today was all assessments all day, but after school the Physics Olympics team was working on prototyping electric cars for the next meet. This is one student and a prototype (we usually prototype with K’nex). Every February the electric car event specifies motors, voltage, size, and other aspects of the car, and scoring criteria. This year the goal is to build an efficient, fast car, and the score will be based on the time to go a specific distance and the total voltage of the batteries. Specifically,

Score = 100/(V x t)
where V is the magnitude of the sum of the potential difference ratings on all batteries in circuit, and t is the magnitude of the photogate time in seconds.

So, we want lightweight, low friction, one 1.5-volt cell, and it is important that the car go straight (not veering by more than 1.5 m over a 10-m track). We’ll work on perfecting the design over break and in the new year.


IMG_20121219_14145969 Some lovely kinematics graphs in preparation for tomorrow’s assessment.


Net Torque Equals Zero

20121219-073948.jpg68 (This is for yesterday, only I fell asleep right after dinner and didn’t post it then) This is one of my favorite AP labs. Static equilibrium! I set up six stations each with a meterstick that has some masses resting on it or hanging off it. Two trusses, two supported by two tension forces, and two with one end supported on a rod clamp and the other pulled in the other direction by a tension force. Two have unknown masses, two have unknown tension forces, and two have unknown meter stick masses. All the tension forces have a spring scale attached, but the “unknown” tension forces have paper wrapped around them so they can’t be read. Students have to take whatever measurements they can (I give them protractors, rulers, one more meter stick, and a plumb bob) on two of the stations and determine the unknown value. They must also provide me with equations for net force and net torque.

Applying the Model


67 With some effort, students got through a couple of problems from one of Kelly O’Shea’s packets. There is still a lot of confusion on units, but I am trying to blast through the rest of CAPM this week and planning to start unbalanced forces after break. Wish me luck!

(The rectangular area is obviously not 91 m, but the girl who wrote 16 was on the opposite side of the board from the girl who was writing most of the work. Similarly, the triangular area is not infinity, but 8 m.)





66 Today some conceptual students did their first “practicum.” They timed how long it took the slow-acceleration disk to roll one meter down its ramp. Then they had to determine how fast it had been going when it passed the 1-m mark. While they timed it, I took video. While they debated how to find the answer, I did video analysis. They told me the answer was 0.16 m/s and then Tracker told me the answer was 0.164 m/s. So I told them that they got it right. They were a little stunned, I think, and I hope they were also proud of themselves for doing it. Next step: more complicated questions.




65 There is something very satisfying about doing some algebra. This is an old AP free-response problem.

Coming to a stop


64 I skipped yesterday. Today we were working on accelerating situations. The photo was taken of the interactive whiteboard after going over a question. One class totally got it. Another class totally didn’t. One class hasn’t gotten there yet, and half of them have been in “Keystone Exams” yesterday and today, so we’ll have to do some catching up. Maybe tomorrow, when over 200 kids from our school go to the local DECA competition! (December is not a great month for powering through curriculum.)




62 Today the conceptual classes were learning to enter formulas into Excel to get the computer to do multiple tedious calculations.


Physics Olympics Meet #2


EXTRA! We had our second Physics Olympics meet of the year yesterday. I am proud of this group of kids (and five others, not shown) for placing third in the DC Circuits Mystery Lab event, in which one of the things they had to do was determine the capacitance of a capacitor using the time constant, with a stopwatch and a voltmeter as their tools. We learned that technique less than a week ago, and haven’t gotten to the theory of it in AP Physics yet. Heck, we haven’t started E&M in AP Physics yet. I am also pleased that the problem-solving team solved all their problems correctly, including the one that had to be solved symbolically rather than numerically. But I am also angry at them for ignoring the instruction on significant figures. They had too many sig figs and that brought our score down.

We also took third place in the aluminum foil tower event, in which students had to build a tower with a 60-cm piece of foil which would support a ping-pong ball. Tallest tower wins. We got over 1 meter, but I don’t know by how much.

Unfortunately, we did not score very high in the build-ahead events, so overall we were in 7th place for the meet (out of 9 schools). But we are still in third place overall for the league to date, so he have hope. In the next meet, there will be an electric vehicle event, the efficient bridge event (we use the IIT rules and our event is the local qualifier for their contest), and the physics relay! The relay involves both athletic requirements and problem-solving, and is quite a challenge. My school used to dominate the relay, since I had athletes in my Physics 2 class. I’m hoping we can re-establish our awesomeness this year with the relay.

Final scores:


Olympics 2


61 Tomorrow is our second Physics Olympics meet of the year. Students were practicing “straw arm” in which they have 25 McDonalds straws and some pins and they have to build a cantilever out from a table to support a 100-gram mass. This is another arty photo on my nice black lab table.