I went to the chiropractor today and they made me stand on this contraption which was basically a big scale with 1 scale for each foot (i.e.: you simultaneously place each foot on different scales). Not being a fan of weighing myself, I am not too sure as to what my exact weight is, so I had to ask the chiro if my total body weight was the sum of the two numbers on each scale? He said no, it wasn’t. I’m hoping he’s right, and falling back on my college physics classes I suspect he is, but I don’t trust him (not a fan of chiros either). On the odd chance that his scale defeats the laws of physics though, I need to go on a diet!
So I guess my question is: What are the physics behind this? Why is you apparent weight higher (sum of the two scales) if you have a foot on different scales, than if you have both feet on the same scale?
Assuming that the chiro didn’t us a chainsaw to sever you perfectly in half, each scale is weighing more than 50% of your body weight. Your right arm is still attached to your torso so it’s weight will still register on the left foot scale and vice versa, for example.
The device is used to determing how balanced your body is, I believe. In theory if you weigh more on one side than the other it means your spine/hips/neck are out of wack and need adjustment.
For actual physics calculations you’ll have to wait for some more knowledgeable doper than me.
Physics was a long time ago for me, but for the reference of yourself and others is this the device in question? If so it seems to be to basically be the chiropractic equivalent of ‘the machine that goes ‘bing’’ rather than something I’d use to accurately report my weight or any other phsyical measurement.
Poking around on that site might give you a few hints on dealing with chiropractors as well. Not saying that this particular individual isn’t on the level but that it doesn’t hurt to know something about what he or she may be proposing.
Well I am not so much interested in the field of chiropractic sciences itself. The whole “being at the chiro” thing was just the my story’s setup as to what led me to wonder about the physics behind weighing oneself simultaneously two scales. My thoughts on chiropractors is a whole other matter altogether that would be more suitable for the Debates portion of this mesage board. But thanks for the link, I will check it out!
Uh, no. The scale only measures the force pushing down on it. There is no way you can produce a force greater than your weight merely because you are standing on two different measuring devices. The sum of the two scales must equal your body weight (unless the scales are calibrated weirdly).
As good a chiropractor as this person may or may not be, I wouldn’t exactly trust their grasp of physics. The sum of the weights on the two scales is your weight.
The above posters are correct. The two scales probably won’t each weigh one-half of the total, but the total will be your actual weight.
I hear the Atkins diet is surviving scrutiny pretty well.
BTW; 230 lbs.
Peace,
mangeorge
Or that your wallet, which is carried in one hip pocket or the other, is too heavy.
Apparently a good rule of thumb for measuring the quackiness of a chiropracter is to ask him whether germs cause diseases. But if he has properly calibrated scales and he says that the two figures don’t add up to your total weight, it could also be a warning sign.
I don’t know anything about chiro-tools but I do know about scales (having been a former ‘technican/service engineer’ at a scale shop). Two scales will sum to your total weight.
The only reason they might not is because the (your) load isn’t directed exactly down but instead a bit to the side. Ideally, this shouldn’t matter but there are physical limitations to lateral loads a scale can handle.
If the two scales were sloped, so that you are pushing out sideways on them, then they would indicate more weight. For example, if they were each sloped inwards at 45 degrees, so that you had to push outwards as well as down to stand on them, then they would together indicate 1.4X your weight.
Ok, so how come if I weigh myself on my scale at home, I weighed 15 pounds less than on that chiro thingamajigger? How? Why? Grrrrrr!!! In light of this fact I can either go with one of the following scenarios:
The chiro’s “scale” was not acurate and could not be considered *sensu stricto * a true scale.
or
The sum of what you weigh by placing each foot on seperate scales must be greater than what you weigh on a single scale.
In the unlikely event that the chiro’s scale IS right, AND that the sum of those two weights equals my true body weight, then I am definately becoming anorexic.
I think this whole weight issue is a good one for Cecil. Also, my point about all this question was not to discuss chiropractors or their charlatanic practises. All I am interested in is this weight issue!!
It’s probably a combination of measurement errors. 1) Typical bathroom scales are not terribly accurate. Typically +/- a few pounds. 2) Using two scales to make one measurement like your chiro did introduces a greater error than using only one scale. 3) Your chiropractor is a boob. Assuming no measurement error, the sum of the readings on the two scales will exactly equal your true body weight.
It’s either 1) or 3) your home scale is not accurate. It cannot be 2). It also may be a combination of 1) & 3), i.e. your home scale is light and the chiropractors scale is heavy.
And as jnglmassiv pointed out, most scales are designed to handle only vertical loads. If standing on two scales forces you to include any sort of horizontal force, that would probably throw the scale off. This is why standing on two ordinary bathroom scales next to each other might give you an odd reading.
Regardless, why are you obsessing over a number? You should really just worry about how you look and feel. Are you happy with how your clothes fit? Then screw the scales. If you’re not happy, go ahead and watch your calorie intake and add some exercise.
