Circular Motion FBD Clicker Questions

In physics today, we spent a long time discussing clicker questions that ask students to identify the correct freebody diagrams for three situations:

1. Objects swinging through the bottom of pendulum swing

2. A car cresting a hill

3. A roller coaster passing through top of loop.

In total we probably spent 45 minutes in discussion. [Note: Next time I want to have quantitative demo setup for first, so that after debate we can see tension is larger than the weight. I would have to think hard about how to set up the cresting hill demo to measure the normal force.]

Anyway, I really let groups discuss these questions. On first one, our first hurdle concerned forces in direction of motion. That wasn’t too difficult because it’s an issue we’ve wrestled with in 1D, but definitely had a fair share of students picking those as their initial response. Even though students argued well that since we know acceleration is inward the net force must be inward, there was a big contingent unconvinced. This is where having quantitative demo would help, but only after getting arguments on table. Mostly they were unconvinced because of wanting their to be a stronger downward force. What was convincing to many is the idea that you would be worried about breaking the string if you had a lot of velocity. Having higher velocity causes greater tension, and the greater tension would break the string. I tried to connect this idea to, it’s because of the greater tension force upward, that the pendulum gets turned upward.

The cresting hill was not made easier just because of the insight with the previous one. While many still argued that Fnet must point toward center, meaning weight was larger than normal; a still strong contingent still argued for upward force being larger. It took several arguments to clear this up: first realizing that the sensation of car going up was not a force. Before cresting hill, there was some upward velocity (even liquid in your belly might still be going up! as you crest the hill). But real crux was figuring out what made the normal force decrease–it was this upward motion that caused car to “almost leave the ground” and therefore press less into the ground. Since you are less pressed into the ground, the normal force goes down. We contrasted this with hitting bottom of hill, the cars downward motion before hitting bottom would lead to tires pressing hard into ground.

In this question, the hold out contingency was crucial because it forced arguments to be clarified and for counter arguments to be addressed. It also forced us to ask, “How did that happen” not just “it must be true”. By talking about moments leading up to cresting and bottoming, we were attending to mechanism by which normal force could increase or decreases.

The third one was also hard. At this point, most students caught on that the net force must be directed toward center, but not all. Some students wanted more upward force to keep you from falling. But most wanted weight to be down and some other force less than weight to be up–still a net force down. Many students were eventually convinced of right answer…. Two downward forces. The main argument was that you should flip the diagram in your head to see that track pushes “in”. But the question  remained, “Wouldn’t that mean the car would fall?” Eventually the idea surfaced that it’s the velocity that carries you forward as the force tries to pull you down–I made connection to projectiles. But I wish I had asked, “have we studied any other kind of motion where an object has a force in one direction, but it doesn’t go in that direction because of velocity carries it as that force acts?”

After all that, I wish we had gone to solve one or more problems around those clicker questions. But our agenda has us pivoting to orbits and gravity. It wasn’t too bad of a pivot ending the last question talking about projectiles. But still, I want to use clicker questions to motivate problem-solving more.

Overall, One thing that was nice during these discussion was that students were listening and responding to each other rather than  just saying what they thought. It helped that I asked for people to state whether they agree or disagree and with whom. Students responded by often saying why they agree, which doesn’t always happen. I also asked students to not only tell me the right one, but to tell me one you know is wrong and why. That’s a lower barrier to entry. You don’t have to know right answer to contribute. Often this leads students who picked that one to speak up. Last thing I did was push for students who changed their mind to say why they did. What were you thinking at first? What did you hear that convinced you other wise?

I still don’t have all students engaged in whole class discussion. About a third of class contributes 90% of the whole class chatter. Most students engage in small groups as contributors. I need to think of how to pull more students in.

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