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.