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Weight and Mass, the common mistake


mooeypoo
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Hi guys,

 

Something's always bothered me, ever since I finished Physics major in highschool.

 

According to my teacher - and the rest of the scientific community - Kilograms (and pounds) are units of MASS. To describe the units of weight in physics, we use Neuton. Therefore, a weight of something can be 10 neutons, while its mass can be 3 kilograms.

 

Why is it then that the misconception of "Kilos means Weight" is so widespread in the world? SO widespread in fact, that no one even knows it is a mistake to call Kilograms and Pounds units "Weight"?

 

Why is it when my mom goes to the grocery shop and buys a Kilo of tomatos, the salesman WEIGHS them. It makes no sense to me... why would such a mistake be implemented in people's minds in the beginning? Why couldn't everyone just say it right?

 

When I asked my friend, for instance, if she would have the same scale of Kilograms on the moon than she does on earth, she told me no - that she would have a sixth of the earthly ammount. It took me almost half an hour to convince her that she would in fact have the same "Kilograms" (hence, mass wouldn't change), but only her weight will - hence, Neutons.

 

It's so weird. There's absolutely no reason why everyone would get this so wrong... and yet they do..

 

Be glad to hear your thoughts on the matter :)

 

~moo

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the language used by people in their day to day lives reflects their own experiences, which is life on earth only. Where weight is always proportional to mass.

 

If societies started living on the moon as well as on earth then language would start developing different words for them.

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But there are more things that are similar - like Liter and Kg in water.. with that kind of logic (I would assume) that people would be saying "A Kilo of Water" since its easier than saying "A Liter of Water"...

 

Sounds like a good enough notion though, thanks Neil.

 

It tends to get frustrating though, to try to convince people of something that is utterly their mistake! People don't believe me! :)

 

~moo

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But for anything other than water, a liter is not a kilogram. So it wouldn't make any sense for people to measure volume by mass for one thing and volume for everthing else. But in common experience, weight and mass are proportional in all cases and therefore interchangable.

 

Oh, and it's NeWton, by the way. Show the man some respect!

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And the irony is that you have it wrong as well... pounds is a measure of weight. Pounds are units of force just like newtons. kilos are the only measure of mass, all others (newtons, pounds, stones, etc.) are of the force exerted between an object and whatever planetary body you happen to be on.

 

And kilos in the sense that day to day people say is considered a unit of weight as well, just one that happens to be proportional to weight when on earth, like Neil said.

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There are pounds-weight and pounds-mass, which is one and the same near the surface of the earth and then changed when necessary by adapting the pounds mass to new accelaration due to gravity of whatever body is pulling on it.

 

So, long story short, both kilograms and pounds mean weight in common usage. But mass and weight are entirely different things. Mass, the amount of matter that a body has, doesn't change, weight does depending on how gravity influences that body.

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You can even take post #3 farther.

 

"How far away from downtown are you?" "Oh' date=' about 45 mins."

 

Time is clearly not a measure of distance, yet everyone understands exactly what is meant.[/quote']

 

That's a great point. I was thinking yesterday how the cool thing about not being SI is that it's easy to estimate the time a trip will take by car by looking at the mileage.

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exactly. When I say my grandpas house in the country is an hour away, people can make a good estimate (they assume I'm travelling at 100km [60mi] making it about 100 km away lol)

 

But yah, you're right. this is gone on long enough lol.

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pounds is a measure of weight. Pounds are units of force just like newtons.

 

Reaaaaally??? I had no idea! Wait.. uhm.. I was sure since weight is the force that gravity has on an object, it is being presented only with Newton units.. interresting.

 

Well.. to my defense (I feel very defensive today.. ;) I only started with the pounds/ounces a few months ago when I moved to the USA. I always used Kg... the entire pounds system seems rediculously built to my taste. Same goes with Feet and Inches, but that's just me :P

 

~moo

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There are pounds-weight and pounds-mass' date=' which is one and the same near the surface of the earth and then changed when necessary by adapting the pounds mass to new accelaration due to gravity of whatever body is pulling on it.

[/quote']

 

The unit of mass in the English system, that mirrors the use of the kg and Newton, is the slug. No kidding. http://en.wikipedia.org/wiki/Slug_(mass) 1 slug*1 ft/sec2 = 1 lb

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The unit of mass in the English system, that mirrors the use of the kg and Newton, is the slug. No kidding. http://en.wikipedia.org/wiki/Slug_(mass) 1 slug*1 ft/sec2[/sup'] = 1 lb

 

That's somewhere with very little gravity. A slug's mass here on Earth is actually about 32.2 pounds, and we all know there the 32.2 comes from. (g)

 

1lb*(g=32.2ft/(s*s)) = 1 slug = 32.2 lbs

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darn, and there was me thinking Snail had a claim to fame!

 

I guess the English lang doesn`t actualy help much, or at least its Colloquial usage, we may say something like "it`s been a Long time"

it`s little subtle bits like that that doesn`t help matters.

 

(Oh Look! two "That"s)

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That's somewhere with very little gravity. A slug's mass here on Earth is actually about 32.2 pounds' date=' and we all know there the 32.2 comes from. (g)

 

1lb*(g=32.2ft/(s*s)) = 1 slug = 32.2 lbs[/quote']

 

g doesn't enter into the definition, just as it doesn't for the definition of the Newton. A force of one pound will accelerate a 1 slug mass at 1 ft/s2. A force of 1 N will accelerate a 1 kg mass at 1 m/s2

 

You have a mass equal to a force, and that's a little ironic, given the thread title. ;)

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Hahahaha. That is ironic, I should have pointed it out. It's near the surface of the Earth so it's all the same.

 

(BTW: I'm not saying I don't absolutely despise this system, and I'm not defending it in any way, shape, or form. I just want to make that abundantly clear. This system is actually on my hitlist.)

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the entire pounds system seems rediculously built to my taste. Same goes with Feet and Inches, but that's just me[/b']

Don't worry, it's not just you. A lot of people in this country hate the measurement system and would prefer metric be used everywhere.

I remember when I visited Alfred University a tour guide who was asked how far it was to something made the estimate in meters instead of yards. I am going to try to do that - just use metric and assume everyone knows what I'm talking about.

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I cant measure myself in metre / centimetres... I know I'm more then 150 cm but thats it lol.

 

I'm good with kg's though, I'm about 67 kgs I think...

 

I always using km's for distance measurement, acres for land size, sq.ft for house size...

 

Thats what its like in canada lol. about half in metric, half in imperial but of course any kind of government thing is in metric.

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