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: g

It is hard to reply on this day of tragedy. Tens of
thousands probably died just accross the river today,
including somebody I know works in the area. I hope
Herb Gottlieb "from New York City" lives far away
from the affected area. But, as they say, life must
continue. This is not a place to discuss the event.

Joseph Bellina wrote:

I understand the point you are making Ludwik, but you speak as if you
know what a force and how if affects motion...something which is in a
very confused state for your students, if they are like mine. I would
rather work out carefully the notion of vector how acceleration is
related to the change in velocity before I confuse the issue with
force. In most students minds, force produces velocity not
acceleration, and you in effect move back to supporting their confusion
between velocity and acceleration by bringing in the notion of force too
soon.

The so-called "Aristotelian idea of no force no motion"
has to be faced no matter what your sequence is. I see
no harm in starting to discuss it at the same time as
acceleration is introduced.

If it was up to me I would discuss static forces (their
vector nature etc.) before kinematics. Then the F=m*a
would be introduced. What is wrong with this sequence?
Ludwik Kowalski


On Mon, 10 Sep 2001, Ludwik Kowalski wrote:

In learning dynamics students will become aware that the sign
of acceleration is always the same as the sign of the net force.
Now we are discussing an object moving along a vertical line.
Here g is negative when the y axis is pointing up and g is
positive when the y axis is pointing down. As emphasized
by others, we are free to choose the orientation of the y axis.

The difficulties described by Michael are likely to be reduced
when elements of dynamics are presented to students when
they learn kinematics. It is not a crime to start talking about
the "no F no a" idea at the level at which F is conceived only
as "a pull or push of some kind". We should not be separating
kinematics from dynamics too much. Students know that
objects are attracted down and this is sufficient to see why g
must be down. Then "down" can be made either + or ? by
introducing a convenient y axis.

In other words "the down" is absolute while "positive and
negative" are arbitrary.
Ludwik Kowalski
a, Jr. 219-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556