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Mass and Weight - which do scales show?

by autodidude
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autodidude
#1
Aug21-11, 10:47 PM
P: 333
Whenever you step on a scale, digital or spring, is that your weight? Or has the scale been calibrated to take into account the Earth's gravity and the number shown is your mass?
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quietrain
#2
Aug21-11, 11:04 PM
P: 651
should be weight? since your mass is constant, your weight is mg which is affected by the "g" of different places

thats why i find it weird that people say my weight is 70kg when it should be 70N?
cepheid
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Aug21-11, 11:08 PM
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Quote Quote by autodidude View Post
Whenever you step on a scale, digital or spring, is that your weight? Or has the scale been calibrated to take into account the Earth's gravity and the number shown is your mass?
Scales measure weight, (actually they measure displacement of a spring or something similar and make assumptions about the properties of the system in order to determine the force needed to produce that displacement cf. Hooke's Law). But the numbers have been calibrated to show the mass of an object (on Earth) that would produce the measured force. At least, that's true of the kg (metric) markings on the dial. I guess for the imperial markings it could be interpreted either way, since (correct me if I'm wrong) an object with a mass of 1 lbm will have a weight of 1 lbf.

EDIT: A more direct answer to your question with fewer unnecessary complications would be: Scales measure weight. If you stood on a scale on a different planet, the reading would be different from what it was on Earth.

EDIT 2: But since the numbers on the dial have been calibrated for Earth, if you stepped onto a bathroom scale made on Earth while you were on a different planet, then I guess what it would show you would be an incorrect value for your mass!

EDIT 3: But the numbers on the dial would only be wrong if you interpreted them as being mass units. If you interpreted them as being kilograms-force and pounds-force, then they would be showing you your correct weight. Sorry, things kept occurring to me!

elfmotat
#4
Aug21-11, 11:16 PM
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Mass and Weight - which do scales show?

It actually depends on where you live. If you're anywhere outside of the US, your scale will probably use kilograms. In that case, it is telling you your mass. If you live in the US then your scale will probably use pounds, which is a unit of weight.

The scale directly measures your weight, and can indirectly tell you your mass.
sophiecentaur
#5
Aug22-11, 08:26 AM
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Any 'weighing machine' will involve the measurement of force (implicit or explicit).

A. You can measure the weight force of an object by hanging it on a spring and seeing how much the spring distorts (bathroom scales). That is, definitely, a weight (fo'rce) measurement.
B. You can compare the weight force on one pan of a 'balance' with the weight force of a known (calibrated) mass. You are still using the weight force but, because there is another, known, mass involved, you will know the mass of your test object - whether you do the measurement on Earth at the poles. on the equator or even on the Moon. This measurement is independent of the value of g (or apparent g). You still need 'some g - or possibly some acceleration.

C. Another way to find mass is to let the mass vibrate up and down (or side to side) on a spring. The period of oscillation is independent of g and, if you know the spring constant (stiffness) then you can deduce the mass. But you are still using force to distort the spring. This method would work even in a spacecraft.
jewbinson
#6
Aug22-11, 10:09 AM
P: 127
This has probably already been said, but...

"mass" is the amount of matter you have. It is to do with the number of atoms that make up your body.

Your "weight" is to do with measuring your mass in a gravitational field. So slightly closer to the centre Earth, I will weigh more, because the gravitational field there is stronger.

Note that both mass and weight are fundemental properties of our universe, because they involve mass (matter) and g (gravity), which most physicists accept as fundemental properties of matter.

For comparison (as an example), if you get a giant rock that weighs 10000N (about a tonne) on the surface of the Earth, and stick it in space away from gravitational fields, then it weighs nothing (well actually almost nothing but maybe a tiny bit because of its own gravity - I think). If you get me in a super-heat-and-pressure-resistant spacesuit and stick me somewhere near the surface of the sun, then my weight will be huge.

