strawberries in tubs on the patio - they're not yer 'common or garden' slugs, these are 'slugs -on-tour'.

[*_ash_*]

brill question!

 

oxygen does weigh something

see atomic mass

http://chemistry.about.com/od/elementfacts/a/oxygen.htm

 

and so does hydrogen

see atomic mass

http://www.lenntech.com/periodic/elements/h.htm

 

its impossible for it to be weightless as both gases have a mass.

 

Bold, I agree Katy. Especially for the arguments created in the 3 pages so far.

 

That Stephen Hawking series that's on sky at the mo would be interesting for people who like this thread, but are restraining from posting 'FACTS'. My knowledge in this is about GCSE level standard - maybe slightly more, if I was being cocky...

 

 

I must say, aside from facts, or the topic, I did genuinely laugh out loud at the 'gospel' comment earlier

[Goon]

Weight = mass X gravity.

 

Nearly right but not quite. It should read:

 

Weight = mass X acceleration due to gravity.

 

Mass is a measure of the inertia of a body; generally, how difficult it is to change a body's velocity.

 

Acceleration due to gravity is a measure of how much force per unit of mass a gravitational field can apply to a body to change it's velocity.

 

Weight is a measure of how much force the gravitational field is applying to the body.

 

 

Any body with mass will have the same mass anywhere in the universe. It's weight is dictated by the strength of the gravitational field it is inhabiting at any time. It will always have some weight as it will always be in a gravitational field (gravity has an infinite range).