Does a Scale Measure Our Weight or Mass?

[D H]

Bathroom scales that measure apparent weight in mass units of kilograms are not meant for pushing against walls, because you get a force unit in mass units of kilograms which is nonsensical.

You've never heard of the kilogram-force, aka the kilopond, aka the kilopoid?

There are certain classes of engineers in the US who tend to work with the pound (mass) as the unit of mass and the pound-force as the unit of force. That they are working with an inconsistent set of units (F=kma vs. F=ma) is less important to them than the fact these units let's them eliminate g, Earth gravitational acceleration, from many of their equations. Their European counterparts? They're likely to work with the kilogram and the kilogram-force. That the kilogram-force is a banned unit doesn't stop them.

 

[russ_watters]

Every engineer in every country uses thumb-rule equations with rolled-up constants and conversion factors like that. It makes the disadvantages of the i-p system less meaningful.

 

[lynnBINTULU]

Reading some of the above replies, I have come to the following conclusion. [Please let me know if I have made an error.]

On Earth, a weighing scale detects the weight of an object as 686 N. It automatically divides it by 9.8. It then displays the object's mass as 70 kg.

Take the same weighing scale to the Moon.
On the Moon, the scale detects the weight of the same object as 114.3 N (one sixth of that on Earth). It automatically divides it by 9.8. It then displays the object's mass as 11.67 kg.

Realising that the weighing scale has not been calibrated to fit the Moon's gravitational acceleration, we need to multiply 11.67 kg by 6 to return to the object's actual constant mass of 70 kg.

 

[Nathanael]

Reading some of the above replies, I have come to the following conclusion. [Please let me know if I have made an error.]

On Earth, a weighing scale detects the weight of an object as 686 N. It automatically divides it by 9.8. It then displays the object's mass as 70 kg.

Take the same weighing scale to the Moon.
On the Moon, the scale detects the weight of the same object as 114.3 N (one sixth of that on Earth). It automatically divides it by 9.8. It then displays the object's mass as 11.67 kg.

Realising that the weighing scale has not been calibrated to fit the Moon's gravitational acceleration, we need to multiply 11.67 kg by 6 to return to the object's actual constant mass of 70 kg.

Except the mass of the moon is far less than 1/6 of the earth. The gravitational acceleration is 1/6 as large because the moon also has a smaller radius than earth. (It depends not only on mass.)

edit:
Also, since the scale is calculating mass, it may also want to take into account the rotation of the planet it's on. (The force the scale detects would be less than the gravitational force on you by an amount ω^2r where r is the planets radius and ω is the planets angular speed.) Most scales probably don't care about this accuracy (or maybe they do) but it would be something to consider if you're weighing yourself on different planets/moons.

 

[elegysix]

here's my two cents: Mass is never measured. Mass is always calculated.

 

[cowls192]

Reading some of the above replies, I have come to the following conclusion. [Please let me know if I have made an error.]

On Earth, a weighing scale detects the weight of an object as 686 N. It automatically divides it by 9.8. It then displays the object's mass as 70 kg.

Take the same weighing scale to the Moon.
On the Moon, the scale detects the weight of the same object as 114.3 N (one sixth of that on Earth). It automatically divides it by 9.8. It then displays the object's mass as 11.67 kg.

Realising that the weighing scale has not been calibrated to fit the Moon's gravitational acceleration, we need to multiply 11.67 kg by 6 to return to the object's actual constant mass of 70 kg.

As D H have noted, the thing about the scale is that it doesn't really use g = 9.8 (kg/s2) for calculation, as the scale is most likely a spring-based device. In that case, the scale uses F = -kx for calculation.

In a sense, you can measure the "weight" by hanging the scale on the wall and push it with your hands. The scale will still give numbers because it's based on the force applied to the spring.

We say the scale measures our body weight quite accurately, when we assume we are standing on top of the scale, which is in the same direction with gravity. It can be difficult to realize that we are constantly accelerating toward the center of the Earth when we are standing still. Obviously, if we "fall from the sky", we would be accelerating!

