We started off the hour with a review of Newton's first and second law (okay, actually we started off the hour with a discussion of Halloween costumes, and also the comparative advantages of various lightsabers). I then wrote on the board the third law: For ever action there is an opposite and equal reaction. The kids copied it onto their papers.
I had thought it would be really cool to have something like an air hockey table so we could boing the pucks around, but transporting the one from our basement wasn't a possibility (my high school physics teacher used an air hockey table under a strobe light to illustrate vectors, and I always remember that as one of the coolest things that happened in high school science). Annabeth recalled that we had an air hover puck that had its own battery-powered motor that allowed it to "float" on any smooth surface sort of like this one. The room we're in for class is carpeted, so we couldn't use it on the floor. We tried to get a little table top action, but the puck kept flying off the table; the back would pop off and the batteries would fall out. But the kids had all seen or played air hockey before, so they had an idea off how it worked -- if 2 pucks collided they'd go zipping away in opposite directions.
Next, I had everyone put on safety glasses for our main demonstration. The safety glasses turned out to be a Huge Deal. First of all, one of the younger boys had to go get his own pair from his mom (I'd brought enough for everyone, but he wanted his own). He arrived back in the class while I was explaining that our family owns so many pairs of safety glasses because we're very conscious of eye safety (and one of the girls was disclosing that her father was cutting wood over the weekend WITHOUT safety glasses, and her mom kept telling him to put them on). The boy explained that his dad was wearing safety glasses RIGHT NOW in the lab he worked in (dad is a PhD chemist); the son was obviously really, really excited to wear glasses like his dad. And as the class progressed one of the girls kept exclaiming, "I feel like a real scientist!" Sort of a change of pace from working with the middle schoolers last year. Although one of the boys in this class wore them on top of his head instead of over his eyes (I tried correcting him. Later I saw him standing to one side rubbing his eye as though he'd bopped himself and looking like he was starting to cry. I asked him if he'd hit his eye, and he claimed he was fine, and that he hadn't hit his eye. I'm pretty sure he had but was afraid he'd get in trouble, or at least get a big ol' "I TOLD you to put the glasses on," and preferred to suffer alone rather than admit he'd been wrong).
Main demonstration: Put water in a plastic film canister, put part of an Alka Seltzer tablet in water, put lid on, watch lid pop off. A classic.
We started by just putting the Alka Seltzer in water (I used half tablets) and timing how long it fizzed -- about a minute. Then I dumped that out, put the same amount of water in, put another half tablet, put the lid on, then started discussing something or the other (it's an enthused bunch full of ideas of other things we should try) and after about 17 seconds POP! the lid went flying off, causing some kids to jump. One boy asked if we could try it again, but not put the lid on so we could see what happened ... um, isn't that what we did first?
Anyway, forging ahead, we went outside so the the kids could try this on their own (although the room has a high ceiling, it seemed to be wise to move this outside). A mistake I made: I should have gone over the procedure multiple times, writing it out on a portable white board or something. Because once we got outside everyone was sooooo excited that it was a little hard to control who was doing what when.
I put the kids in groups of 2 or 3 (I assigned these basis on Annabeth's advice about who would work well together while actually getting something done). I had a couple of kids who said they didn't want to actually DO anything, and I told them it was okay, they could just observe their partner.
Round 1: Fill canister halfway with water, put in half tablet, put on lid, countdown to lid popping off (we didn't have enough watches or clocks for everyone, so I reminded them that Galileo used his pulse and they could simply count steadily).
Round 2: Fill canister 1/4 to 1/3 way with water, put in half tablet, put on lid, countdown to lid popoff. Does this take longer or shorter? (I had tried to engage them in predicting which it would be.)
Round 3: Fill canister 2/3 to 3/4 with water, put in half tablet, put on lid, countdown to lid popoff. Faster? Slower?
Bonus round: Fill canister halfway, put in half tablet, put on lid, put lid-first into plastic tube that's sealed at one end, hold facing some direction other than at me, shoot canister out of tube.
Okay, the above is the THEORY of what we were doing. In reality, the kids were so excited that they kept coming up with new things they wanted to try ("can we put in a whole tablet?" "No, the reaction would be so fast you can't get it covered and put in the tube before it would blow -- REMEMBER YOU GET ONE VARIABLE, AND THAT'S THE AMOUNT OF WATER -- EVERYTHING ELSE IS A CONTROL!!!"). The Alka Seltzer tablets kept falling apart into tinier pieces, we had water sloshed everywhere, at the lower water levels the lids sometimes didn't blow off (not put on well enough? slower reaction meant the gas could leak out enough that it didn't need to POP?), kids were filling the plastic tubes with water and throwing canisters and Alka Seltzer in to see what would happen, one boy flipped the canister upside down on the pavement with the plastic tube over it and waited for the entire thing to go flying (okay, actually that was partially my idea) ... the girl who didn't want to participate was plastered on the ground in the field next to the parking lot counting down with her lab partner as they waited for their canister to blow (I think they were pretending to be a bomb squad). The boy who didn't want to participate was jumping with excitement shouting to his lab partner that it was his turn to put in the tablet and put on the lid .... You know, I'm not sure they got the point of what was SUPPOSED to happen, but they sure experienced a lot exciting discoveries. I know that I discovered that several dozen Alka Seltzer in a quart or two of water, over the course of a half hour or so, gets amazingly sticky. Who knew?
And then I happened to glance at my phone to see the time, since I estimated this would take 15-20 minutes and we could then go play on the playground, and discovered class was OVER. Oops. Everyone grabbed something, we zoomed back inside. Maybe next week we'll try to go over what we learned. I'm pretty sure it was memorable.