Weight & Mass

PC_Dave

stickyfiddle said:

Sesh said:

stickyfiddle said:

Philly_Q said:

Isn't the difference that mass is fixed but weight depends on gravity?

The weight is the force experienced due to gravity, so the answer may be forcemeter.... if, indeed, there is such a thing as a forcemeter.

In the real world I'm sure Jack would use a scale, since he's not likely to be going anywhere where gravity is different.

That's what the teacher said, apparently. Loads of people have been arguing that scale is wrong because it doesn't read out in Newtons, but that's really completely irrelevant as a basic scale (ie one with a single plate and a spring mechanism) IS a type of forcemeter, used to measure the force between the earth and the object sitting on the scale. 

I can totally see that a see-saw-type weight balance might not be the right answer, but ultimately it's a crappy question.

It is a crappy question. It is hard to gauge what scientific level this is aimed at. If it is for an 8 year old then scale would be correct enough in my opinion. A "weighing scale" should really be called a "balance."

I don't see what a question has to gain by being a smartarse.

It's for 6th grade, apparently, which is 11-12 year olds. 

I completely get what the teacher is trying to achieve, but it's badly worded and misleading. I now have a bunch of teachers on facebook telling me it's not misleading or badly worded. They're wrong.

Just tell them "their wrong" and see what they do.

PC_Dave

Or "there wrong"

Phil_aka_Pip

where's @hungrymark when we need him?

BigMonka

It's not necessarily a bad question, we're just missing the context of it - if the class had spent the last month learning F=ma and the difference between weight and mass then it's quite a good question for finding out if they've understood that.

UnclePsychosis

These questions always end up in a load of pointless bickering because people get bogged down in the semantics of the question rather than getting to the point. The point here, of course, is that mass and weight are not the same thing. Mass is a property of an object that depends purely on how much matter is in it. The weight of an object varies with where that object is.

Forcemeter is clearly the "right" answer because it tells you the answer directly but you could also use a scale and do the measurement indirectly, provided you were able to calibrate it correctly and knew the correct relationship between weight and mass for the environment where you did the measurement.

Its a bit of a shit question but its fairly obvious which one you should pick if you want to get the answer "right"---one of the answers would tell you the answer directly and one of them would get you the answer eventually but not directly. To be really pedantic about it, I could technically use the ruler to measure its weight: measure its dimensions, find out what the density of the components of the object are, do the calculation that way. This is clearly not the right answer but in principle its exactly the same as the "use the scale" answer (use instrument, add extra info, do calculation, get answer).

Finally, to be pedantic once more---the question doesn't tell you what environment you're in. You have no a priori knowledge of the gravitational field strength in your current location. If all you're given is a (calibrated) scale then you cannot work out the weight of that object without adding some assumptions that may not be valid and are not suggested by the question. If you're given a (calibrated) forcemeter then you can measure the weight directly, no assumptions required.

Basically: Forcemeter is clearly the right answer. Scale is only half-right.

Modulus_Amps

if the scale gives you a measure of kg, then it is giving you the mass. that is to say the scale is calibrated to take account of gravity it is intended to be used in, maybe that should be in the instructions.

If the scale gives you a measure with Newtons then it is the weight and does not take gravity into account.

People use" weight" incorrectly all the time, in an engineering field you have to look at the units used.

If I wanted to work out the forces on a truck going round a corner, then I need the trucked to be weighed, so that I know it mass, so that I can then apply the accelerations to the mass to get the forces involved.

Anyway the answer to this question depends on what/how the topic has been taught. I could probably use a, b or d to determine the weight, assuming I know more about the rule of course.

Philly_Q

BigMonka said:

It's not necessarily a bad question, we're just missing the context of it - if the class had spent the last month learning F=ma and the difference between weight and mass then it's quite a good question for finding out if they've understood that.

Exactly.  And on the other hand, if they haven't yet gone anywhere near the concepts of weight vs. mass and they've never even heard of Newton - or Newtons - then I'd expect them to just say "scale".

UnclePsychosis

Sesh said:

stickyfiddle said:

Philly_Q said:

Isn't the difference that mass is fixed but weight depends on gravity?

The weight is the force experienced due to gravity, so the answer may be forcemeter.... if, indeed, there is such a thing as a forcemeter.

In the real world I'm sure Jack would use a scale, since he's not likely to be going anywhere where gravity is different.

That's what the teacher said, apparently. Loads of people have been arguing that scale is wrong because it doesn't read out in Newtons, but that's really completely irrelevant as a basic scale (ie one with a single plate and a spring mechanism) IS a type of forcemeter, used to measure the force between the earth and the object sitting on the scale. 

I can totally see that a see-saw-type weight balance might not be the right answer, but ultimately it's a crappy question.

I don't see what a question has to gain by being a smartarse.

The difference between mass and weight is an important scientific concept and needs to be taught in Physics classes, that's what the question has to gain.

thermionic

I agree - poorly-worded question. Us, with a bit of knowledge, get confused and give stupidly wrong answers (like multiply the weight by 9.8 m/s/s - LOL). However, I bet the 11-12 year olds the question is aimed at will know the right answer when they see it because they've been taught in a certain way.

Sesh

I succumbed and googled forcemeter, and the internet tells me that's a spring balance. Never heard one of them called that before. So the correct answer is "forcemeter" and assuming that the kids have been learning about it they should be able to make that distinction. Perhaps the word "scale" was used as they have probably taught it is not the correct terminology.

FWIW I work with people with PhDs in various sciences. I reckon only 25% would have picked forcemeter.

PC_Dave

Sesh said:

I succumbed and googled forcemeter, and the internet tells me that's a spring balance. Never heard one of them called that before. So the correct answer is "forcemeter" and assuming that the kids have been learning about it they should be able to make that distinction. Perhaps the word "scale" was used as they have probably taught it is not the correct terminology.

FWIW I work with people with PhDs in various sciences. I reckon only 25% would have picked forcemeter.

I think it's mis-spelled. I believe it is "Force-O-Meter"

Myranda

Well... semantics.

A scale gives a measure of the force on the mass within the local gravity - that we choose to write kg or lbs on the scale is neither here nor there it's measuring the weight ...

However force meter will give the answer in newton meters which is a force and thus is the correct unit for weight the fact that weighing a thing on a scale and multiplying by 9.81 will give the same answer seems to be lost on those setting the question.

That's the only difference between the two answers, the units displayed. That which is measured is the same and really the answer is the same and the sale will move the same amount... if you scratched into the scale a new set of datum points and marked it in newton meters you haven't altered the function of the device in the slightest, you're just reading the correct unit for the device rather than the unit we're used to associating with weight.

100 kg isn't a weight - we just think of it as such because of habit and convention. But then, gravity as a mathematical construct is much newer than the word weight and its accepted meaning.

Arguably more people talk about weight in terms of mass than in terms of newton metres and under the accepted meaning and understanding of the term 100 kg in weight is understood by most but 981 newton metres is understood by but a few, so is in effect useless for normal conversation. Perhaps conversationally answers A and D are equally valid - but is this question from a science paper or an english paper? 

Scientifically there is only ambiguity because, as I mentioned previously a scale IS a force metre we just mark the wrong scale on it

chrispy108

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.

UnclePsychosis

@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.

John_P

I'm a physics teacher so I spend way too much time trying to persuade kids that it's wrong to ask someones weight.  

Chrispy108 is of course correct and the devices that give a reading in kg are really measuring force anyway.

axisus

Phil_aka_Pip said:

Who's Jack?

Jack Shit. He don't know nothing

Myranda

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

stickyfiddle

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.

Myranda

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)

stickyfiddle

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.