Weight & Mass

UnclePsychosis

Myranda;1076026" said:

UnclePsychosis said:

@myranda that is a laypersons view, not a physics view, and since the question is about physics I'm afraid you're completely wrong.



*Nothing* in the question allows you to assume a value for the local gravitational constant of 9.81m/s/s . All you people answering "use a scale" are making massive assumptions that are completely unnecessary and potentially completely wrong.



The fact that the general public mangle scientific terms has no bearing on the question or answer.










Wait? A scale Isn't a force-meter with a different displayed unit? So it accurately measures mass? Huh.

Only guessing that the student (or imaginary person in the question) is roughly at sea level is the flaw in my logic... so it's a rounding error rather than being totally wrong. Semantics is what I was talking about so semantics is semantic

Except it's not just a rounding error.

Jack could be at the top of a mountain. Or he could be on Mars. Or he could be in an accelerating vehicle. Or anywhere else where the local gravity is not 9.8something.

One of the answers requires you to make assumptions. One does not. One of the answers is completely right. One is not.

UnclePsychosis

Plus, to return to semantics again:

Scales give you a reading in mass. They therefore *measure* mass. You can use mass to *calculate* weight assuming you know the local gravitational constant.

Scales do not, then, *measure* weight.

Force meters measure force. Weight is a force. Forcemeters, therefore, measure weight.

The more I think about this the more I think that the "use a scale" answer is wrong in every possible sense.

hungrymark

Phil_aka_Pip;1075507" said:

where's @hungrymark when we need him?

I'm shit at physics. That said, as a teacher I'd say forcemeter is correct. The kids are obviously being asked about the difference between mass and weight. Yes, a balance/scale can tell you your weight if you know the strength of the gravitational field that you're in, but that's unlikely to be the point of a question aimed at 11 year-olds. Your teacher friends are right about the wording. It's fine in the context of what they'll have been taught in class so I wouldn't confuse matters.

@UnclePsychosis is right, basically.

Myranda

UnclePsychosis said:

Plus, to return to semantics again:

Scales give you a reading in mass. They therefore *measure* mass. You can use mass to *calculate* weight assuming you know the local gravitational constant.

Scales do not, then, *measure* weight.

Force meters measure force. Weight is a force. Forcemeters, therefore, measure weight.

The more I think about this the more I think that the "use a scale" answer is wrong in every possible sense.

They give "weight" in units of "mass" but they're measuring force - ask NASA really nice to test a scale in the ISS and tell you what the mass of an object is... 

That we have a weird hang up confusing mass and weight doesn't stop the scale functioning the way it does... a scale is a force meter which displays the wrong number

hungrymark

stickyfiddle;1076047" said:

Myranda said:



stickyfiddle said:



chrispy108 said:

I'm a secondary school physics teacher. We have a set of bathroom scales - but with a Newton scale on it.



When you think about it a normal bathroom scales is actually this divided by 9.81 rather than the other way round, as a set of scales measures the force you put on it, not your mass.





Exactly. A scale is a forcemeter designed specifically designed to measure the force of gravity between the person standing on it and the Earth.

That we label it in kg instead of N doesn't change what it does or how it works.





(psst I said that, and was told it's a layperson's view of physics and is completely wrong)





I know. It's not wrong. Referring to kilos as measurement of weight is technically wrong, but a scale is always used to measure weight, which is why it wouldn't give the same result on the moon.

Conversely a traditional balance *would* still work on the moon, as a 1kg lead weight would still balance with a brick with mass 1kg, although both would weigh around 160g (yes, yes, 1600 Newtons) according to the bathroom scale.

1.6 Newtons (assuming GFS of 10). Must try harder ;-)

UnclePsychosis

Myranda;1076103" said:

UnclePsychosis said:

Plus, to return to semantics again:



Scales give you a reading in mass. They therefore *measure* mass. You can use mass to *calculate* weight assuming you know the local gravitational constant.



Scales do not, then, *measure* weight.



Force meters measure force. Weight is a force. Forcemeters, therefore, measure weight.



The more I think about this the more I think that the "use a scale" answer is wrong in every possible sense.





They give "weight" in units of "mass" but they're measuring force - ask NASA really nice to test a scale in the ISS and tell you what the mass of an object is... 

That we have a weird hang up confusing mass and weight doesn't stop the scale functioning the way it does... a scale is a force meter which displays the wrong number

If NASA have a scale on the ISS then it will be calibrated properly and will therefore give them the correct mass. 

