Does a Scale Measure Our Weight or Mass?

[thinker-1]

I am still in doubt.

What does a scale read when pushed by hand against the wall?

Any hint?

thank you

 

[K^2]

I measure the resistance change directly.

Oh really? You have a magical ohm-meter that measures resistance without running a current through material and measuring voltages and currents?

What does a scale read when pushed by hand against the wall?

The force. Like it always does.

 

[thinker-1]

do you think the numbers on the scale will be indicating Newton or they need to be converted to Newton?

 

[kjl]

do you think the numbers on the scale will be indicating Newton or they need to be converted to Newton?

The numbers on the scale ALWAYS display a mass (say, X kg), but what it really measures is force in Newtons (Y Newtons) and calculates (X=Y/9.8) and displays X.



Think of it this way:


If you are standing on the scale normally:

Say you mass 75 kg. Gravity is 9.8 N/kg.
The force you exert on the scale is 75 kg * 9.8 N/kg = 735 N.

The scale actually measures 735 N, and helpfully divides it by 9.8 N/kg (gravity) and displays 75 kg.

If you put it vertically on a wall and push on it, it may read 10 kg, but what it really has measured is 98 N and (not so helpfully) divided it by 9.8 N/kg. In this case, the reading of "10 kg" is sort of meaningless, but if you multiply it by 9.8 N/kg you can calculate the force you are exerting on the scale (98 N).

 

[D H]

It measures weight.
which is mass * acceleration due to gravity

Mass times gravitational acceleration is actual weight. That is not what a spring scale measures. A spring scale measures apparent weight.

Even better, a spring scale measures the compression of the springs in the scale. When a scale is laying flat on a floor and a person is standing on top of the scale, the springs are compressed by that person's apparent weight. If the person lifts the scale off the floor and pushes it against a wall, the scale will still register a weight because the springs are still being compressed.

Note well: What a scale really measures is the compression of a spring. A scale that registered a reading of 0.445 centimeters (or whatever) wouldn't sell very well. That compression is converted to mass assuming a spring constant and a gravitational acceleration. That's not all that much different from other measuring devices: What is truly measured deep down in the guts of the device typically is not what is reported.

 

[rivercats]

If it's a typical bathroom scale, it'll be pre-calibrated to factor in the 9.8 to measure you accurately. Since it's reading in kg, which is a unit for mass, I would guess that the scale is a spring-based scale that actually measures weight, but factors in the 9.8 automatically to give a fairly accurate estimate of your mass. So 70 kg is your true mass.

 

[Redbelly98]

I have skipped over the latter half of this thread -- sorry, but my patience ran out at some point -- and just want to add the following: What a device measures, and what it's readout or display says, can be entirely different things. But when they are different, there is usually some understood relation between the two quantities. For example, there is a simple relation between an object's mass and the weight of that object when at the surface of Earth.

This allows a bathroom scale to measure force, yet provide a reading of a mass. So it measures your weight as 686 N, yet displays that your mass is 70 kg.

 

[xengnowteacher]

Mass confusion

Wow, was I glad to find this thread. As another teacher previously posted, there is great potential for confusion on this: a student has observed on countless food packages and other places that a given item's net weight is X lbs or ounces / Y kg or grams. My Cheerios box says "NET WT 9 OZ (255g)." They have been taught how to convert pounds to kilograms in middle school, probably without "g" in the discussion. The student then hears the teacher say in high school physics and/or chemistry that pounds and kilograms measure two different things, and that the real weight equivalent of a pound is a Newton. Uh-oh...

The explanations (and questions) here have been great, thank you to all, and I think will help me guide students through this. While it is all simple enough once understood, I think extra care is needed with students, as they often tune out when what is being taught seems to conflict with their prior understanding.

 

[cowls192]

I'm going to try my best to clear up everything ppl said in this thread.

SUMMARY:
The "bathroom" scale MEASURES weight, in Newton (N). The same "bathroom" scale is PROGRAMMED to do the magic calculations, and DISPLAYS on its display the MASS of whatever is on the scale.

For example, Jimmy stands on the scale. The scale measures the weight, of about 686 N. The scale does not display 686 N, not even 686. It looks up its program, and the program says "divide this Newton value by 9.80665". Then, after doing the calculations, the scale will display, or "tell Jimmy" (if that sounds better for some folks) roughly 70 kg. In fact, it won't even say "kg", because the unit "kg" or "lb" is often PRINTED on the scale, whether the scale is turned ON or OFF.

More explanations:
What the bathroom scale really measures is the FORCE, or simply PRESSURE, applied to its spring. If you look up wikipedia on "Mass versus weight", it will also explain about balance-type scale, which is not affected by the gravity. What I'm saying is, if Jimmy steps out of the scale, and instead, press down the scale with his two hands, it will display higher "mass" than his hands, because this number value is ultimately the gravitational force applied to the spring. In other perspective, if Jimmy is standing on the scale and it says "70", and his friend Nick stands right behind Jimmy, holds Jimmy's shoulders and push him down, his "weight" may be higher than 70 because the scale simply measures whatever pressure is applied to the spring.

