Weight & Mass

UnclePsychosis · May 2016

*sigh*

For fuck's sake.

Scales work by measuring force, yes. They then---behind the scenes and hidden to the user---convert that force into a mass, by using whatever calibration factor that has been built into them. Put a 100kg object on a calibrated scale on the earth's surface and you get 100kg. Put a 100kg object on a calibrated scale on the moon's surface and you get 100kg.

The point is that you cannot assume that you know what the calibration is. Physics is universal: you won't always be working somewhere where gravity is 9.8 m/s/s. Physics students are taught not to make unnecessary assumptions like this, which is why this question is very simple to anyone who has understood their basic physics courses.

A scale gives you a mass. You might be able to use that mass to calculate a weight, but you might not.

A forcemeter gives you a force, which in this case is directly equivalent to the weight. No calculations or assumptions required.

Honestly, this really is a very simple question. There's one answer which is very obviously right and one answer which is, at absolute best, only half right. Stop overcomplicating it.

hungrymark · May 2016

But they don't GIVE your weight, which is why that answer would be wrong.

Chalky · May 2016

I really don't think it is the others who are overcomplicating it. I just went to buy a forcemeter and it is a spring aligned against a straight scale marked in both Newtons AND Grams.

Seems a bit Zen to say if you read off the Newtons side then it is a different object to reading off the Grams side!

equalsql · May 2016

stickyfiddle said:

Jack wanted to know the weight of an object. What instrument should he use?

My 79 Les Paul Pro... cos when that baby's sitting on it's strap you sure know the meaning of weight.

doogz84 · May 2016

crunchman said:

doogz84 said:

Col_Decker said:

What are those things in the bathroom for measuring your weight called again? Oh yes Bathroom Force-meters.

They're called scales, but they don't measure your weight.

They measure your mass.

Which is not the same thing.

They don't measure your mass though. If you used the same scales on the moon they would only give a value that is a sixth of your actual mass. If they measured mass (which doesn't change if you are on the moon) then they would give the same answer there. They measure weight but they give you the answer in units of mass. It works at sea level (or close to sea level) on Earth because g is a known quantity but they work by measuring the force you exert on them - your weight.

Yeah ok, they measure your weight, carry out a simple calc, and display your mass.

The point is, no-one weighs 100kg.

Sambostar · May 2016

Anyway on the subject of weight, apparently BMI is now a load of bollocks as they reckon now that slightly fatter people. over their BMI live longer, so if you are overweight at least you'll have more time to argue about it. Although personally I think it's because people who are bang in the middle of their BMI either physically work too hard because they are poor, worry too much or are just plain crazy.

The simple truth is that a well calibrated cheap Chinese scale costs a few quid from Amazon and is delivered by Herpes within a few days for nothing, acceleration due to gravity is a known constant. On the other hand, a scientific, certified and calibrated Force O Meter probably costs a small fortune, has to be sourced from a specialist company with delivery charged on top and takes more time to deliver. Add in your time researching where to buy one, rather than some cheap scale off of Ebay or Amazon and the costs soon stack up.

See that is the problem these days, no one thinks outside the box. I mean how many decimal places do they need and how much is a decimal place really worth? Bloody pedantic teaches with their free stationary, free coffee and 4 month holidays and pensions and benefits. That is the fundamental problem here, they have no idea how the real world works and these are teaching the fruits of our future? No wonder we have such a overburdened public sector and are neck deep in debt.

I suppose the full answer, to elaborate on that, would also require a full cost benefits and constraints analysis of the longer term economic implications of buying said cheap Chinese scales delivered by Herpes, rather than a well calibrated Force O metre made in the UK. To which the answers probably would be that nobody really knows, just like nobody knows what's outside of space, or even outside of this planet we are all trapped on. You see science is basically just another language as is all about relativity and ultimately it's core extremes and the crux of it is based on faith and philosophy because nobody really knows.

Ner, I'd cut my losses and buy the Chinese scales or at least a scale and calibrated weights, at least then teche might be happier, probably still be moaning about their wage packet, long hours, political interference and pensions benefits though whilst indoctrinating the kiddies, light years after I've figured out the weight and moved on to Borneo.

Myranda · May 2016

Sambostar said:

Anyway on the subject of weight, apparently BMI is now a load of bollocks as they reckon now that slightly fatter people. over their BMI live longer, so if you are overweight at least you'll have more time to argue about it. Although personally I think it's because people who are bang in the middle of their BMI either physically work too hard because they are poor, worry too much or are just plain crazy.
The simple truth is that a well calibrated cheap Chinese scale costs a few quid from Amazon and is delivered by Herpes within a few days for nothing, acceleration due to gravity is a known constant. On the other hand, a scientific, certified and calibrated Force O Meter probably costs a small fortune, has to be sourced from a specialist company with delivery charged on top and takes more time to deliver. Add in your time researching where to buy one, rather than some cheap scale off of Ebay or Amazon and the costs soon stack up.
See that is the problem these days, no one thinks outside the box. I mean how many decimal places do they need and how much is a decimal place really worth? Bloody pedantic teaches with their free stationary, free coffee and 4 month holidays and pensions and benefits. That is the fundamental problem here, they have no idea how the real world works and these are teaching the fruits of our future? No wonder we have such a overburdened public sector and are neck deep in debt.
I suppose the full answer, to elaborate on that, would also require a full cost benefits and constraints analysis of the longer term economic implications of buying said cheap Chinese scales delivered by Herpes, rather than a well calibrated Force O metre made in the UK. To which the answers probably would be that nobody really knows, just like nobody knows what's outside of space, or even outside of this planet we are all trapped on. You see science is basically just another language as is all about relativity and ultimately it's core extremes and the crux of it is based on faith and philosophy because nobody really knows.
Ner, I'd cut my losses and buy the Chinese scales or at least a scale and calibrated weights, at least then teche might be happier, probably still be moaning about their wage packet, long hours, political interference and pensions benefits though whilst indoctrinating the kiddies, light years after I've figured out the weight and moved on to Borneo.

