I agree with what Michael Edmiston says with a few caveats.
The sign on vectors are a real pain for students to get. I blame math
teachers. They tell students that the sign on a number represents a change
in the magnitude, while all of us in physics know that the sign indicates
direction instead. After all, given the two velocities of +10 m/s and -15
m/s and asked who had the bigger velocity, the answer is the -15 m/s.
So, I spend days emphasizing the arbitrary nature of the signs and the sign
conventions. I force students to do some of the problems twice -- once in
each orientation -- to show that they get the same answer. Further, I do not
accept answers that try to indicate the direction using the sign alone.
Negative isn't always down.
In class, thus, I do not speak of the acceleration due to gravity as -9.8
m/s/s, but rather as 9.8 m/s/s downwards. It may be negative or positive
depending on how the problem is set up.
One final thing, unlike many teachers, I never derive the specialized
formulas for falling objects that Michael discusses below. I always have my
students use the same kinematic formulas for all things. I don't really want
them to perceive that accelerating from gravity is different kinematically
than any other sort of acceleration.
Marc "Zeke" Kossover
-----Original Message-----
From: Michael Edmiston [mailto:edmiston@BLUFFTON.EDU]
Sent: Monday, September 10, 2001 1:32 PM
To: PHYS-L@lists.nau.edu
Subject: Re: g
For example, for velocity of a dropped object as a function of time, I can
start with v = v0 + at and then rewrite this as v = v0 - gt, and then when
it comes time to use the number for g, I simply replace g with 9.8.
Please note it would also be valid to write v = v0+gt if we choose to orient
the coordinate system with the positive direction being down.
Therefore, v = v0 - gt is correct if the downward direction is negative; v =
v0 + gt is correct if the downward direction is positive, and the value of g
is always 9.8. It might be that more people associate down as negative, but
not all. In many cases people find it convenient to choose down as
positive.
On the other hand, if your students want to think that v = v0 - gt when the
object is moving downward, and v = v0 + gt when it is moving upward, then we
have a problem. Once the coordinate system is established, then for that
problem the acceleration has to remain fixed at +g or at -g for movement in
either direction.
But I would encourage you to think if g as 9.8 and not as -9.8.
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