College-Prep Physics: Last class, students determined the factors that affected friction. They had said one of the factors was mass and/or weight. Today, we dug a bit deeper and analyzed a few different scenarios (above) to tease out the real factor. They did fine drawing the force diagrams (my annotations in red):
We skipped the interaction diagram this time, since I figured the scenarios were fairly easy. However, a few groups ended up drawing a single combined downward arrow for C and a single upward arrow for D, rather than 2 arrows to represent each object interaction.
Then I asked the class, “For your ranking, which value from the force diagram aided in your ranking?”
It’s not really about weight or mass. It’s not really about the downward force. It’s about the upward force from the surface!
Then we tried using our interlocking bristles model to explain our predictions. The more the surfaces are compressed together, the more the surface bristles interlock, and therefore the more friction there will be.
NGSS Science and Engineering Practices
#2. Developing and Using Models
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
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.
NGSS Science and Engineering Practice #2: Developing Models
NGSS Science and Engineering Practice #6: Constructing Explanations
College-Prep Physics: Much to most students’ surprise, surface area did not have an effect on the amount of friction between surfaces. So how can our “interlocking bumps”-model for friction explain this? It would seem that, for a greater surface areas, there are more bumps in contact, and therefore more friction. However, think about the compression of the 2 surfaces:
This demo is 2 identical metal drawers, one laying flat and the other standing up on one end. Inside both drawers is a 1 kg mass so there is noticeable compression. Although you can’t see it very well in the photo, the “no-bounce” foam is compressed a lot deeper on the right than on the left.
In other words, even though the situation on the right has fewer bumps in contact, it also has a larger compression, making it harder for any pair of interlocking bumps to slide past each other. And vice-versa: an increase in surface area leads more bumps in contact, but a decrease in surface compression, and therefore the total friction remains unchanged.
College-Prep Physics: I was inspired by John Burk’s weekend post about a classroom visit from Eugenia Etkina, in which she flat out says to John, “You guys like Socratic dialog way too much.” So today, I just told the class:
And I just let groups brainstorm and whiteboard their experimental designs first. I didn’t even tell them what materials they would have at their disposals to conduct their experiements — I just wanted to see what they would come up with first and not sway or confuse their thinking in any way.
I was impressed with everyone’s ideas, even if some struggled with control of variables for the surface area case. I particularly loved this board for its simplicity and effective communication via visuals:
(The words on the right got smudged. It says weight to match that of the big shoe.)
College-Prep Physics: Based on our shoe friction graphs from yesterday, students predicted the amount of friction between their shoes and the floor when they are wearing their shoes. Then 1 person from each of the 8 lab groups was assigned to a team for tug-of-war. We tried to predict which team would win based on our estimates for friction. As you can see, it was really too close to call!
College Prep Physics:
- What’s the same about everyone’s graphs? What’s different?
- Why do the graphs have different slopes? What does the slope physically mean?
- Explain how it is possible for two people wearing identical shoes to have different amounts of friction.
- Explain how it is possible for two people wearing different types of shoes to have the same amount of friction.
College-Prep Physics: Continuing our discussion of tug-of-war and the importance of friction…
ME: “So who wins tug-of-war?”
CLASS: “The team with the most friction.”
“So how could we measure the friction force between a person and the floor?”
“Well, how could we measure the friction force between this block and the table?”
“Pull on it with a spring scale until it moves. The scale reading is the same as friction in this case.”
“OK, so how could be measure the friction between a person and the floor?”
“Pull on the person with a spring scale until the move???”
“OK, so I need a volunteer.”
So I loop a large thick rope around the volunteer’s waist and connect the other end to a large 20 newton spring scale. And I begin to slowly pull on the student as we watch the reading on the scale approach 20 N, and then go past it.
ME: “Uh oh.”
CLASS: “You maxed out the scale. Guess we can’t measure the friction.”
I ask the volunteer for his/her shoe, then loop string around it and start to pull the string with the spring scale.
ME: “Looks like the shoe starts to slide at about 2 newtons.”
CLASS: “Yeah, but that’s just the shoe by itself. There’s a lot more friction when someone is wearing the shoe.”
So then we have a discussion about how if we could find a relationship between shoe weight (by adding weights to the shoe) and friction (within the range of our spring scales) then we could use the relationship to extrapolate all the way to the weight of the person. This is a great way to use experiments to determine the value of a quantity we can’t directly measure.
[PS: Tomorrow is a staff PD day, so we’ll be back on Wednesday!]
College-Prep Physics: Alice and Bob are playing tug of war. Based on our study of tension, we know the tension in the rope is the same throughout, which means the pull on both Alice and Bob will always be equal. So then how does anyone win at tug of war?
What would happen if Alice was on roller skates (or my rolling chair) instead? What if Alice and Bob were both on skates? Or on ice?
Then we watched several tug of war videos. Does the losing team go flying forwards due to the pull of the rope or does something else happen?
(We watched just the first match in the above video)
Why is tug of war so difficult on a slip-n-slide? How does the person on the the right cheat?
College-Prep Physics: As you can see, I had a number of groups struggle with drawing the free-body diagram for the question given. Namely, most groups left out the upwards force of friction.
ME: “Based solely on your force diagram, what would happen to the book?”
S: “It would fall.”
ME: “But it doesn’t. Why?”
They thought that just squeezing the book horizontally between the log and wall would keep it from falling. So then we did a few things:
1. We simulated the scenario using our hairbrushes. The two wooden brushes rubberbanded together represent the book, while the two black plastic brushes on each side represent the log and the wall. So what prevents the wooden brushes from falling? (Friction!) Which direction must it act? (Up!)
2. We simulated the scenario without friction. The whiteboard in the middle represents the book. The carts on each side represent the log and the wall. I held on the whiteboard in the air, while I had one of the students press the 2 carts together as tightly as they could in order to try to hold up the whiteboard. As soon as I let go of the whiteboard, it fell to the floor, no matter how hard the carts were pushed against the whiteboard. (I think this demo is more easily understood than the frictionless wall demo I did previously.)