Date: Fri, 19 Feb 1999 01:35:48 -0800 (PST)
Message-Id: <l03102803b2f23f59bdc3@[209.142.19.163]>
In-Reply-To: <199902180809.AAA20999@mail.inreach.com>
To: pinhole@exploratorium.edu
From: Ronald Wong <ronwong@inreach.com>
Subject: Re: converting grams to newtons
Amy said:
>My 8th graders are studying forces, and doing various labs using spring
>scales. All of our spring scales measure only in grams, but I would like them
>to be able to record their answers in newtons. Could anyone help me out with
>how to do this conversion? Thanks!
If your students can live with an error of 2%, then just have them divide
their answers in grams by 100 and they'll have the approximate value in
newtons. Mechanically, all they have to do is move the decimal point over
to the left two places.
If you need a little more precision, then the factor that is commonly used
is 0.0098 newtons/gram instead of 0.01 newtons/gram. The assumption is that
the object is at "sea level".
------------------------------
Things to Keep in Mind - I:
Technically, grams are units of inertia and newtons are units of force.
Inertia and force represent two different quantities and, in general, can't
be compared to one another.
At a given location, the two are proportional.
Unfortunately, the proportionality factor differs from place to place. The
factor is greater at the poles at sea level (0.00983 newtons/gram) than at
the equator at sea level (0.000978 newtons/gram) and is less at the top of
the TransAmerica building than at street level. Anywhere inside a hollow
earth it would be *zero*.
So much for a systematic way of equating a mass with a force.
------------------------------
Things to Keep in Mind - II:
The fact is that spring scales do NOT measure mass. They measure force.
If you were standing on the earth and placed a 100 gram mass on the end of
your scale and adjusted the scale so that it indicated 100 grams and then
went to the moon, you might assume, based on the idea that the scale is
measuring mass, that the scale would still show a mass of 100 grams. It
won't. On the moon the pointer of your scale will indicate that the mass is
now approximately 17 grams.
What has changed of course is not the mass but the weight of the mass. It's
the weight of the mass that the scale is measuring and not it's actual
mass. Weight involves a gravitational force and this force has changed by a
factor of 1/6th as the mass was transported from the earth to the moon.
------------------------------
Whoever purchased the scales for your class didn't realize that these same
scales are available from the vendor with the correct units of newtons on
them.
You might consider having a class activity where your students create the
correct scale for your scales and tape them over the old scales. If one
accepts an error of 2% this simply means that 100 grams is 1 newton, 200
grams is 2 newtons, etc. A small bit of tape to cover the zeros is all that
would be necessary in this case.
This may sound a little cavalier, but you'll find that the percentage of
error in many, if not most, lab activities is frequently greater than this.
Pedagogically, it's never too soon to introduce students to the idea that
at the very root of scientific investigation is the element of uncertainty
with it's attendant lack of precision. Here's your opportunity to do just
this with your students.
Have fun - ron