 

[A.T.]

It can be difficult to realize that we are constantly accelerating toward the center of the Earth when we are standing still.

What? If you stand still you aren't accelerating. Unless you mean proper acceleration, but that is away from the center of the earth, not towards it.

Obviously, if we "fall from the sky", we would be accelerating!

Yeah, with emphasis on "if". Falling is different than standing still.

 

[olivermsun]

As D H have noted, the thing about the scale is that it doesn't really use g = 9.8 (kg/s2) for calculation, as the scale is most likely a spring-based device. In that case, the scale uses F = -kx for calculation.

Doesn't it still need g to give you a number in kg instead of N (which is what F is in)?

 

[olivermsun]

Also, since the scale is calculating mass, it may also want to take into account the rotation of the planet it's on. (The force the scale detects would be less than the gravitational force on you by an amount ω^2r where r is the planets radius and ω is the planets angular speed.)

The rotation is usually figured into (nominal) g (and in fact it's reflected in the slightly flattened shape of the rotating earth!)

 

[cowls192]

What? If you stand still you aren't accelerating. Unless you mean proper acceleration, but that is away from the center of the earth, not towards it.

Even though the mass is not "moving", it is said to be in acceleration, just "infinitely" slow rate. Most of the matters in Earth has weight because of the gravity. If, as you say, there is no "acceleration", then it would be weightless, since F = m * a --> F = m * zero, which would simply equal to zero.

You will find in many physics problems that even if you are standing still, "motionless", acceleration still has to be considered when calculating force and work done (such as riding seesaw alone or simple pulley).

Doesn't it still need g to give you a number in kg instead of N (which is what F is in)?

I'm assuming you're pointing out about the scale. Typical bathroom scales use the spring force, F = -k * x to measure its approximate equivalent of F = m * g, so the measurement depends on the strength/resistance of the spring.

You see, "standing still on the scale" means you are not applying force to your body. I mean, if you're standing on the scale, and the friend behind you "pushes" you down toward the ground, your weight will "increase" because external force has been applied while the friend's body mass was not on the scale. The only thing i can think of right now that can apply force to yourself by yourself is jumping up and down the scale.

Sample example would be, a 70 kg man equals 70 * 9.8 = 686 N whether he is standing on the scale or not. Since the man is 686 N, we can find k of the scale, if we know the x, the displacement of the spring. Let's say when the man stood on the scale, the spring compressed by 0.02 m. Then k equals 34300. Now, here comes a 60 kg lightweight champion, who pushes the scale with both of his hands while his feet are not placed on the scale, and he manages to compress 0.02 m (same as 70kg man standing still).

My point is that the mass of the person does not matter for exerting spring-based force. Whether a lighter person pushing hard or a heavier person pushing soft, spring system is based on the displacement and the strength/resistance (k).

 

[Doc Al]

Even though the mass is not "moving", it is said to be in acceleration, just "infinitely" slow rate. Most of the matters in Earth has weight because of the gravity. If, as you say, there is no "acceleration", then it would be weightless, since F = m * a --> F = m * zero, which would simply equal to zero.

Uh, no. Having zero acceleration just means that the net force is zero; it does not mean something is weightless.

 

[A.T.]

Even though the mass is not "moving", it is said to be in acceleration, just "infinitely" slow rate.

Who says that? Can you provide a reference? And what is "infinitely slow"?

Most of the matters in Earth has weight because of the gravity. If, as you say, there is no "acceleration", then it would be weightless, since F = m * a --> F = m * zero, which would simply equal to zero.

It seems that you are thinking about proper acceleration, that an accelerometer measures:
http://en.wikipedia.org/wiki/Proper_acceleration

However, the proper acceleration of a person standing still on the surface is 1g upwards, not downwards as you said in the previous post. And 1g is not a "infinitely slow rate".

 

[cowls192]

And 1g is not a "infinitely slow rate".

