Re: Controversial Exam Questions - Not Ohm's Law

[Rick Tarara <rbtarara@SPRYNET.COM>]

I think you've introduced a 'third' definition of weight for which I would
guess there is little support.  The choices as I understand them:

1)  Weight is the gravitational attraction of the earth on a massive object.
(If the object is on the moon or Mars, substitute the moon or Mars for the
earth.)  This definition leads to the discussion of apparent weight such as
an 'apparent weightlessness in orbit.'

2)  Weight is the Net force as viewed within the frame of reference that
causes the contact between an object and what supports it.  For someone
standing on a scale, this is still a downward force (since the scale
actually reads the upwards force of the scale) but includes the CENTRIFUGAL
component due to the earth's rotation, and any effects due to other
gravitational attractors.  Because this view looks at the centrifugal force
as 'real' in the frame of reference where the object is at rest, then the
actual weight in orbit is zero.

3)  The now introduced definition of weight as the downward force of an
object on it's support--a contact force.  I'm not sure what the source of
that force is supposed to be if not (1).

If we go back to the question, then (1) and (2) still lead to choice B
except that the (2) group would not have the force exactly 500N since this
is an accelerating frame.  {I've always thought that the 3rd law was the
pedagogical problem with this approach.}  Definition (3) would lead to
answer A, but is this really a viable definition?

I may be misrepresenting these 'definitions', but as I interpret them, I
really see little problem with the exam question.  'B' is certainly the
answer I would expect and this is  more or less the way I would ask the
question in the context of my courses.

Rick

(Off for a weekend vacation--ungraded exams to remain at home!)

----- Original Message -----
From: "Michael Edmiston" <edmiston@BLUFFTON.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, May 12, 2000 11:03 AM
Subject: Re: Controversial Exam Questions - Not Ohm's Law


> Leigh, how can you tell that B is the correct answer?
>
> > A woman is standing on a flat section of ground. Her weight is 500 N.
> > Newton's > third law states that there must be an equal and opposite
force
> to her
> > weight, > which is
> >
> > A. the Earth exerting an upward force of 500 N on the woman.
> >
> > B. the woman exerting an upward force of 500 N on the Earth.
> >
> > C. the woman exerting a downward force of 500 N on the Earth.
> >
> > D. the Earth exerting a downward force of 500 N on the woman.
> It seems to me that choices A and C are a third-law pair using the
> definition that "weight is her contact force," whereas choices B and D are
a
> third-law pair using the definition that weight is "the force of gravity
on
> her."  If the question is referring to the A/C pair, then the force
> described in the question text is C, and A is the answer.  If the question
> is referring to the B/D pair then the force described in the question text
> is D, and the answer is B.
>
> What do you see in the question that tells you they are describing the B/D
> pair?  Since it says she is standing on the ground, that might, in fact,
> imply the A/C pair, because the B/D pair does not require that she be
> standing... it requires no contact... it only requires an idea of what is
up
> and what is down in order to make sense of the spatial directions being
> used.
>
> If two different definitions of something are in current use, why do you
say
> an individual cannot have a bias toward one of them?
>
> Michael D. Edmiston, Ph.D.              Phone/voice-mail:
419-358-3270
> Professor of Chemistry & Physics        FAX:
419-358-3323
> Chairman, Science Department            E-Mail
edmiston@bluffton.edu
> Bluffton College
> 280 West College Avenue
> Bluffton, OH 45817