Archive | October 2014

Day 40: Factors Affecting Friction


College-Prep Physics: First, we brainstormed possible factors that might affect the maximum strength of static friction between two surfaces. Then students designed their own experiments to determine which of those factors actually mattered. Finally, we tried to use our “interlocking bristles” model to explain our results.

— Weight/mass: Definitely affected friction. Why? The bristles interlocked more, making it tougher for them to slide past each other. This is easily demonstrated and felt using toothbrushes.

— Surface Area: Surprisingly, this did NOT matter (with the exception of groups that used highly irregular surfaces like carpet, felt, and cork). Why? Well, a larger surface area means more bristles in contact, which should mean more friction. But a larger surface area also means the surfaces are less compressed, which would reduce the the friction. This is easily demonstrated with weights and foam.


So the net effect is no change in friction.

— Surface material: Changing the material of either surface also affected the maximum amount of static friction between the surfaces. This is similar to changing the material and arrangement of the toothbrush bristles.


NGSS Science and Engineering Practices
#2. Developing and using models

#3. Planning and carrying out investigations


Day 39: Coffee Filters, Air Resistance, and Desmos


AP Physics C: Students designed their own experiments to determine the type of dependence between speed and air resistance. It was the traditional coffee filter lab, but groups could collect data however they wanted. Some groups used motion detectors, others used meter sticks and stopwatches, and others used video analysis. Then we put the new regression feature in Desmos to the test. Our results so far:


NGSS Science and Engineering Practices:
#3. Planning and carrying out investigations
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking

Day 38: Who Wins at Tug-of-War?

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.

Tug-of-War (3)

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).


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

Day 37: Force Diagrams, Part 2

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 — FN and Ff — 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 — NA on B and fA on B  — but that doesn’t emphasize that they are all forces. Modeling makes a similar emphasis with energy — using Ek and Eg 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.

FORCE — When is it present? — How does it work?

wpid-img_20141028_103008726.jpg wpid-img_20141028_103018418.jpg

You can get a copy of the both handouts here:


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

Day 36: Plickers


College-Prep Physics and AP Physics C: Today we tried out Plickers. It’s a student response system that uses paper clickers (hence the name plickers?) and the camera on the teacher’s smartphone/tablet to record their responses. The picture above is screenshot from my smartphone’s app: You can see the histogram of responses (top-right) and the class list with responses (top-left) overlayed on the camera view which flashes students names as it recognizes their responses as you scan the room with your phone’s camera. Each student has a unique QR-code card that they hold in different orientations to indicate a vote of A, B, C, or D. So a camera scan of a code identifies the student and how they are voting.

The inspiration for using Plickers started with my Global Physics talk this past Saturday. During the discussion, I talked a bit about Preconceptions in Mechanics, and Andy posted a link to his blog post about cards vs clickers. I reread Andy’s post that afternoon, and decided to make low-tech response cards for my students to tape into the inside back-cover of their lab notebooks:


Shortly after I posted that picture to Twitter, I got a reply from Lisa:

which prompted me to check out Plickers.

The Good: It’s set up very well for peer-instruction type discussions. While I scan the room with my phone’s camera, I can project the Plickers website from my desktop computer and show/hide a real-time histogram of students answers, or show the class roster which displays a check mark next to students who have registered their answers.


You can ask questions on the fly, or create a deck of questions in advance (like the one above).

The Bad: You can’t add pictures to questions or import a PowerPoint slide deck. But you can easily switch back and forth between applications if you want to show the class histogram after voting. Or just put up windows side-by-side:



While Plickers lets you go back and review the response history of a question, including individual student responses, it cannot generate a student report showing a history of all questions for a particular student. This isn’t a feature I need or use, but it is a feature that traditional clicker systems have, so I figured I mention it.

Time is another consideration. While scanning cards with the phone is rather quick, it’s still quicker to go the low-tech route with colored index cards or the 4-sided card I was originally going to use. But I’ll say that the histogram is really powerful, both for me and the students. Sure, I could say “Most of the class is voting B” or “Looks like there’s a 50/50 split between B and C” but for students to actually see the histogram (and to see it again after a peer-instruction re-vote) carries more impact.

In the end, I think the good outweighs the bad. I’m having the kids tape their Plicker cards onto the inside back cover of their lab notebooks.


UPDATE 28 OCT 2014: We did a set of Peer Instruction-type questions in AP Physics C today. There were a few hiccups — the network dropped a few times (an issue on our end, not Plickers) and some kids kept mistakenly covering up part of the QR code with their hands. And it does seem to take several seconds longer than just using colored cards and doing a visual scan. BUT the histograms are cool and provide some motivation and interest.

And something interesting happened today. When I did the Peer Instruction “vote – discuss – revote” cycle, rather than gravitate toward the right answer, the class split about 50/50 between the right answer and a wrong one. Now what? Discuss and revote again?






(The correct answer is C.)

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:


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.


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

oprah car

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

Much better!



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

Day 34: Levis Jeans

Levis Tension (3)Levis Tension (2)

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?


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:


Even metal wire stretches!


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

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 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!


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

Day 31: Equality of Normal Forces

College-Prep Physics: On Friday, we established that the table must be pushing up on the book. Today, we explored a different scenario to determine if normal forces between objects we equal in size. (Based on a similar sequence in Preconception in Mechanics.)

VOTE #1: Compare the forces between the wood stick and the car. (target)


I set up a slow buggy driving into a wood dowel that is hanging down from a ringstand clamp. If you remove the tire treads, the buggy wheels will continue to spin, showing that the buggy is continuously pushing against the dowel.


Some students says the forces are equal, some say the buggy is pushing harder because it’s trying to roll into the stick, and some way the stick is pushing harder to keep the buggy in place.

I don’t give the answer, but give them the next scenario instead.

VOTE #2: Compare the forces between the hand and the spring. (anchor)

Most kids say they are the same. It helps to think of a small, motionless board in place between the hand and the spring. Since the board is at rest, the hand and the spring must be pushing equally on the board. Now gently slide the board out from between the hand and the spring. Have any of the forces changed? So how do the forces compare? If I push harder on the spring, what happens? Are the forces the same now? How does the spring know how hard to push? (A lot of kids talk about the spring adjusting or compensating until the forces are equal. Some even refer to the spring lab we did previously. While the forces are ALWAYS equal, even while the spring is moving, I let that detail slide because we’ll return to the dynamic case in another lesson.)

VOTE #3: Compare the forces between the stiff and loose rubber band. (bridge)


Again, most kids got that the rubber bands pull equally because the ring is at rest. How is this possible when one rubber band is stretched more than the other? What happens when you try to make one of the rubber bands pull harder? What happens if the ring is removed and the rubber bands are tied together? Are the forces still equal?

VOTE #4: Compare the forces between the rubber hose and the car. (bridge)


Now I have the slow buggy drive into a piece of flexible rubber hose. The slow buggy works well because the hose will visibly flex and while keeping the buggy in place.


Again, students say the forces are the same. How does the hose “know” how hard to push? What would happen if we replaced the slow buggy with the fast buggy?

VOTE #5: Compare the forces between the wood stick and the car. (target)

We return to the first scenario and re-vote. Students make the connection that the wooden stick still bends and the force between the car and the stick must be equal. Then I quick run through the book scenarios from the previous lesson and ask them to compare the forces (the same, the same, the same, …)


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