Path: utzoo!attcan!uunet!motcid!svoboda
From: svoboda@motcid.UUCP (David Svoboda)
Newsgroups: sci.electronics
Subject: Re: Electonic scales question...
Message-ID: <4145@ash31.UUCP>
Date: 17 Jul 90 19:08:29 GMT
References: <5907@videovax.tv.tek.com>
Organization: Motorola Inc., Cellular Infrastructure Div., Arlington Heights, IL
Lines: 39

bill@videovax.tv.tek.com (William K. McFadden):
> 
> It seems that since the carpet has a springy pad underneath, the scale will
> read too low.  This is because the scale will only measure the compression of
> its internal spring, and some of that compression gets used up by the carpet.
> Now, if the scale had pointy feet on the bottom, it would have less carpet to
> compress, hence giving a more accurate reading.
>

Wait a second.  Think about this for a moment.  What EXACTLY do you mean by
"compression get used up by the carpet"?  We are talking about forces here.
As long as the bottom of the scale is not accelerating wrt the earth, the
*force* imparted on the scale by the mass of the person is constant, 
regardless of what surface it rests upon.  To be more specific, the force 
imparted is

	m(kg) * g(m/sec^2) = f (Newtons)	eqn (1)

Now, the compression upon a compressed spring is f = kx eqn (2), where f is 
in Newtons, x is compression in meters, and k is the spring constant in 
(Newtons/meter).

For a stressed member, an an identical stress force is felt by each point
or region on that member.  That's why a rope with a thin spot breaks there;
the thin spot feels the same stress as the rest of the rope, and the stress
is not "used up" by any other portion of the rope.  Likewise the region of
stressed material, including the spring and carpet, feels a constant force
throughout.  Now, since the compression of the spring is proportional to the
*force* on it (2), the reading on the scale will be correct regardless of the
surface it rests on, since the reading is proportional to the spring
compression.

I do not submit a reason for the scale not to work on carpet; it has me
puzzled.

           Dave Svoboda, Motorola CID, RTSG, Arlington Heights, IL
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