Recently I find that every time I say something like “the best way to approach this problem is…”, kids find a clever, elegant, or just different way to approach it. The force addition diagrams above are a great example.
The kids had some pretty awesome problem solving going on today.
I’ve probably written about this before, but Kelly sold me on force addition diagrams last year and I don’t think I’ll ever look back. So far I haven’t come across a situation that I’d rather use components on. This was from the problem below, we spent a whole hour on it. At this point this group was still wrestling with how to use the FAD to solve the problem.
My students are struggling on the difference between equal and balanced. This is what I wrote on the board, but not sure I like it. Thoughts?
Today I led the classes in a series of Newton’s 3rd Law demonstrations. We learned the above demonstration from Eric Gettrust, our Modeling Instruction leader, which he published in The Physics Teacher. The purpose of this demo is to model the interaction between a person and the Earth, showing that the attraction force itself is equal though there is a greater effect on the smaller object. Close up below.
I also do car crashes (picture below), tug of war with two force detectors, and the class favorite, chest bumps with force plates.
We have done very little problem solving with BFPM, so naturally I left my students to work on it themselves. I had an all-day meeting for science articulation, and the directions I left were for each group to whiteboard problem 2, circle up, debate and choose a best answer, take a picture and send it to me. Then repeat with problem 2. I can’t wait to check in with them tomorrow about how it went, specifically discussing 3b. Other classes below. Oh yeah, the problems are from Practice 3 here.
In this scenario a sled was being pushed at a constant velocity 2 m/s, then the following question was asked;
c. Describe, in terms of the amount of force he would have to apply, what the player would have to do to make the sled move with a constant velocity of 3.0 m/s. Assume that the frictional force between the grass and the sled remains the same under all circumstances. Illustrate your answer with diagrams and/or graphs as appropriate.
It continually astounds me that even though we have had days of conversations about constant velocity and balanced forces, the idea that a higher speed requires more force is firmly rooted in their brains. Another group gave pretty much the same reasoning below. The only way we can expunge misconceptions is to address them head on!
Yesterday we took 10 minutes at the end of class to collect force of gravity vs. mass data. We whiteboarded it at the begining of the hour today, with a very convincing result that the slope is the gravitational field strength of earth, 9.8 N/kg. At the end of the hour we collected spring force vs. distance stretched data for springs, and we’ll whiteboard that after the long weekend.
It’s telling to the power embedded misconceptions that even in a research based classroom where we addressed this idea for 3 days, students still want forces to be balanced at the top of the path of a projectile.
Today we were again whiteboarding from the standard modeling WS1 in Balanced Forces. We didn’t get to this one today, so I look forward to the discussion on it tomorrow. It is for the problem below.