Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: The sign of g



At 16:52 -0500 9/3/02, Rick Tarara wrote:

Just to emphasize a piece of Hugh's note.

Never write the kinematics equations with 'g' as I've seen some books do to
create a set of equations for free fall. This is unnecessary and confusing.
If the (constant acceleration) kinematics equations are derived and used,
then they are written in terms of an acceleration 'a'. IF the motion is
free fall, then the vertical acceleration has a magnitude of 9.8 m/s^2 near
the earth's surface with a direction +/- that is totally arbitrary but IS
determined once one makes a sign convention for up and down. 'g' doesn't
appear here--either in the words used and certainly not in the equations.
Keep it that way!

Thank you! I should have said that, but didn't. I suspect my
willingness to allow the usage of g in the acceleration position of
the kinematics equations (probably just laziness on my part--I didn't
want to take on that problem, since most books do it routinely), is
partly responsible for the fact that a few students each year, get
the idea that all accelerations are g, whether gravity enters into
the problem or not.

I forget who pointed out, that this is a good reason for not teaching
acceleration before teaching about forces, but I agree
whole-heartedly. I think a much more logical approach is to talk
first about constant velocity motion, then introduce the idea of
momentum, followed by force, which leads to Newton's laws and then
acceleration becomes a natural concept, which helps to give substance
to Newton's laws. Our school's introductory syllabus, with a few
minor modifications, is basically this sequence, at least in one of
the two introductory courses we teach.

Hugh
--

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

(919) 467-7610

Let's face it. People use a Mac because they want to, Windows because they
have to..
******************************************************