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AP-C: Gauss’ Law Quiz key

AP–KEY–Gauss’ Law


AP-1: Momentum test results and improvement opportunties

Test results from the momentum test were somewhat disappointing, in fact generally lower across the board that many of your performances leading into the holiday break.  As I say this, please note that some of you did very well, and I noted several students who beat the class average for the first time this year.  Good job!

That said: 1)This was a very moderate exam by any objective standard. 2) Many of the most commonly missed items were things that really should not have been missed, revealing a lack of diligence on your part.  That’s harsh, but frankly, this one is on you.

My analysis of the most commonly missed items on the test show that students performed very poorly on the lab analysis questions (You did do the lab, right?  You didn’t just copy someone else’s work?  You know how to interpret a velocity vs. time graph?) and failed utterly at the use of the delta operator where negative signs were involved.  For many of you the questions pertaining to just these two topics constituted the majority of your points deducted.  Finally, many of you are still failing to recognize circumstances where Newton’s 3rd Law is at work.  Remember that discussion from the final exam?  All those questions we dissected the other day?

I am giving you each an opportunity to earn back some credit and learn from your mistakes in the process.  Each student will be eligible to earn back up to 1/3 of the points lost on the exam. Note the word “earn.” To participate you must:

  1. Attend a tutorial session between now and the end of school Tuesday, Jan. 27.  My schedule is below.
  2. At the tutorial you will be given your scantron and a blank test.
  3. By yourself, using only your notes, your brain, your online textbook and/or the one of the class set of books in my room, you will write up full and complete corrections for every missed item on the test.  I will regrade your work (this may take some time to make it in the gradebook) and award you a proportionally correct amount of credit back, up to 1/3 of your missed points, with a max of 20 points.
  4. It will remain silent in my room while this occurs.  If you wish to employ my services on a question, you will lose the opportunity to earn back credit on that item (but it may help you get others correct–you need to weigh the benefit).

A strong caveat: Do not put this off.  There are only 26 seats in my classroom and I will only allow 26 students at one session.  If you wait until the end, you may be turned away.  First come, first served.

Mon. (1/19) No School No School



Wed. (1/21) 7:40 3:30 – 5

In Library

Thurs.(1/22) 7:40 After Duty (4pm)
Fri. (1/23) 7:40 None


Mon. (1/26) 7:40 3:30 – 5
Tues. (1/27)


3:30 – 4:30

Wed. (1/28) None 3:30 – 4:30
Thurs.(1/29) None Duty (None)
Fri. (1/30) None None


AP-1: Momentum HW #23

#26: This is your challenge problem and it is very hard, possibly beyond many of your abilities.  Do not fret!  It’s a bonus! Also, I cannot imagine a question this difficult, using the required knowledge base on your AP exam this spring.

This is a great question, and, I think, very much AP like in that it requires you to apply lots of different physics to solve the final part of the problem.

The diagrams in Figure 9-13 show a brick weighing 24.4 N being released from rest on a 1.00 m frictionless plane, inclined at an angle of 30.0°. The brick slides down the incline and strikes a second brick weighing 35.8 N.

There are actually three parts to this problem.  1) Sliding down the ramp.  2) The inelastic collision between the two objects.  3) Sliding with friction, coming to a stop.

Part 1)

You must apply your knowledge of ramps/inclines to find the speed of the object as it reaches the bottom of the ramp.  This “final velocity” will serve as object 1’s initial velocity during the collision.

Part 2)
You must perform a conservation of momentum calculation on the inelastic collision, using your solution from part 1 as your initial velocity.  You are looking for the final velocity of both (they move together, it’s the same) after the collision.  This final velocity will become the initial velocity in part 3 (see a pattern, yet?)

