why bathroom scales LIE!!!

[Spaz Funbag]

actual Title (too long):
The ultimate Theory of Everything or why your bathroom scales always show you more weight than you could "possibly have".

Many people complain about the fact that their scales tell them a weight "that cannot possibly be".

My explanation: as you step onto the scales, springs inside compress a bit, so you sink down -> gaining speed. Now as we all know, an increase of speed always results in an increase of mass. Therefore, the force downwards is greater, pulling you down even stronger. Thus, you go down even faster., gaining more mass in the process. Now you are stuck in a loop of accellerating and gaining mass.




Eventually you will go through the floor.




OK, now I am well aware of the fact that this is not the case. It's just that I have been carrying this around with me for some months now and want to know your opinion/talking about it.

Second, more serious question: is my train of thought right? I mean, that I would "weigh more" when moving faster? Mass increases, but is it still inertial mass = gravitational mass?





If this is considered spam or useless talk or OT feel free to tell em so and/or rename, move or delete.

[Alisto]

Whatever you're standing on is exerting some normal force. (In the scale example, you also have a spring force) If you do not gain mass quickly enough to overcome whatever force the surface can apply, you will decelerate as the surface deflects under you.

Now by this, you decelerate and lose mass, which means that the normal force ends up pushing you up. Then you gain mass and so on, so you're oscillating on an incredibly small, undetectable level.

Or something.

[3.14159265...]

Sum this: 1 + 1/2 + 1/4 + 1/8 + 1/16.......

You keep adding, but the final answer is finite.

Welcome to the idea of convergence.

Also the effect of gaining speed, is TINY, really TINY, as relativistic mass doesn't come into play till you are a good fraction of the speed of light. So not much affect, but I guess you can make some fat ppl happy

[Berge]

You can watch the needle on your scale (if its not digital) as you first step on with a good bit of force. It swings back and forth around your real weight just as described above. But, where it settles is what you actually are.

[cmacis]

I think I heard that it makes a difference if the scales are on a hard floor or carpet because of the dampening of the carpet.

Anyone care to test this?

[Walking Dave]

I think I heard that it makes a difference if the scales are on a hard floor or carpet because of the dampening of the carpet.

Anyone care to test this?

I have, and 'tis true. On my well-padded carpet, my digital scale is off by a full third.

[3.14159265...]

Also you can lose weight by going to the places near the equator

[DonChubby]

No you wont weigh less, however the gravitational force on you is less, since you are further away from the center of the earth.
So, practically, you weigh less, but your mass wont decrease

[skeptical scientist]

You can watch the needle on your scale (if its not digital) as you first step on with a good bit of force. It swings back and forth around your real weight just as described above. But, where it settles is what you actually are.

I knew this was so, I just never realized it was because of the effect of relativity changing your weight because of your speed... I always thought it was just because the springs inside the scale made natural oscillators. I guess you learn something new every day...

[3.14159265...]

I don't think it is...

[Gordon]

It's not.

Edit: Here ffs
http://home.howstuffworks.com/inside-scale.htm

Edit 2: I thought this looked familiar. I had a question on a test first year that went like this:

Estimate the difference between the true mass of a person, and the mass of a person as measured by a bathroom scale due to the bouyancy of air. The density of air is 1.2kg/m^3 at room temperature and pressure.

This class was actually a lot of fun, here's the exam for anyone who cares
http://web2.uwindsor.ca/courses/physics ... 51-05f.pdf
Solutions
http://web2.uwindsor.ca/courses/physics ... 1tfa05.pdf

[Alisto]

I think I heard that it makes a difference if the scales are on a hard floor or carpet because of the dampening of the carpet.

Anyone care to test this?

I have, and 'tis true. On my well-padded carpet, my digital scale is off by a full third.

This doesn't make any sense.

[Gordon]

I think I heard that it makes a difference if the scales are on a hard floor or carpet because of the dampening of the carpet.

Anyone care to test this?

I have, and 'tis true. On my well-padded carpet, my digital scale is off by a full third.

This doesn't make any sense.

Yes it does. The scale and yourself make a system, the carpet absorbs weight better than, say, a tile floor. This is the reason they're called bathroom scales, but having a hard surface (as is usually found in bathrooms) gives you a better estimate of your weight than on a carpet.

