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]

Newton's third and centrifugal forces.



Since I don't fully follow Leigh's reasons for his approach, let me detail
how I would deal with me standing at the equator--maybe it will help to be
explicit about this.

There are only two forces acting on me. The Earth's gravitational force and
the ground pushing up. The earth's force is GMm/R^2 and I do exert a force
of GmM/R^2 on the earth. I am accelerating and therefore need a net force
towards the center of that rotation of magnitude v^2/R. That force is
provided by the earth's pull on me. In order that there be this net force
towards the center, the force of the ground on me must be less than GMm/R^2.
It is. Thus there is a net force towards the center. Why doesn't the
ground push up with as much force as the earth pulls down? Because of my
tendency to want to fly off in a straight line (N1) which could be made more
obvious if we sped up the earth's rotation considerably. There are no
centrifugal forces here. The net downward force (from gravity) provides the
needed centripetal acceleration.

Now I feel somewhat lighter and a scale would read my weight to be less than
GMm/R^2. The latter because if I am standing on the scale, IT is providing
the upwards push which is less due to my tendency to fly off. I feel
lighter because I am accustomed to EXPERIENCING my weight as the upward
resistance to the downwards motion I would have due to the earth's pull if
the ground/floor/scale were not there.

These same arguments can be extended to other rotating frames where ONLY a
net force towards the center is actually acting. I realize that the problem
is that if you are IN such a frame you would 'experience' an outward force,
but it is customary in intro courses (and I think most Newtonian approaches)
to always look at these rotating frames from outside.

Leigh, in your approach do you have a centrifugal force acting on the books
sitting on the car seat as the car goes around a curve? In most courses
this is a classic example to show Newton's First law in action when viewed
externally.

FWIW

Rick



*****************************************************
Richard W. Tarara
Department of Chemistry & Physics
Saint Mary's College
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

FREE PHYSICS INSTRUCTIONAL SOFTWARE AVAILABLE
see descriptions at:

http://www.saintmarys.edu/~rtarara/

New Win9.x and PowerMac packages now available.
*******************************************************