Actually, we started out with a review of gravity from last week. I used some questions from Science Jim's Bite-Size Physics: Force like: Which of the following are most attracted to each other by gravity? A) apples and bananas, B)beagles and chihuahuas, C) Earth and you, or D)All of the above. Sort of mildly silly, but got the point across.
From there I asked who all had read or seen Magic School Bus Plays Ball, since we were talking about the same topic. Several kids had, but some hadn't. Some came up and looked through the book as we were doing our various activities.
We started by listing ways friction was a friend or foe (idea from Science Action Labs -- Physical Science). We discussed car brakes, crickets, violins, snakes, walking, standing, things that overheat due to friction, etc.
We then did an experiment with a board and various shoes, seeing how high we could incline the board before the shoes slid down the ramp. We talked about how shoes that had more points of contact with the board tended to have more friction.
Next, we tried pulling a heavy metal tray from my kitchen over various surfaces. I hooked a spring scale to the tray so we could see the force used. This also involved explaining Hooke's law about springs, plus what the heck a "Newton" is. Besides running the tray over the carpet, I had brought in a fluffy rug and a yoga mat. After that we ran it a few times over the same surface, but with different amounts of weight on top -- we used books the kids had in their backpacks.
I showed on the board how they could start writing a lab report for this experiment -- what they would write for their hypothesis, materials, procedure.
We took a brief look at bearings. First of all I stuck some pens under the tray while it sat on the yoga mat, and demonstrated how easily it moved on the pens. Then I set up a demo rather like this one (Annabeth and I had done this for a Junior Girl Scout badge back when Girl Scouts had cool badges worth earning). And discussed how the bearing gave the item fewer points of contact with the surface below, just like with the shoes. And, no, even though your grandpa puts grease on bearings doesn't mean that grease is a necessity for bearings to work.
I didn't do much explaining of the whys of friction, although I let the kids know that it's something scientists are still working to understand more about.
Finally, we explored air friction by making parachutes out of plastic grocery bags, taping them to little paper-cone-people. I had taken along drinking straws so we could shoot the people in the air, but since we were done a few minutes early we trooped outside, tossing them over the 2 story balcony on the way out to enjoy the last warm, sunny day of the week. It was really cool to watch them floating down like a bunch of gigantic baby spiders. Some of the kids were still playing with them on the playground, and one girl intended to show her little brother how to make one that evening.
I'd reflect on all of this more, but a rat keeps running across the keyboard, typing nonsense, so I'll leave it at that.