[SpitValve]

Basically, the carpet is an extension of the springs in your scales. You can treat it as just having a looser spring underneath - basically meaning it's not calibrated right.

[Solt]

Oscillations in the spring? lol!

You might have extra "weight" when you first step on the scale because of the impulse that the scale's upper surface imparts onto you in causing you to slow down, which translates to extra momentary normal force on the spring. But that extra force disappears the moment you stop moving, and the scale eventually settles on your true weight.

Errors may come from the permanent plastic deformation of the spring over time (springs aren't perfectly elastic!) thus leading to a loss of calibration. Also, the force response of a real spring is not perfectly linear, meaning it is probably less accurate on the extreme ranges, maybe 5% off. More likely is that it is possible for your weight to fluctuate over a period of days because you are constantly inputting/outputting mass in the form of water, food, urine, and feces.


According to Einstein you do indeed gain mass when you speed up, but this effect is ridiculously small at the speeds that we deal with on earth (not considering sub-atomic particles). The effect is so small that there's no real point in accounting for it in engineering applications. Perhaps a physicist could tell us to what order of magnitude we'd need to know the mass of, say, the space shuttle to be able to detect this change at 28,000 km/h, the fastest any man made object has ever gone. I bet it's really small, and when it comes to a scale, the weight of the random impact of air molecules on the surface is much greater.

[Alisto]

Basically, the carpet is an extension of the springs in your scales. You can treat it as just having a looser spring underneath - basically meaning it's not calibrated right.

This is one of those times where mathematically it works out, but intuition says no.

Things like this drive me crazy.

[miles01110]

No you wont weigh less, however the gravitational force on you is less, since you are further away from the center of the earth.
So, practically, you weigh less, but your mass wont decrease

Uh, actually you do weigh less. Weight is defined as your mass times the acceleration of gravity. Since the acceleration of gravity is less at the equator by virtue of being further away from the Earth's center, you weigh less at the equator than you do at any other latitude. Your mass does not change, however.

[Gelsamel]

Wel...

1) unless you're going at extremely fast speeds it's really negligible.

2) the Spring's force "grows" faster then your mass "grows" so it isn't a never ending cycle... ie. the Spring Wins.

3) Because the Spring wins it slows you down, once the scale stops bouncing up and down for a bit you will have stopped! Try it, you'll notice in the end you'll no longer be moving, your weight force and the springs force will be in equilibrium and sum of forces will be naught.

This has all probably been said but I didn't read any of the previous posts. Sorry if I reposted stuff.

[Alisto]

Damn it, I was right.

Springs in series all exert the same amount of force. If we model the scale as a spring (and we determine weight by measuring the change in length) and the carpet as a spring...

Scale on rigid surface:

W = kx

Scale on carpet, modeled as a spring with coefficient k2:

W = kx = (k2)x

The scale is in an identical state in both cases. The scale should not register any difference in weight.

I knew I was right. Screw you guys for making me doubt myself. :p

[miles01110]

Damn it, I was right.

Springs in series all exert the same amount of force. If we model the scale as a spring (and we determine weight by measuring the change in length) and the carpet as a spring...

Scale on rigid surface:

W = kx

Scale on carpet, modeled as a spring with coefficient k2:

W = kx = (k2)x

The scale is in an identical state in both cases. The scale should not register any difference in weight.

I knew I was right. Screw you guys for making me doubt myself. :p

What? Springs in series are weaker than the individual springs, while springs in parallel add.

[DonChubby]

No you wont weigh less, however the gravitational force on you is less, since you are further away from the center of the earth.
So, practically, you weigh less, but your mass wont decrease

Uh, actually you do weigh less. Weight is defined as your mass times the acceleration of gravity. Since the acceleration of gravity is less at the equator by virtue of being further away from the Earth's center, you weigh less at the equator than you do at any other latitude. Your mass does not change, however.

Touché

[Alisto]

What? Springs in series are weaker than the individual springs, while springs in parallel add.

You're half right. If you're calculating a total k for all springs in the system, springs in parallel add. Springs in series would be (k1*k2)/(k1+k2)

But we don't care about finding an equivalent k, we only care about what happens to the scale itself. No matter what we do to the surface beneath the scale, the scale itself still has to support the full weight of whatever is on top of it. The scale will read the same weight no matter what.

