🚀 go-pugleaf

   It's early in the morning and I still have another final exam to
type up, so I'll give an answer to the first of Michael Harwood's
questions (and I think the answer is correct); I'll give an opinion
about the second one, and I'll skip the one on why stressed plastic
is white (although we'd never have had Dymo labels without that).

1. What is the difference between centre of mass and centre of gravity?
   (preferrably withexamples of the difference if there is one).

   When describing certain phenomena, it is useful to define a point at
which all the mass of an extended object (or collection of objects) seems
to be concentrated.  This is the center of mass (CM).  There is a
definite mathematical rule for computing where this point is, and the l
ocation depends only on the shape and mass distribution of the object.
When describing certain kinds of phenomena, it is usefel to define a point
at which the gravitational force on an object or collection of objects
appears to be concentrated.  This is the center of gravity (CG).  There is
also a definite mathematical rule for computing where this point is, and
it depends not only on the shape and mass distribution of the object but
also on the nature of the local gravitational field.  In a uniform field,
such as that found near the surface of the earth, the CM and CG are at
the same point.  If the field is not uniform, then the CM and CG will be
at different places.  For example, in the extremely nonuniform field near
a black hole, where the "field lines" converge toward the hole, the CG
would be closer to the hole than the CM.

2. The formula for calculating %error is
        (measured value - accepted value)/ accepted value    x100%
   They had to calculate the total torque on a mobile (made of wooden blocks
   and fishing line). The total torque on each block should be zero for
   it to be in equilibrium, BUT how do you calculate the %error when the
   accepted value is zero?

   My opinion (which I express to students in my labs) is that when measuring
something that is supposed to be zero, it is obviously impossible to find
a per cent error because any error is automatically infinitely large.  We
either try to measure some quantity that isn't zero, or we examine the
experimental result *including* the uncertainty to see if the number is
consistent with being zero.  For example, if we get a numerical result
like .123 +/- .789, the result is zero within the limits of experimental
uncertainty.  If we get .123 +/- .011, then the result is not zero within
the limits of experimental uncertainty.
   It is also important to look at a result relative to what is being
measured.  In some experiments I do in my intro course, a result that
should be zero ends up having a value that may be on the order of 100
(in whatever units we're using).  However, it turns out that all the
measurements are on the order of 10000, so the result is still very
small by comparison.  (And we can make this comparison even if we
haven't yet discussed experimental "error".)

3. What exactly happens to plastic that turns it white when you make a
   sharp bend in it. (this is probably chemistry)

   I'll pass on this one.

Steve Luzader
sluzader@fre.fsu.umd.edu