Part 3)
You need to assess with Newton’s 2nd Law.
{F = ma   only force is Ff and is back/negative
-Ff = ma   Mass is combined mass, a = (delta) v/t
You know the initial velocity is the final speed from part 2, and the final velocity here will be zero (it comes to a stop), Ff = mu*N, N=(m1+m2)*g
Solve for t
Use t to find x by kinematics


AP-C: E-fields practice

Worksheet–E-field practice (point charges)


AP-C: Regarding the nature of the EM force (photons)

I did some reading to try to address the question from class regarding the nature of the EM force and the exchange of photons between charge carriers, creating a change in momentum (a.k.a an impulse) on those objects.

This discussion seems very relevant, but may require some additional reading.  The main idea is pretty easy to follow:

In an attempt to address the specific issue of how the virtual (exchange) photons are “radiated” from the source charge (Are they unidirectional or going everywhere all the time?), the best answer I can find for that particular question is that they are simply not emitted when another change is not present.  Only when another charge is “nearby” are photons exchanged between the objects.  Of course, this idea is problematic since the EM force has an infinite range, albeit one that tapers off with an inverse square of distance.  So, this implies that every charged object is constantly interacting with every other charge everywhere (since the range is infinite), but only those objects closer receive substantial numbers of photons and large changes of momentum as a result.  After a fashion then, I suppose one could argue that the photons are being radiated in all directions since they are interacting with all charge everywhere at all times, even when relatively isolated.

Of course I may be somewhat wrong, as I said in class, this theory was only sparsely addressed in my undergraduate work and I have never studied the specifics, but this is my reading of the research I obtained for you.


UIL: Bus Schedule for Princeton (1/10/15)

Take the bus for your first event.  All students must ride the bus to the event.

Shuttle 1
Holland is in charge/heat count
Meet at school @ 5:55, departs 6:10.  If your late, you are out.

All Speech/Debate (Holland)
Number Sense (Devito)
Current Events
Computer Apps (Rice)

Shuttle will return to PHS immediately after drop off.

Shuttle 2
Boehringer in charge/head count
Meet @ school @ 7:20, departs 7:35

Journalism (Hamlin)
Calculator (Gomez)
Comp. Sci (Boehringer supervise)
Science (Bradstreet)
Spelling (Castleberg)
Lit Crit (Rutledge)
Accounting (Boehringer supervise)
Social Studies (Atkinson)
Math (Kirk)
Bus will leave to return to campus @ approx 12:30 (after 11:30 tests release) so completed students may return to school if they wish.

  • Parents/Legal Guardians may pick up students after contests.  Parents must enter the facility and sign student out with PHS staff member.  Parents/Guardians may only pick up their own children, even with a note.  Sorry, no exceptions.
  • Bus will come back to Princeton for final pick up at 6pm and return to PHS.  ETA depends on tournament outcomes. Estimate 7-7:30pm.


AP-1 Final Exam Data and Plans for Spring

Howdy, AP Physics 1 students.  By now you have probably seen your final exam grades.  I am quite pleased with your performance, in general.  Not to say that there isn’t work to be done, but many of you are showing great promise in your mastery of elementary physics and good potential as far as your hopeful success on your AP exam in May is concerned.  Allow me to explain a little about your curve.  As I told many of you in class, the standards on the AP exam are what are called a norm-referenced test.  In the truest sense of a “standardized exam” the graders working for the College Board put in a great deal of statistical wizardry to ensure that students who score, say a 4, in any given year, are equally academically prepared versus students who score the same in the following year, and the year after that.  Because each test year is different, the “cut points” which separate a 4 from a 5, and a 3 from a 4 (etc.) are different every year, but still somewhat close.  Since your final exam was based entirely on AP test questions and/or AP quality questions (for the most part), I am very comfortable applying the type of “curves” that you might see on your AP exam to this winter final exam.  That said, the data below is based on averages of the now-defunct AP Physics B program, since there is no real reference for this new class that you are taking.  If you have any questions about how the curve (and your grade was generated), please let me know.  I will try to walk you through it step by step, below.

