## Senin, 02 September 2013

### Mail Time! Some quick questions about AP Physics

It's that crazy-arse time of the year, at which point I look at the calendar and say, "Oy, my next day without an obligation is the Wednesday before Thanksgiving."  So in honor of the start of school, here are a few quick questions from readers with quick answers.

From Joseph:

Hi Greg. I was examining some of the grading done on the AP as well as how you graded some of your tests. I ... saw that you adjust the multiple choice scores slightly. The example I'm looking at says you multiplied the MC score by 1.304 after rounding then added that to the total FR points for the RAW AP score. Why did you adjust the MC? Does your strategy vary test to test?

Hey, Joseph.  Both sections should be weighted to one minute per point.  Since on that test I gave 23 questions in 30 minutes, I multiplied the mc score by 30/23 to add to the free response.

From Youri, who has two questions:

1. I can't remember but on the AP do they go by significant figures or by given answer to 3 decimal places?? Can you clarify that for me.

Use 2 or 3 sig figs.  Not worth throwing a fit over with the students, though, compared to the other classic battles like using units or describing a solution thoroughly.

2. Do you have any cool demos I can do for kinematics, using a pasco track, carts, I have a fan for the cart, a labquest, a motion detector a force sensor...basically what you told me to get. I want to do a demo but I am not really sure what would be the most appropriate and useful to the kids???

Use the fan cart on the track with the motion detector and do qualitative and quantitative demos.  Like, what will the x-t or v-t graph of this motion look like?  Can someone make the cart create this graph?  What initial velocity will get the cart to stop at the top of this inclined track (given the cart's acceleration)?   Or, just what is the cart's acceleration given the v-t graph?  All sorts of fun stuff.  Choose an end-of-chapter problem, and scale it to a cart on a track.

From Jessica:

Random question. I have a student who is solving force problems with tangents instead of sines and cosines to break forces into components and determine magnitudes of force components. His math works. But I rarely see tangents show up on rubrics. Does it matter? As long as his math is sound? Or will it lose him points to not show force components in sines and cosines?

Hey, Jessica!  His approach is fine, as long as the physics is correct.  Nonstandard but correct and clear approaches always earn full credit.  (As I sometimes say rather cheekily, that's why the College Board hires physicists to grade the exam rather than lawyers.  The rubrics are meant to be interpreted intelligently, not inflexibly.)

Good luck to all this year... please email questions as you have them.

GCJ