I have found that the same discussion about Torques (in place of Ang. Mom)
seems to register more easily as a first exposure for students. Ang Mom
considerations then follow more easily.
IE., replace your P's with F's and evaluate the torques Ri^Fi.
Bob Sciamanda
Physics, Edinboro Univ of PA (Em) http://www.winbeam.com/~trebor/
trebor@winbeam.com
----- Original Message -----
From: "John Denker" <jsd@av8n.com>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Sunday, February 26, 2006 2:06 PM
Subject: [Phys-l] angular momentum (in a straight line)
| Hi --
|
| I had a student with an interesting misconception: He was stuck on
| the idea that angular momentum was associated _only_ with things that
| went around and around and around.
|
| To deal with this, I drew the following diagram:
| http://www.av8n.com/physics/img48/angular6.png
|
| The left half of the diagram shows a particle going around and around
| in a circle. It has constant angular momentum relative to the center
| of the circle.
|
| The right half of the diagram shows a particle moving in a straight line.
| It has constant angular momentum relative to any fixed pivot-point
| (including the pivot-point in the picture).
|
| The angular momentum in all *six* cases is the same:
|
| R1 /\ P1 = R2 /\ P2 = R3 /\ P3 = R4 /\ P5 = R5 /\ P5 = R6 /\ P6
|
| He looked at the picture for a couple of minutes, and then said "Got it."
|
| He had been confused about that and about six other things, which made
| it hard to diagnose what the problem was ... but the six other problems
| evaporated as soon as the straight-line angular momentum issue was
| understood.
|
| ============
|
| I'm not sure how common this misconception is, and in general I don't
| like to spend time discussing misconceptions ... but somehow I found
| this one particularly interesting. I think it's because the student
| was really trying to do the right thing by _visualizing_ the _physics_
| rather than just plugging into some formula.
|
| The mental picture he came up with was not quite right, but you gotta
| respect him for trying.
|
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|