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Re: [Phys-l] Independent Variables + syllabii



At 10:34 +1100 11/2/06, thecraftyphantom@australia.edu wrote:

As a note, some of the syllabus is good such as

�� define weight as the force on an object due to a
gravitational field

and

�� define gravitational potential energy as the work done to
move an object from a very large distance away to a
point in a gravitational field Ep = − Gm1m2/r

While these two are common (the former moreso than the latter), they, too, are not without problems, and having them defined in this restrictive way can only be a virtue in making it easier to give a standardized test to everyone at the end of the year.

Defining weight as the gravitational force on an object gives the impression that when you are "weightless" (a very common term, at least superficially understood by most people), that there is no gravity acting on you, and of course that is not true, even in earth orbit, where NASA, in its infinite wisdom chooses to define the environment as "microgravity," which it is of course not. We have just been through the discussion of the fact that one cannot feel the effects of a uniform gravitational field, so when someone feels weightless it doesn't mean they are in a region devoid of gravity.

Far better, IMO, to define weight as what the bathroom scale reads when you step on it, regardless of where you are or what circumstances you find yourself in when standing on the scale--in other words, the net force supporting you (or any object) in your (its) present state. This way, we find that weight can vary with circumstances--something that most people have observed many times, especially on carnival rides--but the force of gravity doesn't, unless you take yourself to a place where the gravitational field is very different than where you are now.

As for the definition of GPE, I think I know what they mean, but from far away from what? And work done by what to what? Assuming that they meant something like "the work done in moving an object from an infinite distance away from a gravitational force center, such as the earth, to a point closer to that center" do they realize that they defined the zero point of energy as being at an infinite distance from the center, and that the work done in moving the object to the nearer point is negative?

Do the definers realize that they have defined GPE so that it will always be negative? Do they not want to use the more common definition in most introductory textbooks that allows for an arbitrary reference point (zero point) and measuring the gravitational interaction energy relative to that point, so it can be either positive or negative?

This is the problem with people whose knowledge is spotty and whose interests are not in understanding the subject but in standardizing its teaching can create problems for students later on, no matter how sincerely they think they are helping the students. Their hearts may be in the right place, but their brains sure aren't.

Hugh
--

************************************************************
Hugh Haskell
<mailto:haskell@ncssm.edu>
<mailto:hhaskell@mindspring.com>

(919) 467-7610

When you are arguing with a stupid person, it is a good idea to make sure that
person isn't doing the same thing.
Anonymous