The first set of posts I wrote for this series was about momentum because I made such a large shift from how I used to teach to how I currently teach.

In the same vein my teaching of forces has also changed.

In the past my force unit looked like this:

1. Inertia Day! Lots of Demos, initiation into the inertia club with club cards (you hold the card on your index finger with a penny on top and figure out how to flick the card out from the penny)
2. F=ma. Define it, notes, define force diagrams, practice force diagrams. Practice F=ma problems.
3. One day on action-reaction. Gloss over it; “it’s easy”

I cringe writing this out now. It was so boring! Inertia and action-reaction felt like fluff. We don’t need fluff!

Currently, my unit structure is designed with the big ideas in mind. (Because, tenet 3: Order Matters, Language Matters) I was excited to see that the idea that teaching in a structure that models the thinking we are targetting to improve outcomes is actually supported by research, so my model draws on Lei Bao’s frameworks for force:

One of my biggest frustrations was students putting random “F(applied)” on force diagrams. It irked me to no end!

So starting with the framework for Newton’s Third Law, I turned my force unit on its head. The fundamental piece we begin with is:

A force is an interaction between objects

Observational Experiments

We start with the activity from Pivot Interactives where two cars collide.

Students are asked to separately write what they observe about the car motion and also what they observe about the force acting on each car.

After making the observations we discuss.

The primary aspect students recognize is that heavier/faster cars result in bigger forces. That’s all well annd good, but what about the force that each car experiences. Even though they’ve literally just witnessed and recorded it, they still want the heavier one to hit harder than the light one within the same collision! We closely observe this together and see that, indeed, the forces are always the same.

This is what allows us to define a force as an interaction between objects. Without a second object pushing on the ring, the ring won’t squish. Since the force is something that happens between, it must be equal and opposite.

This very small shift has been a game-changer. It is very rare for me to have students putting totally random forces on objects because “it should have one”.

From here we dive into Eugina Etkina’s ISLE cycle.

Students are asked to hold a heavy and a light object in each hand, palms up and then represent those objects with arrows on a diagram. Students are asked to label each arrow with the object interaction. This is a fun one because a lot of kids are quick to label “gravity” but when I inform them that gravity, is not in fact, an object, they have a moment of pause. Eventually all students arrive at the correct diagrams: equal sized forces on each object, bigger forces on the heavier object.

From here I diverge between AP and regular physics. In regular physics we will go directly to the mass vs weight lab where students will ultimately derive the expression F(earth) = mg. With AP we continue to follow a modeling cycle with experiments with a bowling ball down the hallway: rolling, constant force forward, constant force backward. Then I ask how we could have constant velocity AND constant force. Students are quick to say “push down” (and we are fresh off of projectiles where x and y are independent!). Then realize if we alternate “taps” that will do it (balanced forces). Students are asked to represent and reason by drawing a complete motion map, an accompanying force diagram and then look for patterns. In this way students then recognize that balanced forces will result in constant motion (including v=0) and unbalanced forces result in accelerations. For homework students will complete two exercises from the Active Learning Guide from Etkina’s book where they will continue to practice drawing motion maps and force diagrams together in order to find relevant patterns. From here we get ready for labs!

Up next… labs labs and more labs!
Quantitative Experiments with Forces