I think i mixed up with velocity. So, even though the mass seems to be "motionless" since it is not moving at constant speed, it may or may not have acceleration. For example, let's say a car hit a tree (very gently). As you hit the gas, the car is not moving beyond the tree, but it is still accelerating toward the tree, as you can tell from screeching tires.

That's the concept of mass, acceleration, and weight i was pointing out. When a man is standing still on the scale, the man may have zero velocity, but he is still being pulled toward the center of the Earth.

I meant to say that the man is said to be in acceleration, because if not, say an earthquake occurred, that minor vibration is enough to set the man fly away like a pollen (not considering his directions and paths taken in the air).

 

[Nathanael]

but it is still accelerating toward the tree, as you can tell from screeching tires.

I don't think this is correct usage of "acceleration"

Acceleration is change in velocity. The car is not accelerating (ignoring centripetal acceleration).

If there is no change in velocity, there is no acceleration...

 

[ModusPwnd]

I think you are mixing up acceleration and force. There is a force pulling him to the center of the earth, a force equal to mg. Acceleration is a result of the net force and it is when there is a change in velocity. No change in velocity -> no acceleration.

 

[A.T.]

For example, let's say a car hit a tree (very gently). As you hit the gas, the car is not moving beyond the tree, but it is still accelerating toward the tree, as you can tell from screeching tires.

No, it is not accelerating. You are confusing accelerating with making noise.

That's the concept of mass, acceleration, and weight i was pointing out. When a man is standing still on the scale, the man may have zero velocity, but he is still being pulled toward the center of the Earth.

He is pulled down by the force of gravity. But he is not accelerating down. You are confusing a single force with the net force.

..., say an earthquake occurred, that minor vibration is enough to set the man fly away like a pollen ...

Sigh.

 

[olivermsun]

Even though the mass is not "moving", it is said to be in acceleration, just "infinitely" slow rate...If, as you say, there is no "acceleration", then it would be weightless, since F = m * a --> F = m * zero, which would simply equal to zero.

You will find in many physics problems that even if you are standing still, "motionless", acceleration still has to be considered when calculating force and work done (such as riding seesaw alone or simple pulley).

...Typical bathroom scales use the spring force, F = -k * x to measure its approximate equivalent of F = m * g, so the measurement depends on the strength/resistance of the spring.

In many physics problems the "motionless" state exists because there is a precise balance between two forces, one which you know and one which you want to know. The scale, which has a known spring constant, is pushing up with just enough force to balance the downward pull of gravity on your body mass, and as a result there is no acceleration. That is, -kx = F_spring = -F_gravity = mg, and then you can recover m if you know g.

You see, "standing still on the scale" means you are not applying force to your body. I mean, if you're standing on the scale, and the friend behind you "pushes" you down toward the ground, your weight will "increase" because external force has been applied ...My point is that the mass of the person does not matter for exerting spring-based force. Whether a lighter person pushing hard or a heavier person pushing soft, spring system is based on the displacement and the strength/resistance (k).

But that isn't how a scale is meant to be used, right? You're supposed to put the mass on the scale, in normal Earth gravity g, wait until everything settles out — and then you read the number on the dial.

 

[NascentOxygen]

When I step on a scale I see that it says...70kg. We typically call this value weight? However, kg is used for "mass" and not "weight". So when we say 70kg do we mean that my mass is 70kg OR do we mean that my weight is 70 N?

The scales measure force, and are conveniently calibrated in units of kg wt. So it means that your weight (on Earth) is 70 kg wt, to within acceptable accuracy. Most bathroom scales are not rated or calibrated for extraterrestrial use.

See where I'm coming from?

No, but I can see where you're going ...

 

[Buckleymanor]

The scales measure force, and are conveniently calibrated in units of kg wt. So it means that your weight (on Earth) is 70 kg wt, to within acceptable accuracy. Most bathroom scales are not rated or calibrated for extraterrestrial use.

Balance scales don't have to be, 70kg wt, on Earth measured by a balance scale, will be the same when measured by the scale on the moon.

