Elevator question #1: Forces on the student

The poll question to the right asks only for the downward forces acting on the student.  For the purposes of our discussion, let's first consider ALL forces acting on the student.

It is important to focus only on the forces acting directly on the student himself.  Forces that act on the elevator, on the scale, or on the building are utterly irrelevant to this question. 

So, we begin with earth's gravity.  We know the earth applies a downward force on the student.  That downward force is generally called the student's weight.

Since there are no electric or magnetic fields here (and, if you're really going to be a lawyer about it, since we're not dealing directly with a nucleus), any other forces on the student require contact with the student.  What else is touching the student?  Only the platform scale.  And since he's standing on the scale, that scale must push upward. 

What about the "normal force?"  A normal force is defined as the contact force of a surface pushing perpendicular to that surface.  Because the scale provides a surface for the student to stand on, the upward force of a platform scale *is* a normal force... on a free body diagram, a label of "Fn" or "N" rather than "Fscale" would be completely correct.  Just recognize that when we apply Newton's second law, whatever value we find for the normal force is the reading on the scale.

What about the force of the student pushing down on the scale?  Well, that force certainly exists... but that force does not act on the student.  In order for Newton's Laws to work, we must consider only the forces that act ON the student, not forces exterted BY him.

What about the force of the cable attached to the elevator, or the elevator's weight?  Isn't the entire elevator accelerating, not just the person?  Yes... but we must choose a single object for the purpose of using Newton's second law.  Had we chosen the entire elevator with the person inside, then there would be an upward tension from the cable, and a downward weight.  But we were asked about forces acting on the person, not the elevator. Newton's second law applies just fine to the person alone, even though the elevator exists. 

And as for the "force of motion" or the "force of acceleration"... these things do not exist. A force must be attributed to some agent pushing or pulling, either through contact or through a gravitational (or electric or magnetic) field.  Sure, forces can cause acceleration, but the "force of acceleration" is not a meaningful or correct phrase.

Next, I'm going to ask about the MOTION of the elevator... stay tuned.

General Physics exam review, a "clicker" exercise, and trying out SCRIBD

In addition to AP physics, I teach a general high school physics course for juniors and seniors.  This course is intended to be accessable to all college bound students, even those not gifted in or particularly interested in math and science.  In fact, a significant number of students who didn't take our freshman course are required to take and pass this General Physics course.

Now, everyone has a different perspective and philosophy about general high school physics.  My approach -- which isn't necessarily the RIGHT approach, but has been successful for me -- is to teach a limited number of topics nearly to the depth demanded by the AP physics B exam.  By March 1, we have covered most of physics B mechanics.  (How come I claim to call this "general" and not "AP" physics?  Because my AP class covered this same set of topics by November 1.)

My trimester exam, coming up on Thursday, is 100 points worth of AP-inspired free response problems.  Some are directly off of old AP exams.  Others are modified slightly, but still maintain the spirit of AP problem solving.  Students of all abilities usually do quite well on this exam.  I always say that if this class could take an AP exam limited only to the topics that we cover, they would be getting mainly 5s, with no one below a 3.

If I am going to give such a comprehensive and rigorous exam, I must prepare my students for it the same way I prepare my AP class for their national exam.  Of course, most of this preparation comes via the coursework we've done all year.  But I spend the last week before the test in review mode.

The primary review method I've used this week involves a multiple step process:  a homework problem, sometimes a review quiz over that problem, and an in-class "clicker" exercise.  The "clickers," or "classroom response system," allow me to present a series of questions as an extra credit competition.  Class members are divided randomly into groups of two.  Each group submits a single answer to each question.  They earn one point for a correct answer, plus another point for every group who gets the answer wrong.  This scoring system encourages collaboration with the groups, while discouraging reliance on one or two students who might carry the whole class along.

I plan on posting a couple of these clicker exercises over the next week.  Thing is, it is often difficult to post quizzes or class handouts on this site.  Equations and diagrams don't copy well into google's "blogger."  But I think I've found a place where I can very simply upload my word documents, and where you can easily download them as well.  Check out "scribd.com".  This site looks to me like the text equivalent of flickr.  I was asked to create a username and password -- I'm "gregcjacobs," though I won't tell you my password.  The site is free, and I haven't received any spam from them yet.

I've posted two documents on scribd that I'd like people to try to download, just to see if this works. 

The first is a homework assignment:  it's an elevator problem, based very loosely on 2005 AP physics form B problem 1.  (This problem could also be given as a 10 minute quiz, or as a test problem.)  Click on the link, and see if it works.

The second document is a multiple choice exercise for the clickers that is based on the elevator problem. 

Please post a comment, firstly saying whether you could access these files an a format that you can use; also, feel free to comment on the utility of the exercises, or any thoughts related to this general exam review strategy.

GCJ