Re: Centrifuge in "World Book"

[John Clement <clement@HAL-PC.ORG>]

Heavier implies greater weight.  It is the less dense substance that comes
to the top, and the more dense goes to the bottom. Note that the solids
Styrofoam and pumice will both float to the top.  This description
reinforces the common student misconception that light things float and
heavy things sink.

By the way the definition of weight in the same encyclopedia defines it as
the force due to gravity, and then proceeds to say that the SI unit of mass
the gram and kg are used as the units of weight.  It does not mention
Newtons.  Now granted, there used to be scales that measured weight in g and
in N, but this terminology has been abandoned.  The gram weight, meaning the
weight exerted by a gram of substance at the surface of the earth, is
inaccurate, and extremely confusing to students.  I realize that there is a
debate over how to define weight, and that this has not been resolved in
this list.  However, if an encyclopedia uses a particular definition, then
it should stick by it and be consistent.

John M. Clement
Houston, TX

> I do not see what is wrong with the quoted description.
> A tube in a centrifuge is positioned in such way that its
> top is closer to the axis of rotation while its bottom is
> further always from that axis. What am I missing?
> Ludwik Kowalski
>
> John Clement wrote:
>
> > After perusing the article on centrifugal force in the Wold Book
> > Encyclopedia, I glanced at the centrifuge article.  It says it
> "causes the
> > heavier liquid or the solid particles to move to the bottom of the
> > container, leaving the lighter substances on the top."  Don't
> they have any
> > good scientists on their staff to proofread this stuff!
> >
> > One wonders how any student can learn science when this sort of
> nonsense is
> > published in both the texts and the encyclopedias.
> >
> > John M. Clement
> > Houston, TX