# Day 17: Forces – Revisions for Next Year

College-Prep Physics: Three weeks in, and I’m already thinking of changes for next year. As I’ve said before, I’m using Preconceptions in Mechanics as the foundation for our forces unit. Pictured above is the recommended sequence, which I’m finding out isn’t quite working. Changes I’ll make:

• Do Gravitational Forces 1 first. Understanding the nature of gravity and how it acts will help make normal forces easier to understand, particularly when analyzing stacked masses. Also, gravity is the force everyone knows about. Why ignore its existence until much later?
• Integrate system schemas and free-body diagrams into the HW problems. PiM doesn’t do them (which makes the book more flexible for fitting into instructors’ different teaching styles). But without SS and FBDs, we have a less structured way of analyzing the forces in the HW problems, which is likely causing some confusion. This means that I cannot simply photcopy the PiM HW sheets as is, but I will need to re-write them and add supplemental problems.

• Newton’s 3rd Law is treated throughout all the lessons. The static case is developed first, with the dynamic case delayed until later. In fact, 3rd Law reasoning is essential to developing the proper models for each force.
• The voting and discussion that happens around the anchor-bridge-target scenarios. (I personally need to work on getting more students involved, though.)

Next year’s anticipated sequence:

1. Bowling Ball and Mallets (get at qualitative model for forces and motion)***
2. Dueling Fan Carts (qualitative model for NET force and motion)***
3. Gravitational Force I
4. Gravitational Force vs. Mass Lab (simple lab, one data set, focus on making good graph)
5. Normal Force (ball-and-spring model leads into spring lab)
6. Spring Force vs. Stretch Lab (simple lab, 2 data sets, focus on taking data over a wide range of levels, each spring will likely NOT have same set of levels).
7. Tension (Equal tensions along rope leads to forces in tug of war.)
8. Friction Forces
9. Friction of Shoe Lab (1 data set, focus on range of levels AND multiple trials per level, graph all trials to see uncertainty in data)

So far for THIS year, we’ve only done 1,5, and 6. Looks like I’ll have to head back and do 2, 3, 4, then introduce schema and FBDs, followed by 7, 8, and 9.

*sigh* This teaching thing would be so much easier if I just assigned textbook reading and end-of-chapter problems.

##BFPM

*** UPDATE 7 OCT 2013: Now thinking of delaying the bowling ball and the fan carts until after the sequence of forces. Trying to separate the static and the dynamic cases. Kids get the static case — balanced forces means constant zero velocity. Having all the forces under our belt will allow for a deeper analysis of the dynamic cases (bowling ball and fan carts).