First I created cut points by averaging cut points from old Physics B tests in recent years for which I have data:

2004 2009 2013 2012 Average % of total
5 115 112 123 129 119.8 67%
4 91 85 97 100 93.3 52%
3 63 57 70 71 65.3 36%
2 49 40 52 52 48.3 27%

You should assume any scores below the threshold for a “2” would earn a 1.  These scores are out of 180 total points on the test.  The percentage scores at the right are the important element for my calculations.

After a great deal of reflection and some math wizardly of my own, taking into account that you were responsible for roughly 1/3 of the content (vs. the Physics B tests above) and the number of items on your test (55), I generated the following cut points to be commensurate with the AP scoring guidelines above.

Mr. B’s Cut pts #out of 55 Gradebook grade Delta
70% (5) 39 93+ 23
60% (4) 33 83+ 23
50% (3) 28 73+ 23
40% (2) 22 63+ 23

Notice that each group block has the same percentage increase on the low end (+23 points), ensuring that the score changes are as fair as possible for all students.  I then applied a formula that would take your actual, percentage score, and scale it appropriately within your score block.  For instance, a student who earns a 5 with 40 items correct will get a lower grade than a student who earns a 5 with 45 items correct, but all of this must be accomplished within a spread of 7 possible points (i.e., 93-100).  Below you will find the result of that proportional scaling.  You will also see the frequencies of each score out of all AP Physics 1 students who took the exam this week.

Raw pts out of 55 Raw % AP Eqv Scale % Frequency
19 35 1 58 1
20 36 1 59 2
21 38 1 61 2
22 40 2 63 3
23 42 2 65 5
24 44 2 67 2
26 47 2 70 6
27 49 2 72 3
28 51 3 74 3
29 53 3 76 7
30 55 3 78 5
31 56 3 79 5
32 58 3 81 2
33 60 4 83 5
34 62 4 85 5
35 64 4 87 6
36 65 4 88 10
37 67 4 90 4
38 69 4 92 4
39 71 5 93 2
40 73 5 94 4
41 75 5 94 2
42 76 5 94 1
43 78 5 95 1
44 80 5 95 4
45 82 5 96 1
48 87 5 97 2

Next you will see the individual breakdown of all AP scores (1-5) and the percentage of students who earned each rank:

AP Score Frequency % of scores
1 5 5.2%
2 19 19.8%
3 22 22.9%
4 33 34.4%
5 17 17.7%

I’m really very pleased by this last bit of data as it indicates that if an appropriate AP test was given to you today, about 75% of you could, conceivably walk away with college credit by a score of 3 or better.  I am very heartened by the fact that more than half of you would have earned a 4 or a 5.  Great work!

Now, the bad news:  All of you, even those with top scores can do better.  In analyzing the questions missed I found some patterns.  Some are my fault for nor giving enough emphasis to certain topics, but some are on you as well.  In some cases I saw a clear pattern that you were not applying things that you had mastered to address the problems or situations you were given on the exam.  Never fear, we will address all of these deficiencies next spring, starting with spending at least half of our first class back in Jan. by dissecting your individual performances on the exam.  What did you miss?  Why did you miss it?  How are we going to avoid that pitfall in the future.  The good news is that a change of just 5 or 6 questions will put most of you over the cut point into the next score group (e.g., a 4 becomes a 5!).  I have every confidence that you can succeed.

Topics for the Spring will include:

  • Momentum and collisions (what happens when things run into each other and/or explode into many pieces)
  • Rotational Motion (everything we have studied to far, but now moving in circles)
  • Gravity and Planetary motion
  • Harmonic Oscillators, Waves, & Sound
  • Static Electricity
  • DC Circuits

My hope is that we will conclude new material by Spring Break, but that never happens in my other AP class, so I doubt it will for us, but that’s what we are working towards.  I hope to have at least two weeks of review time prior to your AP exam in May.


Tutoring during finals week





Mon. (12/15)


3:30 – 5

Tues. (12/16)


3:30 – 5

Wed. (12/17)






Fri. (12/19)



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