Like mentioned earlier, the scale assumes many things. Some of which are:
(1) Your whole body is on the scale,
(2) You are not "pushing yourself down" on the scale, (i can't imagine how to do so)
(3) You are on Earth, (and the gravity of Earth is 9.80665 and so forth)

After these assumptions, the story about Moon is that when Jimmy actually brings that bathroom scale to the Moon, we also assume that Jimmy didn't change the program inside the scale. So the scale will still calculate with 9.80665, which is a value valid for Earth, not on Moon. And if Jimmy were to really "weigh" himself on the scale, the scale will "measure" about 1/6 of 686 N, or just tell Jimmy 11.66 kg. Yes, this is an inaccurate "mass" of Jimmy, since mass is supposed to remain same regardless of gravitational force.

Language, language. You have to know the difference between "mass" and "weight". If you do, you are in "physics" mode. In commercial / everyday usage, people use both of these terms INTERCHANGEABLY. I mean, how else can we say "Let's measure your mass"? Most people will commonly say "Let's weigh your (weight)". I've never heard ppl saying "Let's mass your weight" or anything in that fashion. The thing about "net wt." is an example for such usage. In linguistics terms, it's all about prescriptive grammar and descriptive grammar. Physics' definition of mass and weight are "mispracticed" by common population. So, when the package says "Net wt. 10 oz", it really means the "physical mass" of the product (without any packaging).

The thing about kg versus lb is that lb (or pound) has been used interchangeably again, with pound-mass and pound-force. Bottom line is that when ppl use the word pound to mean "body weight", as this is specifically how the thread started, the number is really pound-mass that is already calculated for that purpose (whether from the doctor's office or a device purchased from a store).

 

[Nugatory]

I don't see why you would multiply it by 9.8 when gravity was already factored into give you the reading of 70kg. That's why I was saying that it would make more sense if we would say 70N.

The Earth's gravity will pull on a 70 kg mass with a force of 686 Newtons.
The spring scale is carefully designed (usually by printing the right numbers at the right points on the dial) so that when a force of 686 Newtons is applied to it, the indicator will point to the "70". Then you print the letters "kg" on the dial... and you have a scale which reads "70 kg" when a force of 686 Newtons is applied. Thus, it accurately reports the mass as long as we stay within the Earth's gravitational field.

 

[D H]

Wow, was I glad to find this thread. As another teacher previously posted, there is great potential for confusion on this: a student has observed on countless food packages and other places that a given item's net weight is X lbs or ounces / Y kg or grams. My Cheerios box says "NET WT 9 OZ (255g)." They have been taught how to convert pounds to kilograms in middle school, probably without "g" in the discussion.

There is no g in the discussion. The pound is a unit of mass. The pound mass is defined as exactly 0.45359237 kilograms. One source of confusion with customary units is that the pound is also a unit of force. The pound force is defined as exactly 0.45359237 kilograms * 9.80665 m/s².

The student then hears the teacher say in high school physics and/or chemistry that pounds and kilograms measure two different things, and that the real weight equivalent of a pound is a Newton. Uh-oh...

That's another source of confusion, one created by scientists. The use of the word weight as a synonym for what we now call mass goes way back, back before the word mass became a word in English.

I blame it on the French. The French are, for better or for worse, the keepers of the International System (SI) of units. The French also have a standardized vocabulary. If the official meaning of a word is changed, the people had better pay attention to that new meaning. Fines can result if they don't. The official meaning of weight changed in 1901 from a synonym for mass to a kind of force. English has a much better approach: Invent a new word. We should have told the French thanks but no thanks, but we'll call it grue or some other brand new word. But we didn't, and now we have this scientifically invented controversy between mass and weight.

 

[namanjain]

I am still in doubt.

What does a scale read when pushed by hand against the wall?

Any hint?

thank you

it gives you:


the normal exerted by you
__________________________________ (it's division)
g

 

[cowls192]

Here's another good example of "misuse":

"Karly lost 20 pounds of weight in 3 weeks!"

Well, we physical folks know that Karly really lost her "mass".

Source: OpenStax College textbook (http://cnx.org/content/col11406/1.7)

 

[D H]

That is not misuse. It's being clear. How else are we to know that Karly didn't lose ₤20 from her wallet?

In lay usage, weight and mass have been and are synonyms. Weight is a much older word than mass -- and it meant "mass" before "mass" was a word in the English language. Words have multiple meanings, particularly in lay usage. There is absolutely nothing wrong with using weight to mean mass in a lay context.

The misuse is in texts such as the one cited above that try to teach us that that old, old usage is wrong. In English, scientists do not get to dictate what is and is not proper usage of an old, old word in a non-scientific setting. That can happen in other languages such as French, but not in English.

 

[PhanthomJay]

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. Instead, such devices for measuring forces are calibrated in Newtons in the SI countries, or pounds in the USA.

 

[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