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From: Leslie Marra (lgmarra@bellsouth.net)
Date: Tue Jun 25 2002 - 18:59:36 EDT

• Next message: MTweedy@aol.com: "force and motion questionnaire"

think of a way other than arrows to visually represent forces. The length
of the arrow gives a visual of the magnitude of the force. Perhaps if the
forces were labeled with words in parenthesis on top of the arrows? Or
maybe if the measured amount (Newtons) of each force were labeled for each
arrow?
I think the most important aspect of understanding this notion of balanced
and unbalanced forces lies in the actual experience with the ball and the
track.
When the ball sits on the horizontal track, the forces are balanced. That
is, the weight of the ball (pull gravity x mass of ball) is equal to the
force of the track holding up the ball. The forces are both perpendicular
to the track and acting in concert on the center of mass of the ball as
well.
When the ball sits still on the horizontal track the NET FORCE is 0
(zero); because the pulling down force of gravity (on the ball) is exactly
equal to the pushing up force of the track (on the ball). Also, there are
no forces acting side to side on the ball at this point. NET force is the
force "leftover" if you combine all of the directional forces.
When the track tilts the pushing up force of the track no longer pushes
perpendicular to the track, no longer exactly opposite to the pulling down
force of gravity. The shift in angle of the center of mass combined with
the consistent pull of gravity causes a shift in energy from the pushing up
force of the track to the pulling force of gravity, or in effect - the ball.
Note the significance of "center of mass" as pointed out in the workshop.
Here's what I wrote (drew) in my notes:

(1) In this situation, balanced forces meant zero net force and a still
ball.
Things change when the track is tilted:
(2) Forces on the (center of [mass] of the) ball are no longer balanced.
(3) When the perpendicular line drawn downward from the center of [gravity]
is no longer perpendicular to the track, the forces on the object are
unbalanced.
(4) Keep in mind the thought of "center of mass". The forces are acting on
the center of mass.
(5) Tilting the track in effect lessens the pushing up force the track has
on the ball; however, the pulling down force of gravity stays the same.
(6) Forces don't just disappear: In effect, the pushing up force lost by
the track is gained by the pulling down force of gravity, gravity "wins" and
the ball rolls down.

The "reason" there is more net force on the ball than on the track once the
track is tilted is the ball's center of mass has been thrown off balance and
gravity is pulling down on the ball harder than the track is pushing up.

The principles of potential and kinetic energy and inertia are in play here
as well.
********
Can you better explain the unbalanced forces again with respect to moving
objects? I know they can be balanced (even though my misconception logic
says otherwise) but I am still not sure I understand HOW or WHY.

The simplest explanation I can give is: An object moves BECAUSE an
imbalance of forces has occured. And, only an imbalance of forces can alter
the movement of an object in motion or at rest.

I also made an observation: At least via the video coverage available, both
the cameraperson and the teachers have spent more time (except in the 6th
workshop classroom) with the all-boy groups, as well as allowing more boys
to speak during the class discussions. As a psychologist I actually do not
find this surprising, just continually interesting, given the research in
this area. I wonder of anyone else noticed this?

Actually, I thought I noticed some interesting gender bias issues as well -
and wondered if I had done the same in my classroom! It seemed the 6th
workshop classroom had more heterogeneously grouped "groups", which would
make it difficult to focus on boy groups. I've noticed more boys getting
called on, and more boys' ideas/suggestions being pursued than girls. I am
also taking another ACPB workshop called "The Next Move", and have noticed
similar concerns in those videoes. I have witnessed serious bias/favoritism
incidents in my teaching experience, and do not see these as such. My
perception is that the female teachers in these workshop series have no idea
they're favoring the boys slightly; however, I do wonder if the little girls
are conciously aware of it.

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