BMI has always been bollocks ... someone like Jonah Lomu comes out as ultra-mega obese. It's a rough guide for sedentary people...

UnclePsychosis said:

*sigh*
For fuck's sake.
Scales work by measuring force, yes. They then---behind the scenes and hidden to the user---convert that force into a mass, by using whatever calibration factor that has been built into them. Put a 100kg object on a calibrated scale on the earth's surface and you get 100kg. Put a 100kg object on a calibrated scale on the moon's surface and you get 100kg.
The point is that you cannot assume that you know what the calibration is. Physics is universal: you won't always be working somewhere where gravity is 9.8 m/s/s. Physics students are taught not to make unnecessary assumptions like this, which is why this question is very simple to anyone who has understood their basic physics courses.
A scale gives you a mass. You might be able to use that mass to calculate a weight, but you might not.
A forcemeter gives you a force, which in this case is directly equivalent to the weight. No calculations or assumptions required.
Honestly, this really is a very simple question. There's one answer which is very obviously right and one answer which is, at absolute best, only half right. Stop overcomplicating it.

You're very irritable... I was only saying that the question is not as clear cut as it would like to be. Get over yourself.

UnclePsychosis · May 2016

It is absolutely clearcut to anyone with even a basic understanding of physics. That's why I'm getting irate.

This is a question aimed at 12 year olds with a very obvious right answer. Maybe the people who need to "get over themselves" are the ones who can't handle getting the answer wrong?

Chalky · May 2016

UnclePsychosis;1078157" said:
It is absolutely clearcut to anyone with even a basic understanding of physics. That's why I'm getting irate.

This is a question aimed at 12 year olds with a very obvious right answer. Maybe the people who need to "get over themselves" are the ones who can't handle getting the answer wrong?

The point is that it is aimed at 12 year-olds! They are very able to google forcemeter, look at the first Amazon entry and see that is a spring aligned against markings of both Grams and Newtons. Its a forcemeter and a spring scale! Yet the question wants them to differentiate between the two descriptions of the SAME object!

Its a pisspoor question.

Teacher "You got the answer wrong Johnny"
Johnny says "The box says its a Scale and a Forcemeter miss. So I picked Scale".
Teacher "No you're wrong".
"But it says its a Scale on the box miss!"
"@UnclePsychosis says it is a Forcemeter so that is the right answer".
"How does he know better than the people who make the thing miss?"
"Because he just does. Now don't make him angry because I've mislaid his dried frog pills..."

)

Maynehead · May 2016

The question is not that tricky really...

It is designed to test that the student knows the SI unit of weight, as well as the units measured by the four common instruments listed. If the student knows all of the above then their task simply becomes one of matching the unit to the instrument.

A student answering the test should be expected to know that the unit of weight is newtons, and that the units measured by a force meter is also newtons. Therefore the most appropriate tool for the job is the force meter.

UnclePsychosis · May 2016

Ahh, yes, clearly, the right answer is wrong and the wrong answer is right not because of the fundamentals of physics but because of amazon.

Yes, how silly of me.

ESchap · May 2016

This was never going to end well on this forum!

)

hungrymark · May 2016

What people (apart from @UnclePsychosis it seems) aren't grasping is that the question is being asked in the context of the curriculum within which that knowledge is being taught. It is simply asking if they know that weight is a force, using the example of a scientific instrument. Nothing more.

@chalky, you may well be able to buy a forcemeter with grams next to newtons but the grams measurement will only be accurate under the same gravitational field strength as the instrument was calibrated, which is presumably sea-level. At any other field strength, grams will not be accurate. In any case, the question isn't asking whether you can measure mass with a forcemeter, so even if you were right your point would be something of a strawman.

The question is fine. The answer is forcemeter.

UnclePsychosis · May 2016

The dual use thing is irrelevant anyway. When you read the gram scale you're using it as a scale. When you read the N scale its a forcemeter.

stickyfiddle · May 2016

I wished I hadn't asked now...!!

I completely agree that Forcemeter is unquestionably correct. I think my original point is that it's a crappy question.

Anyway, my weekend starts now so I have drinks to get down me...

Chalky · May 2016

Quick! This device is going to explode if I don't measure the weight exerted on this pressure pad! Hand me that spring balance!

Uh-oh, can't do that, you need a forcemeter.

But we've got 10 seconds!

Well we're going to die then because using a scale would be scientifically wrong. See, we're in Aberdeenshire on top of all this granite so...

BOOM!

(Standing at the pearly gates)

Why oh why didn't you just hand me the spring balance?

Because someone on a guitar forum said it would be the wrong answer!

And it was this important?

Well it seemed like it at the time!

There, there, I hear St Peter has dried frog pills for everyone...

UnclePsychosis · May 2016

Here's a tip, @chalky - - - If you ever do any exams you'll find that getting the answer "technically" right is fairly essential. Particularly so when the exam is in multiple choice format.

Now, if you don't mind I'm off to join @stickyfiddle for a drink.

Chalky · May 2016

If I was there I'd buy you one

UnclePsychosis · May 2016

Just so long as you didn't ask the barman what he was using as a measure :-P

hungrymark · May 2016

3 pages on a question set for 11 year olds only to conclude that the 'right' answer was right all along. Good grief.

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