Re: Newton's 3rd law? was Re: inertial forces (definition)

[Robert A Cohen <bbq@ESU.EDU>]

On Thu, 21 Oct 1999, John Mallinckrodt wrote:

> I appreciate your point.  Indeed, my reaction (or overreaction, if you
> will) was primarily to the original poster's apparent insistence that you
> *had* to show the centripetal force on an FBD.  Perhaps I didn't make that
> clear enough.  (And I still say it's nonsense!)

I found Rick Tarara's post perfectly reasonable.  Perhaps it is because I
interpret "centripetal" as "directed toward the center".  In other words,
if an object is going in a circle, there must be a force directed toward
the center.  The force diagram, naturally, should indicate a net force
directed toward the center, should it not?  This net force might be the
result of several forces or just one force, but it must be there just the
same.

The problem with students, as I see it, is that they interpret
"centripetal force" as being one of those four fundamental forces of
nature, separate from the gravitational, electrical and nuclear forces.
The same is true for the centrifugal force.  In my class, I prefer that
students use "force directed toward the center" and "force directed away
from the center" for centripetal force and centrifugal force.  Then, when
I ask which force is directed toward the center of the motion, I am less
likely to get the "force directed toward the center" as the answer.

P.S. I also thought Rick's explanation of the 3rd law was quite
reasonable.  All too often, students think the normal force acting up on
an object is the 3rd law counterpart to the gravitation force pulling down
on the object.

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| Robert Cohen               Department of Physics       |
|                            East Stroudsburg University |
| bbq@esu.edu                East Stroudsburg, PA  18301 |
| http://www.esu.edu/~bbq/   (570) 422-3428              |
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