Stage 0: Lots of lecture – not much else to say about that.
Stage 2: Start with the question: Which is more difficult – pulling a truck or pulling a lineman across astroturf? Students will immediately have opinions one way or another. (Usually they think that pulling a truck is harder because it’s “heavier”.)
I split the students into group and have each group agree on which object will be more difficult to pull and guess how much force each will require. (This is my favorite part of the lesson – so often students have no concept of how much force a Newton is. If they ask for some guidance, I inform them that I weigh about 500 N. Students find this shocking. They think 500 N is a HUGE amount of force.)
Once each group has their guesses solidified, we then watch this video and the students work to explain why a flea was able to pull something so heavy (no friction!):
I have the students make a list of factors they think might affect an object’s friction and have them test them out (different weights, different surfaces, etc.) This leads us nicely into coefficients of friction and other discussions.
Usually at this point, students are still convinced the truck will require more force – after all, pretty much all of our tests confirmed that heavier objects ARE more difficult to pull. We try it out (I let them each try to pull my car using an ENORMOUS force sensor that one of the math classes built last year). They then try pulling each other across astroturf – most of the students were not strong enough to pull ANY of their classmates across the turf.