I find it is best to state the value of g as 9.8 m/s^2, not -9.8 m/s^2. In
other words, I always associate the symbol g with the magnitude of the
gravitational acceleration. Then, when writing equations, I explicitly put
the minus sign in front of g when necessary.
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