Day 57: Balanced Force Lab Practical
College-Prep Physics: Today we did a balanced force lab practical to tie together all our work on forces. It’s similar to the ones I’ve written about in past years. However, this year we used the green buggies and whiteboards instead of wood blocks and carpet/rubber. (This is because this year, in previous labs, some groups already worked with wood blocks and carpet/rubber.)
Given only a green buggy, a whiteboard, a spring, a 200-gram mass, and a ruler:
- Determine the spring constant of your spring.
- Determine the weight of your green buggy.
- Determine the force of kinetic friction between your buggy’s rubber tires and your whiteboard.
- Determine the coefficient of kinetic friction between your buggy’s rubber tires and your whiteboard.
- Predict the force of kinetic friction when 500 grams is added to your buggy. Have your teacher test your prediction!
UPDATE 2014 DEC 3: We found that when the 500 gram mass is added to buggy, the buggy rolls (rather than slide) when pulled. A binder clip on a rear wheel works great to lock the wheels so the buggy slides.
NGSS Science and Engineering Practices:
#2. Developing and using models
#3. Planning and carrying out investigations
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking
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Day 54: Kinetic Friction
College-Prep Physics: Last Thursday, students investigated the factors that might affect kinetic friction and how kinetic friction compares to static friction.
Today, students looked at the relationship between normal force and kinetic friction. Is the relationship proportional, like our previous experiments with static friction? If so, how do the slopes for kinetic friction compare to that from our static friction experiment?
NGSS Science and Engineering Practices:
#2. Developing and using models
#3. Planning and carrying out investigations
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking
Day 52: Balanced Forces in Motion
College-Prep Physics: We’ve been looking solely at forces in static cases. Now it’s time to look at moving cases. So I asked students to complete the following chart on a whiteboard:
What’s true about the forces in the constant speed cases? In the speeding up and slowing down cases?
Then I tried to address the misconception/difficulties kids have with constant speed = balanced forces (wouldn’t it just not move?) by demo’ing dueling fan carts. First with fan carts off, then with both on — what happens after I push?
NGSS Science and Engineering Practices:
#2. Developing and using models
Day 44: Tug-of-War!
College-Prep Physics: We discussed the results from Wednesday’s friction lab:
What does the slope mean? How does the slope relate to the “grippiness” of the two surfaces?
Then we repeated the experiment with our shoes to determine the “grip factor” between our shoes and the tile. Then used the grip factor to extrapolate to how much friction there would be when we’re wearing our shoes.
Theoretically, the tug-of-war team that wins is the team with the most friction. So we used the friction calculations of the students who wanted to participate in the tug of war to determine the winning team.
Time to test our prediction!
##BFPM
NGSS Science and Engineering Practices:
#2. Developing and Using Models
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking
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.
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NGSS Science and Engineering Practices
#2. Developing and using models
#3. Planning and carrying out investigations
Day 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?
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!
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NGSS Science and Engineering Practice #2: Developing Models
NGSS Science and Engineering Practice #6: Constructing Explanations
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 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, …)
##BFPM
NGSS Science and Engineering Practice #2: Developing Models
NGSS Science and Engineering Practice #6: Constructing Explanations
Day 30: Does the Table Push Up on the Book?
College-Prep Physics: Today we did another round of voting (a la Preconceptions in Mechanics) to answer the question “Does the table push up on the book?”
One snafu that happened this year that didn’t happen last year: Because we studied gravitational forces first, kids were confused by the question and thought about the gravitational attraction between the book and the table. This was something I did not anticipate. So I had to clarify the scenario (explaining that table’s gravitational force on the book pulls the book down rather than push the book up as per the question).
Last year, that confusion wasn’t an issue because we did normal forces first, which is the suggested sequence in preconceptions in mechanics. But I was dissatisfied with that sequence because there were questions about normal forces between individual objects that are stacked on top of each other. We were talking about the object at the bottom of the stack having to support the weight of the objects on top. Those complex scenarios are easily analyzed using system schema and free-body diagrams, but we hadn’t talked about gravitational forces yet.
So, despite the confusion this year, I still think gravity should be done before normal force. So for next year, I’m revising the questions. I’m going to start with the hand on the spring question, since the answer is obvious and we just wrapped up the spring lab. Hoping that question puts kids in the proper mindset, then I’ll move to the table on the book question. And instead of the foam question, I’ll replace the foam with springs. (My foam never really deformed much anyway.) 
Here’s the revised slides I’ll try next year:
##BFPM
NGSS Science and Engineering Practice #2: Developing Models
NGSS Science and Engineering Practice #6: Constructing Explanations
Day 29: Spring Lab Debrief
College-Prep Physics: Yesterday, students designed their own experiments to determine the relationship between the stretch of a spring and the force applied to the spring. Today we shared out our results.
This year, I chose to use 2 different springs: one with a pre-load (red) and one without (green). My goal is to drive home that not all trends will go through the origin. Some groups still tried to fit a trend through the origin, though. Looking back, if I had been monitoring groups better, I would have asked those groups to go back and take more data for small stretches….hopefully continuing the linear trend down to the y-intercept.
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NGSS Science and Engineering Practice #4: Analyzing and Interpreting Data
NGSS Science and Engineering Practice #5. Using Mathematics
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