This is the whole point of the question. A scale is a forcemeter which, if calibrated, gives you the correct mass. The scale should be calibrated for whatever gravitational field you're in. You can only calculate weight using a scale if you also know the current gravitational field strength or can work it out somehow. That is not an assumption that is necessary, or readily obvious. 

If I sent you to the planet zog with a scale and a forcemeter, told you nothing other than "both devices are fully calibrated" and asked you to measure the weight of an object---you couldn't do it with the scale. 

Lets rephrase the question: 

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

a) Thing which you can use to calculate the weight of the object provided you just happen to know the local gravitational acceleration

b) Thing which you can use to measure the weight of the object provided you just happen to know the precise density of it under the current temperature and pressure and the local gravitational acceleration

c) Lightmeter

d) Thing which tells you the weight directly

hungrymark

Myranda;1076103" said:

UnclePsychosis said:

Plus, to return to semantics again:



Scales give you a reading in mass. They therefore *measure* mass. You can use mass to *calculate* weight assuming you know the local gravitational constant.



Scales do not, then, *measure* weight.



Force meters measure force. Weight is a force. Forcemeters, therefore, measure weight.



The more I think about this the more I think that the "use a scale" answer is wrong in every possible sense.





They give "weight" in units of "mass" but they're measuring force - ask NASA really nice to test a scale in the ISS and tell you what the mass of an object is... 

That we have a weird hang up confusing mass and weight doesn't stop the scale functioning the way it does... a scale is a force meter which displays the wrong number

And because it displays the wrong number you can't use it to measure weight unless you know the gravitational field strength of where you're using the thing! The point of the question isn't to assess their knowledge of how scales and forcemeters work - it's to see if they know the difference between mass and weight. It's that straightforward.

Turning it round, how would the people who say it's a badly-worded question rephrase it?

Myranda

UnclePsychosis said:

Myranda;1076103" said:

UnclePsychosis said:

Plus, to return to semantics again:



Scales give you a reading in mass. They therefore *measure* mass. You can use mass to *calculate* weight assuming you know the local gravitational constant.



Scales do not, then, *measure* weight.



Force meters measure force. Weight is a force. Forcemeters, therefore, measure weight.



The more I think about this the more I think that the "use a scale" answer is wrong in every possible sense.





They give "weight" in units of "mass" but they're measuring force - ask NASA really nice to test a scale in the ISS and tell you what the mass of an object is... 

That we have a weird hang up confusing mass and weight doesn't stop the scale functioning the way it does... a scale is a force meter which displays the wrong number

If NASA have a scale on the ISS then it will be calibrated properly and will therefore give them the correct mass. 

This is the whole point of the question. A scale is a forcemeter which, if calibrated, gives you the correct mass for whatever gravitational field you're in. You can only calculate weight using a scale if you also know the current gravitational field strength or can work it out somehow. That is not an assumption that is necessary, or readily obvious. 

If I sent you to the planet zog with a scale and a forcemeter, told you nothing other than "both devices are fully calibrated" and asked you to measure the weight of an object---you couldn't do it with the scale. 

Lets rephrase the question: 

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

a) Thing which you can use to calculate the weight of the object provided you just happen to know the local gravitational acceleration

b) Thing which you can use to measure the weight of the object provided you just happen to know the precise density of it under the current temperature and pressure

c) Lightmeter

d) Thing which tells you the weight directly

A scale wont work in zero (or micro) gravity... 

Mass doesn't change... but a scale will change its value based on altitude ... a balancing scale wont even work in space - though would work on the moon.

scales work by a mass acting under gravity... it's a mass and an acceleration .... it's a force. No gravity, no acceleration, no force... but there IS still mass. ... 

Force meter and scale (not balance scale) are the same thing and even a balancing scale requires there to be gravity... no gravity no scale... but the mass hasn't gone away or changed... even though the scale has changed its value based on altitude the mass hasn't changed... so it's measuring a force and displaying them in a way that people understand rather than accurately 

hungrymark

@Myranda, you're right about how forcemeters and scales work, yada yada yada, nobody's doubting that. However, you're misjudging the point of the question and how it relates to what they're being taught.

Sambostar

Weight is a force (N) = mass (kg) x acceleration due to gravity (9.81m/s2 odd), so using a scale, assuming the scales have been calibrated for kg on planet earth and not Mars, just use a scale and multiply the reading by 9.81m/s2).  All a bloody 'Forcemeter' is, is a calibrated fishing spring scale with Newtons instead of Kg on it anyway, if that is what they mean.

