From: Ben Pittenger (benpittenger@earthlink.net)
Date: Thu Nov 21 2002 - 07:00:26 PST
Date: Thu, 21 Nov 2002 07:00:26 -0800 From: "Ben Pittenger"<benpittenger@earthlink.net> Subject: Centripetal force Message-ID: <Springmail.0994.1037890826.0.25505600@webmail.pas.earthlink.net>
Shannon,
I get a student volunteer to walk a straight line across the room in front of
me. (if I remember, I tell them to always face the direction of their
motion). Then I have him/her walk back. On the second pass, I grab the
sleeve of their sweatshirt and gently yank on them to change their course a
little, then let go. I get the class to figure out that s/he was traveling in
a straight line and changed direction when the outside unbalanced force acted
on him/her, then traveled in a new direction in a straight line. On the third
pass, I yank the student twice, with some straight-line motion in between. I
make sure that I turn, always facing the student (I face along the line of
force). Then on the next pass I yank three or four times. Again, I get the
class to observe and say that at each yank, the force changes the
straight-line motion; they say the direction of the force and the direction of
the change of motion. By now the students are getting antsy and even the
slower ones are starting to get it. At the last pass, I keep ahold of the
volunteer's sleeve and capture her/him into rough circular motion with an
obvious series of yanks which may or may not eventually change to smooth
circular motion.
I find this demonstration helps the class visualize the concept that the
centripetal force is perpendicular to the direction of the object's motion.
It also shows that the force changes direction as the object changes
direction. With the right student volunteer, it also helps demonstrate that
once the force is released, the object travels in a straight line once again.
The hardest part of the demo is to pick a student who will quickly understand
that they are to walk in a straight line when not being pulled, and change
direction while being pulled. Many of my middle school students who are
willing to volunteer also are hams who don't listen so well when in front of
the class. But even the process of instructing the volunteer is helpful.
Often, when I am clear with the instructions for them to change direction only
when acted upon by an outside unbalanced force, the rest of the class sees
when they aren't responding properly. The class will correct them. When it
works well, the student always faces in the direction of his/her motion; once
in circular motion, their shoulder is always towards me.
It's a fun exercise. I've done it a couple of times with the volunteer's eyes
closed. That helps reduce anticipatory reactions.
I also like spinning a ball in a bowl and get the students to figure out that
the bowl is causing a force on the outside of the ball, pushing the ball
toward the center of the bowl (even though there is nothing in the center of
the bowl, and no strings attached).
Ben Pittenger
This archive was generated by hypermail 2.1.3 : Mon Aug 04 2003 - 16:18:08 PDT