Day 35: Toothbrushes and Friction

College-Prep Physics: Today will likely be our last round of voting for while. As per Preconceptions in Mechanics, we started this round of discussion on friction with a Pre-Instruction quiz. I set up a toy buggy (without the tire treads) connected to a friction sled by a rubber band to help visualize the scenario:

Day 35: Toothbrushes and Friction

In previous years, I’ve used a pair of hair bushes to model friction between surfaces. But the black bristles made it hard for everyone to see. So I took PiM’s advice and bought a class set of toothbrushes.

Day 35: Toothbrushes and Friction

And gave everyone a toothbrush so they could interlock brushes with a partner and observe.

Day 35: Toothbrushes and Friction

“And you get a toothbrush! And you get a toothbrush! Everybody gets a toothbrush!”

Much better!

Day 35: Toothbrushes and Friction

##BFPM

NGSS Science and Engineering Practice #2: Developing Models
NGSS Science and Engineering Practice #6: Constructing Explanations

Day 35: Toothbrushes and Friction

Day 44: Bridge from Coulomb’s Law Computational Model to Electric Fields

Today, we started class with students sharing their computational models. I then used one of the students models that attached a vector for the net force to the point charge to illustrate how their model showed the direction and relative magnitude of the electric force at every point through which the point charge moved. I then asked what we could do if we wanted to know the direction and relative magnitude of the electric force at every point around a charged object. Once they said that we would need a lot more point charges, I showed them this model:

Day 44: Bridge from Coulomb’s Law Computational Model to Electric Fields

While tomorrow’s lesson will formally introduce the concept of the electric field and its relationship to the electric force, I did use this model as a bridge by introducing the visual of electric field vectors. I also showed them another model with multiple charged objects to demonstrate that the electric field vectors represent a superposition of the electric force due to each charged particle:

Day 44: Bridge from Coulomb’s Law Computational Model to Electric Fields

I tried to write this model in a fairly extensible manner to make it easy to model various configurations of charged objects.

  ##electrostatics ##representations ##tech ##glowscript  

Day 34: Levis Jeans

Day 34: Levis JeansDay 34: Levis Jeans

College-Prep Physics: Which situation is more likely to rip the Levi’s jeans?

Today we did another round of voting to get at the idea that tension in springs, strings, and ropes are constant all the way through. Today’s slides:

During which, we do one of my favorite demos: What does the middle scale read?

Day 34: Levis Jeans

Click the picture to reveal the answer

In the end, we talk about how a seemingly unstretchable rope or spring actually stretches under tension, much like seemingly unbendable surfaces like tables deform under compression. Just like a solid can be modeled as a network of balls and springs, so can rope and string:

Day 34: Levis Jeans

Even metal wire stretches!

##BFPM

NGSS Science and Engineering Practice #2: Developing Models
NGSS Science and Engineering Practice #6: Constructing Explanations

Day 34: Levis Jeans

Day 43: Coulomb’s Law Computational Model Continued

Today, AP Physics 2 students continued to develop their computational model of a the electric force on one or more point charges due to one of more charged objects. Some developed simple harmonic oscillators; others, helixes; others, eccentric orbits. Everyone got at least a basic version of the computational model working. Brandon, Tiffany, and Chris made some of my favorites:

Day 43: Coulomb’s Law Computational Model Continued

Day 43: Coulomb’s Law Computational Model Continued

  ##electrostatics ##representations ##tech ##glowscript  

Day 33: Popsicle Sticks

Day 33: Popsicle Sticks

I’m really bad at calling on students at random. So today while students were taking a quiz, I wrote each of their names on Popsicle sticks. I wrote the period number on the other end of stick for easy identification.

The sticks will also come in handy for determining new random lab groups and seats. I change seats about every 2 weeks, but our SIS’s seating chart randomizer isn’t very random.

 

Day 33: Popsicle Sticks

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Day 32: Interaction Diagrams and Force Diagrams

Day 32: Interaction Diagrams and Force Diagrams

College-Prep Physics: Now that we have gravitational forces, spring forces, and normal forces under our belts, we can analyse more complex situations. Today was a direct instruction lesson* on drawing interaction diagrams and forces diagrams. You might notice some changes from how I drew them from last year.

I’m using agent-object notation on the force diagrams, rather than last year’s force type + agent, in order to combat the misconception that the force diagram represents what the object is doing, rather than what is being done to the object. This also helps with getting the students to focus on the objects that are exerting the forces, because “every force has a source.” To make the force diagrams easier to read and label, we’re not including the force types on the force diagram vectors. Force types are labeled on the interaction diagram only, to help reinforce that a force is a single interaction between objects.

I’m also starting with complex scenarios early, and also asking students to draw more than one force diagram for a given situation. Last year, some students had the misconception that there must always be one force up, down, left, and right. The didn’t realize you could have 2 forces in one direction or no forces at all.

Drawing multiple force diagrams also allows for identifying 3rd Law pairs (the two vectors with circles in #4 above, though we haven’t formally called them 3rd Law pairs).

We also started with numerical values early. Although the scenarios don’t ask a specific question, we determined the values for as many forces as we could based on what was given.

In hopes of avoiding another common misconception, you’ll see that in both scenarios the normal forces aren’t equal to the weights of the objects.

We are only looking at static cases right now. Up next is tension, then friction. After friction, we’ll consider the dynamic cases.

The two scenarios pictured are taken from Preconception in Mechanics, though PiM doesn’t have the students draw interaction diagrams or force diagrams — a fault I found out too late last year. You can get the entire handout here: ForcesSchemaFBDDevelopmentStatic2015

PS: I haven’t been using the HW sheets from PiM at all. Rather, I’ve been using the occasional PiM HW problem as a bell ringer/do now/warm up.

*If you have a more engaging way of introducing interaction diagrams and force diagrams, please share!

##BFPM

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

Day 32: Interaction Diagrams and Force Diagrams