Now, you could use a bloody forcemeter, but assuming the scales which read in kg have been calibrated fro planet earth, multiplying it by 9.81 will give you the same bloody answer anyway. 

Anyway a 'Forcemeter' can't possibly physically exist because spell check doesn't recognise it as an actual word and I thought all kids have to do everything by computer these days.  With questions like that I would shit my pants in class just to be annoying.  They are being pedantic just for the sake of it, I understand their thinking that they want to encourage proper critical thinking in terms of the science but there are better ways to teach.

UnclePsychosis

THEY ARE NOT THE SAME THING. 

They use the same principle but they are calibrated differently. 

One gives you the answer in kg (mass) and one gives you the answer in N (force, or weight). Unless you make the massive assumption that you know the details of the calibration you cannot measure weight with a scale. 

If I gave you a ruler marked in "zogs" and asked you to measure how tall I was in metres, you couldn't do it unless you miraculously happened to guess how many zogs there were in a metre. Even though it is exactly the same measurement method as using a ruler marked directly in metres, you can't assume you can convert between the two without the precise details of the calibration. 

Assuming you know the local gravitational acceleration is the same as assuming you can guess the relationship between zogs and metres. 

hungrymark

Yes. I can't believe this is really so difficult.

The question might as well ask, "do you know that weight is a force?", it's just asking that in the context of basic pieces of scientific equipment. The question is absolutely fine. Anything more pedantic about how scales actually work and the GFS on Mars or whatever is liable to confuse matters. These aren't GCSE kids.

UnclePsychosis

Worrying that anyone thinks that the difference between weight and mass is "pedantry". 

Also weird that people seem far more willing to choose the answer that requires assumed knowledge rather than the one that tells you the answer directly. 

@hungrymark is completely right on this. Weight is a force, thats all the question is checking. 

Clarky

PC_Dave said:

Or "there wrong"

or "that ain't right that ain't"

Clarky

UnclePsychosis said: *Nothing* in the question allows you to assume a value for the local gravitational constant of 9.81m/s/s . All you people answering "use a scale" are making massive assumptions that are completely unnecessary and potentially completely wrong. 

I reckon that as someone called Jack is involved, we need to find out if he is a spaceman or just a regular Earth Jack

Myranda

UnclePsychosis said: THEY ARE NOT THE SAME THING. 
They use the same principle but they are calibrated differently. 
One gives you the answer in kg (mass) and one gives you the answer in N (force, or weight). Unless you make the massive assumption that you know the details of the calibration you cannot measure weight with a scale. 
If I gave you a ruler marked in "zogs" and asked you to measure how tall I was in metres, you couldn't do it unless you miraculously happened to guess how many zogs there were in a metre. Even though it is exactly the same measurement method as using a ruler marked directly in metres, you can't assume you can convert between the two without the precise details of the calibration. 
Assuming you know the local gravitational acceleration is the same as assuming you can guess the relationship between zogs and metres. 

But if you gave me a straight rule marked in kilograms and told me to measure the length of a thing... I could still measure the length. I'd just get an inaccurate answer

That's using a scale in this question... I've never said it's the right answer... just that it's not nearly as wrong as you are implying

Clarky

Myranda said:

UnclePsychosis said:

THEY ARE NOT THE SAME THING. 

They use the same principle but they are calibrated differently. 

One gives you the answer in kg (mass) and one gives you the answer in N (force, or weight). Unless you make the massive assumption that you know the details of the calibration you cannot measure weight with a scale. 

If I gave you a ruler marked in "zogs" and asked you to measure how tall I was in metres, you couldn't do it unless you miraculously happened to guess how many zogs there were in a metre. Even though it is exactly the same measurement method as using a ruler marked directly in metres, you can't assume you can convert between the two without the precise details of the calibration. 

Assuming you know the local gravitational acceleration is the same as assuming you can guess the relationship between zogs and metres. 

But if you gave me a straight rule marked in kilograms and told me to measure the length of a thing... I could still measure the length. I'd just get an inaccurate answer

That's using a scale in this question... I've never said it's the right answer... just that it's not nearly as wrong as you are implying

I don't think your unit of measure works as an argument because they are not representing the same thing..

for example…

miles and km are both units of distance.. they are different standards that represent exactly the same thing

just as km and km/s are not units of the same thing, N and kg are not units of the same thing

chillidoggy

Forcemeter?

Never heard of it.

doogz84

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.

crunchman

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.