🚀 go-pugleaf

Cherie "sunglasses" Lehman writes:

>I forgot to finish what I was saying about the spring scales, John.
>If you were describing the process of hanging the mass from the
>spring scale in order to determine its weight (I know you don't like
>to call the downward gravitational pull on an object weight. 8->  ),
>then I object to your calling it the object's mass quite strongly.
>Yes, yes, the 2 quantities are directly proportional as long as you
>stay in one place.  However, a spring scale calibrated in mass units
>for a particular location -- say the earth -- will not necessarily
>yield the same value at other places -- say the moon or in an orbiting
>space shuttle.  Some sort of balance is a better choice for "massing"
>objects.  A double pan balance yields the same result on both moon &
>earth.  An inertial balance yields the same result on the earth, the
>moon, or in an orbiting space craft.

Cherie,

My point was that since about 1915 we "trained" (arf!) physicists
have understood the ordinary gravitational force to be an inertial
force indistinguishable from the kind of "fictitious forces" that
appear when we are in appropriately accelerating reference frames.
Thus, when we hang a body from a spring scale, what we are, in
effect, doing is determining the force required to give the body
an acceleration of about 9.8 m/s^2.  This happens to be the
approximate "weight" (shudder) of the body when it is "at rest"
near and with respect to the surface of the earth, but from the
known force and acceleration we can easily calculate the inertial
mass of the body.  In fact, we might just as well label the scale
with the results of those calculations.  The only proviso is that
the scale must now be read ONLY when it is in the act of imparting
an acceleration of 9.8 m/s^s to the attached body (as it does when
it hangs in the produce section!)

Yes, inertial balances calibrated in mass units work just as well
on the earth as on the moon.  But notice that they depend on
"gravitational forces" to work in both places.  They don't work in
deep space, unless you crank up a litte "gravitational force" by
firing the rockets or spinning the room.  So what are they REALLY
measuring?

But back to earth...  The distinction between mass and weight IS
important and difficult for many if not most students.  I'm not
recommending that we cloud the issues for students by getting in
to all of the above.  But I would like for US to recognize the
subtleties in statements like, "Spring scales measure weight;
balances measure mass."  It may even help us do our job; it can't
hurt.

(And if you want to get some use out of those sunglasses, hop on a
plane for a day of skiing in sunny Southern California!)

Tssssssssssssss...

John

John Mallinckrodt
Cal Poly Pomona
ajmallinckro@csupomona.edu