 

[Newtonsghost]

Boy there are some confused people posting on this thread going off on all kinds of tangents and displaying a complete inability to understand the essentially simple question posed by sharygob. In essence the reading on the scales is your mass in Kilograms having 'worked it out ' from mass = the force you exert on the scales divided by the strength of the gravitational field you find yourself in. Thats it

 

[d.r.og]

I had this same question, basicly yeah, the profesor is right the scale will mesure Newton, then will get converted to the kg it's proportional to on earth. So the scale truly mesures weight, but shows you mass, not because the gravity factor it ignored, but because it's already taken into consideration and they generated a sort of scale that's proportional. As is to say your 70kg is proportional to 686N so it's correct to say our weighing self. But that doesn't mean what the scale shows you is weight, it shows a proportional value that corresponds to the 686Newton that I'm this case is to 70kg of mas on Earth's gravity.
And your last question kg isn't the same as newto and kg isn't defined by weight a kg here and a kg on the moon are the same, cause they mesure mass, remeber a kg is a thousand grams, and a gram is defined as 1 cubic centimeter of whater.

 

[anurags123]

When I step on a scale I see that it says...70kg. We typically call this value weight? However, kg is used for "mass" and not "weight". So when we say 70kg do we mean that my mass is 70kg OR do we mean that my weight is 70 N?

So if it does mean mass, then my ACTUAL weight is 686N! lol
And if it means weight, then my mass is roughly 7kg.

See where I'm coming from?

And just to let you guys know, b4 u waste ur time, I def. know the difference between mass and weight. I'm just uncertain about what we are actually measuring.

I truly appreciate anyones help! thanks

Here is the response to your question !

It actually measures your weight, but it is calibrated so that shows your mass.

Explanation:
Mass is measure for amount of matter. Unit for mass is 1kg.
Weight of a body is force by which one mass is attracted to another. Your weight is force of gravitation pull between Earth and your body. Unit for force is 1N.
On surface of the Earth, relation between these two is:
G = m g
where
G is weight
m is mass
g is acceleration of gravity and it approximately constant everywhere in the world.
Bathroom scale is calibrated so that it convert measured units of weight to units of mass based on constant g. But if gravity changes, bathroom scale doesn't show correct value for mass anymore.
If you could weight yourself by bathroom scale on the surface of the Moon, it would show that your mass is 6 times smaller than on Earth, and on Jupiter it would show 2.5 times greater mass. In reality, your mass is equal on Earth, Moon, Jupiter and everywhere else. It is weight that changes with gravity and that's why would scale show incorrect values.

 

[C.F.]

The only reason people are confused is because you tell them they are weighing something and the scale gives them units of mass instead of force. This conversation wouldn't be happening if scales measured the right unit for the intended purpose, instead of measuring mass and telling you that's how much the object weighs. It's frustrating indeed.

 

[olivermsun]

The only reason people are confused is because you tell them they are weighing something and the scale gives them units of mass instead of force. This conversation wouldn't be happening if scales measured the right unit for the intended purpose, instead of measuring mass and telling you that's how much the object weighs. It's frustrating indeed.

In the real world, that's how lots of measurements actually get done. We use physics to infer something we can't easily measure from things we can. We don't want a thermometer to report volume or resistance, we want temperature in degrees. In exactly the same way, we may want a scale to tell us mass in units like "kilograms" even though the operating principle is "weighing" (or, perhaps, "balancing").

 

[C.F.]

I'll give you the thermometer and many other measurement instruments (viscometers come to mind). I would pose the question that, in a society that already confuses mass and weight INCLUDING in the commercial and academic arenas, why enable the confusion by making scales designed to weigh something-- their purpose to be used to find weight-- and then give said weight in terms of mass? It makes absolutely no sense whatsoever.

 

[nasu]

The only reason people are confused is because you tell them they are weighing something and the scale gives them units of mass instead of force. This conversation wouldn't be happening if scales measured the right unit for the intended purpose, instead of measuring mass and telling you that's how much the object weighs. It's frustrating indeed.