But the massive rock has over 10 times greater mass than I have (i.e. over 10 times more atoms than I have)

mass is measured in Kg and weight is in N
K^2
#7
Aug22-11, 12:26 PM
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What scales measure and what they display may be two different things. The spring/electronic scales measure weight. They might display mass under assumption of Earth gravity. Check units to be sure.
Gnosis
#8
Aug22-11, 02:12 PM
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Quote Quote by quietrain View Post
...thats why i find it weird that people say my weight is 70kg when it should be 70N?
70N??? You weigh less than 16 pounds?
JerryClower
#9
Aug22-11, 06:36 PM
P: 75
Scales show your weight.
autodidude
#10
Aug25-11, 08:57 PM
P: 333
@ cepheid:

EDIT 1:I'm just talking about scales on Earth :p
EDIT 2: Yes, that's what pretty much what I wanted to know, whether the scale was calibrated to find your mass or not
EDIT 3: So say. 70kg, you'd have to convert that?

elfmotat:
so if it read 150lbs, that's under the force of gravity? And so when you convert from kg to lbs, the constant has something to do with it?


@Gnosis: haha that's why I asked. there was a question asking for the mass given the weight I believe, and I got
a really low mass and thought that can't be right
Drakkith
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Aug25-11, 09:04 PM
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Quote Quote by autodidude View Post
elfmotat:
so if it read 150lbs, that's under the force of gravity? And so when you convert from kg to lbs, the constant has something to do with it?
Yes. You must be under the effect of gravity otherwise you will not be pulled down onto the scale! With a force of 1g you would convert pounds to kilograms by dividing the weight by about 2.2 I believe.
cepheid
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Aug25-11, 11:40 PM
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Quote Quote by autodidude View Post
@ cepheid:

EDIT 1:I'm just talking about scales on Earth :p

EDIT 2: Yes, that's what pretty much what I wanted to know, whether the scale was calibrated to find your mass or not
Well that's the thing. Scales measure weight, so they'll always tell you your correct weight no matter where you are. In contrast, they can only tell you your mass correctly if you're on Earth. Elsewhere, the calibration breaks down and has to be redone.

Quote Quote by autodidude View Post
EDIT 3: So say. 70kg, you'd have to convert that?
I was pointing out a subtlety. If you have a mass of 70 kg, then on Earth you'll weigh approx. 687 newtons (N), or 154 lbs in Imperial units. If you then went to the moon (where surface gravity is only about 1/6 as strong), and stood on the scale, the dial would read 70/6 kg = 11.67 kg. Clearly, if you interpret this as your mass, it is just plain WRONG. Your mass is 70 kg, not 11.67 kg. This illustrates that scales measure weight, not mass. But here is the subtlety: there is a unit of force called the kilogram-force (kgf) which is defined as the weight that a mass of 1 kg would have in the standard Earth gravitational field of g = 9.81 N/kg. In other words, 1 kgf = (1 kg)*g = 9.81 N. Now, let's take your weight on the moon (687/6 N = 114.5 N) and convert that into kgf: 114.5 N * (1 kgf/9.81 N) = 11.67 kgf.

So, IF you interpret the kg numbers on the dial as being units of force (kgf) then they will always show you the correct value, since you're interpreting them as a measurement of weight. The same thing is true of the pounds (lbs) scale. If you interpret them as being pounds-mass (lbm) then the measurement is wrong everywhere except on the surface of the Earth. But if you interpret them as being pounds-force (lbf) then the reading is always correct.

By the way, I wouldn't recommend using kgf, since it's not a standard SI unit. Use newtons as the units for expressing all forces (including weight). Using newtons also helps make sure you distinguish between mass and weight clearly. Using kgf confuses things, since the mass and weight will have the same numerical value (on Earth).
robphy
#13
Aug26-11, 12:07 AM
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scales measure a normal force
http://en.wikipedia.org/wiki/Normal_force
Subductionzon
#14
Aug26-11, 12:09 AM
P: 168
I may be old, but when I went to the doctor's office my weight was measured by a scale that used a weighted balance. It was not weight for weight or mass for mass, but it was a leveraged weight. So using the doctor's scale my "weight" would have been the same on the Moon as on the Earth , which meant it was really measuring my mass in a gravitational field. It would not work properly in a zero g situation.