This is a slight shift from the OP, but close enough I think.
When scaling a race car, you place a scale pad under each wheel. This lets you know the corner weights along with the percentages of weight from side to side, front to rear, and cross weight. Naturally totaling the four corners gives you the total weight of the car. This seems to be similar to using two scales to weigh a person. In other words, it is your total weight if the scales are level and properly calibrated.
One of the problems is that very few people know how to “zero” their el-cheapo home scale.
99.99% of people turn the zero adjustment dial until the pointer is lined-up with zero. This, of course, is done when nothing is on the scale.
This is not a good way to zero a scale. This is because there tends to be quite a bit of non-linearity error from 0% to 5% of full scale. The worst non-linearity error is usally at 0%. Why make the adjustment at the worst place on the scale?
A much better way to zero a scale is to apply a known weight to the scale, and then adjust the zero adjustment dial until it matches the known weight. Ideally, the calibration weight should be in the range of what the scale is typically asked to measure (150 lbs or so). But anything (even 20 lbs) is better than zeroing at 0 lbs. Note, however, that if you do this, the reading at zero will likely be off. But who cares? It will be more accurate when measuring your weight.
First, I wouldn’t waste my time or money on a chiropractor… especially one that uses two scales.
I weight the same 175 lbs. that I did when I got out of the USAF over 45 years ago. (my uniform still fits) And most of those years were spent on some kind of diet. I never could let myself get too many lbs. over weight… it always has been a battle.
That’s exactly the thing to do. Who cares if the scale measures 0 lbs accurately? Whether they know it or not, the user is interested in loaded accuracy.
As an anecdote: I mentioned I once worked for a scale shop. I’d go around to mostly industrial customers calibrating, installing and servicing counting, lab, and pallet scales, among other types. Doing this kind of field service really gives some insight into some misconceptions people have about this stuff.
Anyway, I once went to a doctor’s office to verifiy their Healthometer beam scale. This is the type most people are familiar from their doctor’s. White with sliding pois and a height measuring bar. These are honestly some of the least accurate devices I ever encountered on the job. Most of the parts are stamped steel and there’s a long rod from the bottom to the top just begging to get bent, flexed or misaligned. Shifting the load a few inches throws the reading off by 10’s of pounds. Don’t even get me started about parallax errors.
Anyhow, this particuar scale had something wrong with it. Probably a bent part or sticky joint. The doctor didn’t want to pay to fix it; he was really just interested in a fresh calibration sticker. I did as Crafter_Man said: put 150lbs and corrected the error at this load. I put the sticker on (improper, really), got my signature and left. I was still in the parking lot when my boss calls me (Beep-beep…Nextel). The nurses and doctor were irrate, go back Right Now!
They wanted the scale to read zero when the pois (sliding weights) were to the left…no matter what. I tried explaining why this was about the worst thing but it was no use. I took the sticker off, zero’d the unit and left angry. I caught quite a bit of hell back at the office but it really was the right thing to do.
Two most common reasons for service calls:
Debris under the pallet scale…sometimes huge chunks of 2x4 or large skid fragments, visible from 50’ away. Often, the deck had been pushed against the wall, too.
The power strip the scale is plugged into had been turned off. That’ll be $198, please. Sign here.
I know I’m gonna get yelled at for saying this, but accuracy in a personal scale really isn’t that important at all. Even the “ideal weight” charts give a range of at least a few pounds. It makes no difference if the scale indicates 115 or 110 lbs, as long as you just use it to keep track of gain or loss.
The OP is acting all nutzoid about numbers, but what’s really important is whether or not you’re at a weight that’s good for you, and where you think you look good. What, did you think you came home from the chiropractor suddenly 20lbs heavier?
Anorexia, indeed. Don’t be silly. If it’s friends opinions you worry about, pick an ideal weight (one that you like) and tell everybody that’s what you weigh.
Anorexia indeed… Obviously missed the sarcasm on that one, people!
Having a thyroid problem, being a few pounds overweight has always been an issue for me. So naturally, not weighing myself was a good way to not deal with it (anyone who was a slightly overweight teen can relate to that one). Of course, those years are far behind me now but I have kept the habit of not weighing myself. Thinking, albeit wrongly, that if I don’t know my exact weight, then I won’t weigh as much. Make sense? Anyway, I am not obsessed with being a waif, nor am I really going to purposely give myself an eating dissorder. I was just a little curious since my weight at the chiro was so much more than what I thought it was.
Incidently, I weighed myself on our “good” scale at home, and turns out that the scale at the chiro did have about 20 pounds excess of my weight. Their scale is most likely not that well balanced, and also I was fully clothed (and “booted” at the time).
Well that’s probably more info than you guys needed to know but I thought I’d clear my name, so to speak, about making myself seem like a superficial, weight-obsessed person. The actual number on the scale means nothing to me.
