[Physltest] [Phys-L] Re: sign of velocity (was: California standards test in physics)

[Anthony Lapinski <anthony_lapinski@PDS.ORG>]

I couldn't agree more with this idea. When two objects collide, the forces
are equal and opposite via Newton's third law. Technically, if one is 20
N, the other is -20 N, but that's irrelevant. The signs just indicate
directions. Now, signs are important when talking about net force,
momentum collisions, etc. When possible, I use the term "speed" in my
motion questions to avoid the confusion that 20 m/s is more than -20 m/s.
I want my students to understand the basics of physics, not get confused
by certain technical details.

Forum for Physics Educators <PHYS-L@list1.ucc.nau.edu> on Thursday,
January 6, 2005 at 11:06 AM -0500 wrote:
> I'm going to quibble here.  John is correct concerning the mathematics of
> vectors, but conceptually I think there is some wiggle room here.  Is the
> velocity at the end of the trip really less than the velocity at the
> start?
> (assuming up is taken as positive)  Again, mathematically yes, but
> conceptually I think we could look at it as the upwards velocity at the
> start is 'the same' as the downward velocity at the end.  While we are
> talking about the speed here, the idea is that positive and negative are
> ARBITRARY labels to separate directions (after all we could have chosen
> down
> as positive) and being too pedantic about negative being less than
> positive
> can be confusing here.
>
>
> How mathematical one needs to be here depends on the level of the course.
> I
> would expect my Calculus level class to see the final velocity here as
> less
> than the initial velocity and might ask such a question.  With my Gen-Ed
> class, I would have no such expectation and therefore I don't ask the
> question.  I do ask about the speed at the start and end of this motion
> however.
>
>
> Rick
>
>
> *********************************************************
> Richard W. Tarara
> Professor of Physics
> Saint Mary's College
> Notre Dame, Indiana
> rtarara@saintmarys.edu
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>
> ----- Original Message -----
> From: "John Denker" <jsd@AV8N.COM>
>
>
> > Richard Tarara wrote:
> >
> > > Question 8 has a problem that we all have to be careful about.  Since
> the
> > > velocity is a vector quantity, and mathematically we consider negative
> > > quantities to be less than positive quantities, none of the answers are
> > > mathematically correct.
> >
> > We agree that in general vector quantities cannot be considered positive
> > or negative, and that students are often confused by this.  However,
> there
> > is a big fat exception for one-dimensional vectors.  And since the item
> we
> > are discussing is explicitly one-dimensional, it beats this rap.
> >
> > One-dimensional vectors are isomorphic to plain old numbers, and can be
> > considered positive, zero, or negative.
> >
> > > However, if one argues that a negative velocity IS
> > > NOT 'less' than a positive velocity, just in a different direction,
> >
> > There's no chance of that.  Take a look at the number line.  Negative
> > numbers *are* less than positive numbers.  No choice.
> >
> > > .... The question SHOULD have asked about the speed of the ball
> > > to eliminate this confusion.
> >
> > Yup.  BTW it's not a "confusion".  It's a gross error.  All four
> > choices are so grossly wrong that it is impossible to choose a "best"
> > answer ... except by assuming that the person who composed the question
> > doesn't know anything about physics.  The question has very limited
> > value in assessing students' knowledge, and strongly negative value
> > in terms of teaching-via-the-exam.
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