But otherwise I agree, your typical spring scale is measuring your weight.
Flubertin
#15
Sep4-11, 05:38 AM
P: 2
Quote Quote by cepheid View Post
Scales measure weight, (actually they measure displacement of a spring or something similar and make assumptions about the properties of the system in order to determine the force needed to produce that displacement cf. Hooke's Law). But the numbers have been calibrated to show the mass of an object (on Earth) that would produce the measured force. At least, that's true of the kg (metric) markings on the dial. I guess for the imperial markings it could be interpreted either way, since (correct me if I'm wrong) an object with a mass of 1 lbm will have a weight of 1 lbf.

EDIT: A more direct answer to your question with fewer unnecessary complications would be: Scales measure weight. If you stood on a scale on a different planet, the reading would be different from what it was on Earth.

EDIT 2: But since the numbers on the dial have been calibrated for Earth, if you stepped onto a bathroom scale made on Earth while you were on a different planet, then I guess what it would show you would be an incorrect value for your mass!

EDIT 3: But the numbers on the dial would only be wrong if you interpreted them as being mass units. If you interpreted them as being kilograms-force and pounds-force, then they would be showing you your correct weight. Sorry, things kept occurring to me!
Bathroom scales are confusing: If you were to use a balance then you would measure MASS.

Mass is a measure of the quantity of 'stuff' in a body. So if you sit on a balance and add material to the other side of the balance until you reach the balance point, the system will stay balanced on earth, on the moon, under the ocean etc since the quantity of 'stuff' on each side has NOT changed.

So, imagine that (on earth) you sit on a balance and add 70 kg of mass (from a standards lab) to the other side and it balances then you have a mass of 70 kg. If (on earth) you now stand on your bathroom scales you would find it reports 70 kg. But we call this weight (even though it has the wrong units and is clearly a mass).

Calling it weight is however partially correct because if you go to the moon with the scales and stand on them they will NOT report 70 kg anymore so they must be reporting some sort of weight since the quantity of 'stuff' has not changed but the measure has!

The problem is about 1) language and 2) calibration. We use the term weight and the scales certainly measure weight BUT they have been calibrated (on earth) using a standard MASS so the number you see (ON EARTH) is mass. Thus if your scales report 70 kg then your real weight is (70 kg x 9.8 m/s/s) = approx 700 N !! If you are doing calculations based on things 'weighed' on bathroom scales then you should recognize that this is Mass (hence kg) and NOT weight (N).

In short: bathroom scales measure weight but are calibrated in mass. The use of weight and kg is not correct but is common language usage.
McQueen
#16
Sep4-11, 07:02 AM
P: 265
Quote Quote by quietrain View Post
should be weight? since your mass is constant, your weight is mg which is affected by the "g" of different places

thats why i find it weird that people say my weight is 70kg when it should be 70N?
This wrong , 70 Kg weight would be 70 Kgm or 686 N.
Flubertin
#17
Sep4-11, 09:55 AM
P: 2
Bathroom scales are confusing: If you were to use a balance then you would measure MASS.

Mass is a measure of the quantity of 'stuff' in a body. So if you sit on a balance and add material to the other side of the balance until you reach the balance point, the system will stay balanced on earth, on the moon, under the ocean etc since the quantity of 'stuff' on each side has NOT changed.

So, imagine that (on earth) you sit on a balance and add 70 kg of mass (from a standards lab) to the other side and it balances then you have a mass of 70 kg. If (on earth) you now stand on your bathroom scales you would find it reports 70 kg. But we call this weight (even though it has the wrong units and is clearly a mass).

Calling it weight is however partially correct because if you go to the moon with the scales and stand on them they will NOT report 70 kg anymore so they must be reporting some sort of weight since the quantity of 'stuff' has not changed but the measure has!

The problem is about 1) language and 2) calibration. We use the term weight and the scales certainly measure weight BUT they have been calibrated (on earth) using a standard MASS so the number you see (ON EARTH) is mass. Thus if your scales report 70 kg then your real weight is (70 kg x 9.8 m/s/s) = approx 700 N !! If you are doing calculations based on things 'weighed' on bathroom scales then you should recognize that this is Mass (hence kg) and NOT weight (N).

In short: bathroom scales measure weight but are calibrated in mass. The use of weight and kg is not correct but is common language usage.


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