Where does a scale tells you that the number shown there is the weight?
The scale is calibrated to show whatever people are more interested to know and the units more familiar to them.
In Europe people are, in general, familiar with kilograms so the scale shows the mass in kilograms, if the conditions are right (you are on Earth, close to surface and at rest).
In the US many scales show pounds because this is what people are more familiar with. This is a weight unit. In both cases, a spring (or piezo) scale measures a force, t start with.
Same as a thermometer measures a length or a resistance or a voltage. And then shows degrees Celsius or Fahrenheit or whatever. It's nothing special about scales.
Many instruments measure indirectly the quantity displayed and do some conversion.

Edit.
The post above (olivermsun) went along similar line. Did not see it yet.

 

[ZapperZ]

I'll give you the thermometer and many other measurement instruments (viscometers come to mind). I would pose the question that, in a society that already confuses mass and weight INCLUDING in the commercial and academic arenas, why enable the confusion by making scales designed to weigh something-- their purpose to be used to find weight-- and then give said weight in terms of mass? It makes absolutely no sense whatsoever.

This is rather silly.

Can you show me an instrument of measurement that actually measures what it is supposed to measure? Do you think a voltmeter actually measures "voltage"? If you can't do this, then your complain is actually moot, because nothing that we use actually measures that quantity directly.

And I'm an experimentalist, where what I actually precisely is something that I pay very close attention to.

BTW, this is a very old thread, and @anurags123 is responding to a post from 2008!

Zz.

 

[C.F.]

Can you show me an instrument of measurement that actually measures what it is supposed to measure?

Stopwatches, rulers, balance scales-- the tools used to measure the three base units.

I understand that measuring desirable values sometimes (actually quite a lot of the time) requires an instrument to do conversions from one unit to another. However, the colloquial meaning of weight has become synonymous with mass. I'm an intern at a PVC solvent cement facility, and on the sheets for percent-solid tests it asks for weight in grams. I suppose it may be a pet peeve of mine, but is it really too much to ask for the unit to match the intention?

 

[ZapperZ]

Stopwatches, rulers, balance scales-- the tools used to measure the three base units.

Open a stopwatch. It measures nothing! It only has an oscillation mechanism. So essentially, it tells you how many oscillation it has made! Someone has to calibrate the number of oscillation to a unit of time!

I can replace a ruler with any length of stick. Is that stick now actually useful to "measure" something? Do you think your "1 meter" is something that is inherent in that stick?

Balance scale is no different than any scale, because you need a calibrated mass!

I understand that measuring desirable values sometimes (actually quite a lot of the time) requires an instrument to do conversions from one unit to another. However, the colloquial meaning of weight has become synonymous with mass. I'm an intern at a PVC solvent cement facility, and on the sheets for percent-solid tests it asks for weight in grams. I suppose it may be a pet peeve of mine, but is it really too much to ask for the unit to match the intention?

You cannot connect the sloppiness of where you work, or the sloppiness of how the public do something, with blaming an instrument for not measuring what you want it to measure.

Zz.

 

[C.F.]

We as humans needed to use something to quantify nature, didn't we? Regardless of the arbitrary nature of the existence of units themselves, those tools used to measure seconds, kilograms, and meters have an output unit that corresponds exactly to what they are used to measure; time, mass, and length. There is no conversion necessary.

I don't blame the scale for measuring force and having an output unit of mass. I blame the misuse of the scale and misunderstanding between weight and mass for people's confusion about what a spring scale actually measures. The only solution I can see besides educating the entire public about the difference between weight and mass (again) is to change the output unit of spring scales from units of mass to units of force. Pointing out a problem without offering a solution is simply complaining, and I don't like to complain.

Pragmatically, what is spewing this peeve all over an internet forum going to accomplish? Next to nothing; but it does make me feel better, and that counts for something.

 

[Dale]

What is wrong with this thread? It just will not die a natural death, so I killed it with my mentor-issued digital axe.

I feel a little like a survivor in a really boring version of a zombie movie.