21 May 2014

Hints on "Justify and Explain" questions on the AP 1 and AP 2 tests

Below is a conversation on the AP Community website. I reprint it here because I think it needs wider exposure. The response is from a long-time AP Reader who is held in high regard by many of the other Readers.

Justify and Explain questions...
I need advice, or resources, that will help students answer ‘justify’ or ‘explain’ questions accurately and concisely (our school’s English department has tried to help, but they don’t seem to grasp the problem), and how to assess their answers accurately and efficiently.
Next year, I’ll teach five periods of 40 students  (four of P1 and one of C (M,E+M)), which has always created lots of grading… but now, with the advent of the new exam, there’ll be more, especially of the ‘justify’ and ‘explain’ type.
What do you do to improve your student’s answers and your assessment of them, while saving time?
RE: Justify and Explain questions...
5/20/14 9:30 AM as a reply to Martin R Kirby.
Kirby, I sat next to Rebecca last year and scored that pilot test for Physics 1 and Physics 2. Here are some do's and don't tips for everyone. (1) Don't rewrite the question in an introductory paragraph. You are wasting precious space in the test booklet that gets you no points. Do use a label to shortcut your introduction but don't expect that label to earn you a single point. Rebecca said, "There are no more skinny points." In the old days, merely mentioning things like right-hand rule, Lenz's Law, Kirchoff's Junction Rule, Newton's Shell Theorem and so forth would get you the justification point. No more. (2) The justifcation point now comes from application of the label. Deborah says, "If you can't accomplish this in three sentences or less then you are probably not going to get the point." More importantly, the student is also losing time that could be spent getting more points else where. An example here would be something like,  "induced current will flow in a direction to replace flux loss (or cancel flux gain). In order to accomplish this, the right hand curl rule indicates that current flow must be clockwise." I would suggest that you have your students list maximum of four points in bullet form to begin on test day. The first bullet should be (i) label point that applies to this justification (ii) connect the label to the cause (iii) connect the label to the effect (iv) connect the label to the conclusion through physical application. Many students miss this last point and merely wound up repeating their conclusion. Award points for every bullet that they mention. Take off points for restating the question. I know extinguisment doesn't work but.. I have also considered giving them one essay question the night before just because our kids seem to write physics explanations like a five paragraph essay. Hope this is not redundant from past threads.

I would have added a little more but the post was too long. Here is the rest. You can take some of the old AP Physics B FRQ's and use them to make practice problems. However, the problem with the old AP Physics B problems is that the changes always come at the end, rather than the beginning of the problem. Use the old images and move the change to the beginning of the problem. Consider the 2013 AP B FRQ where you had the catapult type problem and the mass was changed at the end of the problem and students had to explain the change in the range of the projectile. Practice that problem specifically. Tell the students to use "inertia" in their explanation. Do this when you are finishing up the Newton's Laws Chapter. After they do that, return to it again after the Work-Energy chapter. Have them repeat their justification using the Work-Energy Theorem instead of Newton's Laws. Then have a third go using momentum and impulse after you finish the chapter on momentum. Although the pulley makes the third approach more challenging it should help the students to synthesize three chapters of mechanics. Two more thoughts- (1) Look up T.I.P.E.R. questions and try to modify old AP Physics B FRQ's into those suggested formats. But move the change to the beginning and demand physical explanation without using mathematical logic or symbols. Your qualitative/quantitative practice can come from those old images but look at the pilot exam next month before you try this. (2) Start the year with bullets for the rubric and then try to grow your students into more complete and fuller explanations as the year passes. You can try I don't know how to make this an active link yet.

27 February 2014

What Physics Teachers Talk About Out Of Class

Physics Teachers being Naughty

One of the nice things about being a member of the Western New York Physics Teachers Alliance is the email list. Here we can share ideas with colleagues  we may see only once every month or two at our regular meetings. The following is an example of one such exchange (names have been deleted to protect the guilty).

I've got a PVC cannon that can fire ping-pong balls/dog toys, paper tubes/rockets, and dry erase markers.  It's electrically triggered, but I'm still trying to figure out what the max pressure it can take and still electrically fire.  Individual components are good up to 90 psi, but around 70, the sprinkler doesn't actuate.  40 psi will send a dog toy at ludicrous speeds.  I also went out and got an air compressor at Harbor freight, so we can basically fire the thing every minute or two (depending).  Air compressor is loud as dickens, tho.  Actual muzzle velocity calculations/high speed forthcoming.

It can fire dog toys; can it fire toy-dogs (not the plushy kind)?

If you can get them to fit in the barrel.  And not wiggle around too much.

We could get a Doppler-shift demo out it, too. 


17 February 2014

5-g's? Let's not Exaggerate.

Watching the Olympics, I have seen a commercial that annoys me. It features a figure skater comparing what she goes through to other occupations. One is that she "experiences more g-forces than a fighter pilot". Let's examine that claim.

The math

The forces on the various parts of a spinning skater are due to rotation. However, we will look at the acceleration since that what g-forces really are. For a 5g-force, the relationship between rotation speed (RPM) and distance from rotation center (r) is

A graph might make this equation a little more meaningful.

So, we can see that a 5-g force can be obtained for a hand rotating 1 meter from the center at a little more that 60 rpm (or one rotation each second). But the head? Even with a skater's head held straight out at around 20 cm, 5-g's would only be experienced at 150 rpm (over 2 rotations per second). And if we look at a 9-g force which most fighter pilots experience occasionally (and for a lot more than a second), it would take 200 rpm. While the supposed world record for a figure skater spin is around 300 rpm, that skater held her head upright, so her nose might feel a 9-g force, her head would feel a much lower force.


What can we conclude? While it is possible that a figure skater can experience over 5-g's of force on some parts of her body, when was the last time you saw a skater do a rapid spin in competition? So, the ad is true, but very misleading.