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Re: L2-"Negotiating" a curve.

Rick Tarara wrote:



Arlyn,

I suspect you are new to the list (this topic is discussed often ;-). The
problem with labeling a force vector as the centripetal force is that it
instill in students the idea that there is a force in nature called the
centripetal force. We know that for uniform circular motion there has to be
a net force directed towards the center of that motion in order to produce
the needed centripetal acceleration,

snip... (I've been on the list about 6 months - I don't contribute much) *We*
know that but my students (and I suspect the great majority of high school
students) don't know this. They think there is a force directed outward in
uniform circular motion called a centrifugal force because things go straight
out from a circle (or so they think - ask them about the release point of a
football pass or a baseball pitch or anything else that is released from a
circular sort of motion). Each year I have students do a lab where they first
calibrate a simple accelerometer (a test tube about 1/2 full of water) using
linear forces and then put the accelerometer on a turntable. Many students
INSIST that their accelerometer is not working because it reads that the net
force is inward instead of outward. They don't believe their own eyes.

end snip....

but that net force must be identifiable
as a gravitation, friction, a tensional, a push, a pull, etc. or a
combination of such. Look at the motion of a person riding a Ferris Wheel
and try to identify the forces that cause the centripetal acceleration as
the wheel rotates. Those forces keep changing--being sometimes the 'weight'
of the person, sometimes the bottom of the chair, sometimes the back of the
chair, sometimes maybe just friction, and usually a combination of at least
two of the above. Just drawing an force vector towards the center and
labeling it Centripetal Force is insufficient to fully understand and
appreciate such a situation.

snip....

I don't disagree. I insist that students use some notation like "the net force
causes uniform circular motion so the net force is the centripetal force (not
just equal to but IS)". As the student progresses I think it is good for them
to recognize that the source of the centripetal force keeps changing in a Ferris
Wheel. I still don't see where Hewitt blew it unless the complaint is that he
didn't describe the situation in enough detail but then this is a common problem
in textbooks.




Rick


and for Leigh:

The differential was invented a very long time ago and was applied
to tricycles long before the transistor was invented, even before
I was born!

Leigh

I am quite aware of the differential but unless you have different tricycles
than I've seen this doesn't apply. I've never seen a commercial tricycle with a
differential - they all have solid axles with one gear attached to a chain
attached to another gear attached to pedals. Even with a differential there
would be some "slip" as the tires twisted on the vertical axis (unless you are
going to consider the ideal situation where only one point of the tire is in
contact with the road).


Enough for tonight - time to go home. I've been here 11.5 hours already.
--
Arlyn DeBruyckere
Hutchinson High School