My nephew is 3 and weighs around 30 pounds I am guessing. However, I would like to weigh him at home. I have kitchen scales, one flat one that goes up 1kg and one that looks like this picture that goes up to 5kg. enter image description here

What is a safe and practical way I can weigh my nephew at home?


closed as off-topic by ACuriousMind, Kyle Kanos, tpg2114, Bernhard, Jim Sep 1 '14 at 19:31

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    $\begingroup$ have a bathroom scale? $\endgroup$ – Jim Sep 1 '14 at 19:12
  • $\begingroup$ If so, weigh yourself without him. Then weigh yourself holding him. Subtract the two numbers $\endgroup$ – Jim Sep 1 '14 at 19:13
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    $\begingroup$ related: physics.stackexchange.com/q/70839/23473 $\endgroup$ – Jim Sep 1 '14 at 19:15
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    $\begingroup$ Take a strong piece of wood and a piece of PVC pipe underneath. Use your scales to fill a water bucket to 5 kg. Put nephew on one end, bucket on the other. Roll the piece of wood along the ground (using the PVC pipe as a "wheel") until the system is balanced. The ratio of distances multiplied by the weight of the bucket is the weight of the nephew. Measuring these distances accurately now becomes the main source of inaccuracy. The longer the piece of wood, the greater the accuracy. $\endgroup$ – Floris Sep 1 '14 at 19:54
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    $\begingroup$ The obvious method is of course to divide your nephew into small enough pieces that you can weigh each of them on the 5 kg scale and then sum the weights. Due to health reasons, I normally wouldn't recommend this method, but maybe it's ok since you mention that you're not attached to him. $\endgroup$ – jkej Sep 1 '14 at 20:01

I can think of two simple ways.


The first would be to eschew the scale altogether and build a rudimentary scale with a stick and a pivot, and balance out your nephew's weight with a bunch of water in some buckets, or 2L bottles. Knowing the density of water, you could equate the weight of your nephew to a volume of water. If you are interested in correcting for the weight of the buckets, you could weigh those using your available scales. Or you could use a collection of solid objects and weigh these in turn on your existing scales.

In the same spirit, with access to stick and pivot technology, along with a measuring stick, you could just make the lever arm on the side opposite your nephew a factor of 6 times longer (7 to be safe), and then find some objects around the house to balance out this rudimentary scale. That collection of objects (or water again) could be measured on your existing scales. Knowing this mass, and the ratio of the lever arms on the two sides of the scale, you'll get a weight for your nephew.

The major source of error in both cases is likely to be the uncertainty in the location of the center of mass on both sides. I reckon you could get this down to about an inch or so without too much trouble, which would equate to an error in the mass of around 3% if your lever arm on each side was around 3 feet. Not perfect but decent. With less certainty in the position of the center of mass, or care taken, I would expect an error at the 10s of percent level.

Sock scale

In the interest of science, I'll also report a method that I don't recommend. I was trying to think of an elastic medium that you would have access to in your home, which you could use to weigh your nephew incrementally. If you could imagine calibrating a single spring to a small mass, then you could use many such springs to measure your nephew. What spring-like material does everyone have access to many copies of? Socks! So, in the interest of science I took off my two socks and tried to see if socks have a hookian enough response to be useful.

Experimental setup:

Experimental Setup

I used two socks, a binder clip to secure the weights to it, a pen, some string, and multiple 20 fl oz ( = 1.3 lbs of water ) bottles, and tried to see if the response was hookian over several bottles. I got

1 bottle  -- 1.5 distance units
2 bottles -- 2.8 distance units
3 bottles -- 3.9 distance units

I'm being vague about the distance units because I didn't actually have a ruler handy, so instead used the L scale on my sliderule to measure the extensions

Rendered as a plot we see:

socks fit

While this looks decent, I'm troubled by the fact that it doesn't line up well with the zero point extension of the sock, also we were only able to take 3 measurements since with the addition of the 4th bottle, the binder clip gave way.

Regardless of the questions of hookian reliability of the socks, to weigh your nephew at around 30 lbs, you would need something like 25 nearly identical socks, or calibrate all of them individually, and figure out a way to reliably afix your nephew to the socks. Due to the impracticality of the method, I can't recommend this approach, though in the interest of science, and so that others need not follow in my footsteps, I've shared this failure here.

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    $\begingroup$ I suppose if you have the time and money to build a set of scales, you could always go out and buy a bathroom scale $\endgroup$ – Jim Sep 1 '14 at 19:29
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    $\begingroup$ I think we need to see the graph as a function of the number of socks as well. Do you have several similar socks available? $\endgroup$ – Lembik Sep 1 '14 at 20:44
  • $\begingroup$ @Lembik Lol, I'm not sure if you're being serious? Funny question anyways :) $\endgroup$ – Danu Sep 1 '14 at 20:46
  • $\begingroup$ @Lembik at work I only have two. My binder clip won't allow me to attach more than a single bottle to a single sock. I would require better affixing technology. $\endgroup$ – alemi Sep 1 '14 at 20:46
  • $\begingroup$ @alemi glue? staples? paper clips? Any similar and common office supplies? $\endgroup$ – Jim Sep 1 '14 at 20:49

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