[Tchebu]

No you wont weigh less, however the gravitational force on you is less, since you are further away from the center of the earth.
So, practically, you weigh less, but your mass wont decrease

It will if you actually WALK all the way to the equator...

[3.14159265...]

The weighing less at the equator thing, I meant it as a result of the the centripetal force. We did it in a physics class once.

[hotaru]

The weighing less at the equator thing, I meant it as a result of the the centripetal force. We did it in a physics class once.

wouldn't a centripetal force make you weigh more, not less?

[Fluff]

actual Title (too long):
The ultimate Theory of Everything or why your bathroom scales always show you more weight than you could "possibly have".

Many people complain about the fact that their scales tell them a weight "that cannot possibly be".

My theory:

That's what the scale says when you've been eating too many pies.

[Gordon]

No matter what we do to the surface beneath the scale, the scale itself still has to support the full weight of whatever is on top of it. The scale will read the same weight no matter what.

Stop posting in this thread.

[Alisto]

No matter what we do to the surface beneath the scale, the scale itself still has to support the full weight of whatever is on top of it. The scale will read the same weight no matter what.

Stop posting in this thread.

I can interpret this in two ways:

1. The whole thing was a joke, which I am ruining through my failure to understand it.

2. You think I'm wrong and want me to shut up.

I certainly hope it's not 2.

[Hawknc]

Alisto, the "springyness" of the carpet absorbs some of the force of your weight. A hard surface has much, much lower static displacement and thus results in nearly all of the force being absorbed by the springs in the scales.

[Alisto]

Ok, you guys are just messing with me.

Bastards.

[miles01110]

Ok, you guys are just messing with me.

Bastards.

No, actually we aren't. Your misunderstanding of the physical system is flawed at a very fundamental level. This isn't to say that you should stop posting in the thread, but if you keep posting things like this:

But we don't care about finding an equivalent k, we only care about what happens to the scale itself. No matter what we do to the surface beneath the scale, the scale itself still has to support the full weight of whatever is on top of it. The scale will read the same weight no matter what.

Then your stock is going to decrease. Rapidly.

In response to the above passage, imagine that instead of a carpet you put the scale on a tub of water and step on it. It's the same situation carried to an extreme. Since you will be applying force on top of the scale and the scale in turn applies force onto the water, the scale will register almost 0 weight because the water does not exert a normal force back onto the scale. With a carpet it's the same thing, except the carpet does exert a normal force back onto the scale. However, the springs in the carpet fibers absorb some of this force, biasing the measurement of how much you weigh.

[Alisto]

Ok, let's back up. Let's say we have two springs in series with a mass resting on top. Does F = (k1)(x1) = (k2)(x2)?

[3.14159265...]

The weighing less at the equator thing, I meant it as a result of the the centripetal force. We did it in a physics class once.

wouldn't a centripetal force make you weigh more, not less?

No no, think about it, your constantly thrown OUT of the earth.

[Hawknc]

Technically speaking it's your inertia that makes you weigh less, centripetal force keeps you on the ground weighin' something.

[3.14159265...]

The weighing less at the equator thing, I meant it as a result of the the centripetal force.

[miles01110]

Actually I thought this through some more with a friend.

The carpet absorbs some of the normal force exerted back on the scale by the floor, causing the scale to read less.

[Jach]

I find this thread fascinating..

Anyway, wouldn't the FDA or some other government agency make our scales read less so we think that we're lighter?

[skeptical scientist]

Come on guys, please. It doesn't matter what kind of a surface the scale is on, it will read the same. It is not moving, so the total downwards force is equal to the total upwards force. The total downward force is your weight plus the scale's weight, and the total upward force is the same. It makes no difference what surface you're on; the forces still have to balance. Stop messing with Alisto already.

[miles01110]

Come on guys, please. It doesn't matter what kind of a surface the scale is on, it will read the same. It is not moving, so the total downwards force is equal to the total upwards force. The total downward force is your weight plus the scale's weight, and the total upward force is the same. It makes no difference what surface you're on; the forces still have to balance. Stop messing with Alisto already.

No, you are wrong as well. Think the problem through and you'll arrive at the same conclusion.

[Gordon]

I can't tell if sketpical is being sarcastic, I hope so.