Today we tried a little something different…. the past two years the kids have REALLY struggled drawing force diagrams. There are so many problems… So this year we decided to break it down a bit.
We started with only one object, I drew an example last class of a book on a table, we did the force diagram of the book. Today we did three more scenarios – mass hanging from spring scale, and then same thing, but pulling down or pushing up a little on the mass. I am also trying to call it “cylinder” since calling it a “mass” always seems to cause issues.
I drew the interaction diagram and force diagram with them for the hanging cylinder. I told them when they read the spring scale, it is telling them how hard the spring scale is pulling (Ft). Then I let them go. One class did great, one ok, and one (as expected) struggled a lot.
They first had to figure out that the spring scale reading equals the Fg since the cylinder is not moving, and the forces must be balanced. Then, for the next two, most classes at least drew three forces, though the had lots of ideas about how to draw them… this was new, I usually just show this, but I let them struggle with it for awhile. Also they didn’t have much idea what to do with the numbers, so there was all kinds of variety there… I tool many pictures of their (incorrect) boards. In the struggling class, many groups didn’t even draw three forces, often left off the Fg… we have a lot of work to do.
We circled up for a board meeting, and I walked around and pointed things out, starting with the Ft=Fg for the first one. Then we talked about the Fg interaction between the cylinder and earth… and decided nothing about THAT interaction changed (the fact that my hand was now there on the other two doesn’t involve the cylinder-earth interaction) – so the Fg should stay the same across all pictures. Then they started to figure out how to balance the forces. We also had a discussion about whether to put arrows side by side, or to stack them, and decided stacking them made it easier to see if they were balanced or unbalanced.
After this, we went back to the book-table diagram from yesterday and now looked from the table’s perspective. We drew the force diagram for the table, and then identified the N3L pair… must be the same force type, same feeler-dealer,but swapped positions.
Lastly, we did an example (originally taken from @fnoschese I think)… and made it quantitative. Wow this is hard. Really really hard. A couple of kids came after school and I remembered all of the problems… they don’t always mark the weight as Fg, they mess up the force types…. the have trouble identifying what is happening. Well, it is the hardest unit of the year, I have been warning them. We will press on and hopefully the way we are breaking it into smaller pieces will help them.
Overall, I really liked the new sort-of-exploration way to move forward with force diagrams! We will see if it pays off… time will tell!