So I forgot that earlier this week we had some N1L fun. Of course I did the tablecloth and dishes, and most kids tried it as well and really loved it! I also had 1/2 index cards and pennies for the index card/penny on your thumb “trick”. Today, being Halloween, and the Giants victory parade – we have a shortened schedule. My class that is ahead (and I am still missing like 1/3 of the kids) – we decided to take the day off and give the other class a chance to catch up (they will do the FBDs). Some of the boys decided to set up a golf course hole (since one of the students had golf clubs in his costume) – of course I tried and got a hole in one! Take that! And I thought…. hmmmm… how could we turn this into a lab??

Today we whiteboarded some FBDs before taking the Appraisal 2. They did fairly well, but still the misconception of some random force in the direction of the motion keeps creeping up. The students who presented the player sliding into base labeled a F(slide) – but were not sure…. I told them when they are not sure what to label the force, and keep questioning themselves to figure it out, that likely means it is not a force and should be removed! This is one misconception that dies hard…. This year I am also making them write what Case of N1L it is (or just balanced/unbalanced if they prefer).

College-Prep Physics: Started off with this Do Now.

Which went just fine….

So then I threw this at them. (Remember, we haven’t done dynamic cases yet.) And here’s how they voted:

Not surprising. So then I follow up with a demo, but ask them to predict first. And here’s how they voted:

What?!? I’m happy, but confused. And when several kids see my expression, a few kids say, “Wait. That’s just like the previous situation, isn’t it?”

So we test it out. Instead of roller skates, I have one kid sit on a cart. I set up a (spring scale)–(string)–(spring)–(string)–(spring scale) between the two people. Having a spring in the middle is nice because it allows the tension in the string to slowly change as the kid on the cart speeds up. Then I send the kid on the cart back and the spring/strings slow the kid down gradually. And the WHOLE time, the scales read the same. And it makes sense to them now, because they think/visualize the rope as one long spring than stretches and maintains even tension throughout. And when one person pulls harder, the whole string/pulls harder. The spring stretch/compresses evenly along its length. We don’t see one end of the spring stretched out more than the other end.

So then if the tension between the two people is always the same, who wins at tug of war? Why did the kid on the cart slide, but the kid standing on the floor didn’t slide? (Friction!)

So then we modify our diagrams from before.

And then we watch a few tug of war movies I found from another physics teacher who posted them on his class website, paying close attention to friction (or lack thereof).

##BFPM

NGSS Science and Engineering Practice #2: Developing and Using Models

Today in AP Physics 2, we began to investigate the concept electrical potential energy. It is important that students have a strong understanding in electrical potential energy before we start to investigate the concept of electric potential. While students are familiar with other forms of potential energy (gravitational, elastic), a couple of aspects of electrical potential energy provides a challenge. One, unlike gravitational potential energy, the electrical potential energy depends not only on the position of the particle in the field but on the sign of the charge of the particle as well. Two, negative electrical potential energies are quite common. While “zero” gravitational potential energy could be defined such that negative gravitational potential energies are considered, students rarely encounter this in their first-year course.

Students spent most of class today, thinking through the following nine scenarios (from Knight’s Five Easy Lessons) and whether the electrical potential energy of the particle increases, decrease, or stays the same from the initial point to the final point; first individually, then among their groups, and finally as a whole class. It was fantastic to hear students justify their answers in terms of energy conservation, work done on the particle, and the relationship between the direction of displacement and force.

College-Prep Physics: Since our last look at force diagrams, we’ve explored tension and friction forces. So we did another set of interaction and force diagrams, this time incorporating tension and friction forces.

I’m really happy to be using agent-object notation for force diagrams again. But we ran into trouble with labeling friction forces because, when written using agent-object notation, they are labeled the same way as normal forces. So we decided to add the words “friction” and “normal” to our labels to distinguish them. There were a couple of options I had also considered:

Use force type notation — F_{N} and F_{f} — but that doesn’t emphasize the objects that are interacting. Also doesn’t lend itself to easily finding 3rd Law pairs.

Replace F with force type — N_{A on B }and f_{A on B }– but that doesn’t emphasize that they are all forces. Modeling makes a similar emphasis with energy — using E_{k} and E_{g} rather than K and U.

And on this sheet, like the other one, I had scenarios where more than one force acted on an object in the same direction (#2, #3) and a scenario where the normal force wasn’t equal to the object’s weight (#3).

I also asked students to draw force diagrams for 2 different objects in each scenario. This allowed us to see 3rd Law pairs (indicated with circles and triangles on the force diagrams and interaction diagrams).

Finally, we figured out the values of all the forces. Identifying those 3rd Law pairs was necessary in order to find the values for some the forces.

We wrapped up with a summary of the forces we investigated so far. I’m trying to emphasize the mechanism of how the force works — this was the purpose of all those voting questions from Preconceptions in Mechanics.