## Thursday, April 05, 2007

### Playground physics

Science Club will be wrapping up for the year soon, as we're moving into GRUBS and soccer season. This week we decided to head to the playground and have a low-key time doing some exploratory physics with the playground equipment, our bodies and a few simple props. First we rolled a damp tennis ball against a wall and traced its soggy path on the pavement with chalk. Lo and behold, the angle of incidence equalled the angle of reflection. Next we topped a basketball with a tennis ball, courtesy of a little collar of masking tape, and dropped the two together. The basketball bounced and transfered much of its kinetic energy to the tennis ball in the midst of its bounce and wow, did it ever fly!

Next we explored the 15-second swing. We used a stopwatch to time how long it took Fiona to do five gentle swings. Fifteen seconds. How about someone heavier? Some of the kids thought heavier kids would complete five swings faster. Nope, fifteen seconds. How about someone lighter? Fifteen seconds. How about someone doing really energetic swings in a large arc? Fifteen seconds. No matter what we did, it was a fifteen-second swing. Amazing, no? You'd almost think that the constancy of that swing would be a good way to keep time. Hey! A grandfather clock... This made sense to all the kids.

Finally we shortened the chains with a couple of spring clips. VoilĂ , thirteen-second swing.

After that we went to the merry-go-round. Are these contraptions a thing of the past in most places? They sure seem delightfully dangerous compared to most of the stuff that is in kids' playgrounds these days. We built an accelerometer (white board, thumb tack, string, key and an arbitrary scale) and tried to see how much acceleration we could produce. We maxed out at 2 3/4, until we got all the kids to move into the centre. Now we were able to generate almost 4 acceleros. We talked about inertia, and the way the distance of a mass from the axis of rotation affects its rotational inertia. We got the merry-go-round spinning as fast as we could with the children near its edges. On command they all moved to the centre. Wow, it sure sped up. Unfortunately the kids who sort of forgot to hold on firmly were thrown (literally) by the sudden increase in acceleration. Two Barker kids were flung off, neatly (though somewhat traumatically) demonstrating linear momentum.

Finally, we tried the waiter-on-a-merry-go-round trick. Hold a cup of water on a tray (or in our case, a plastic IKEA plate), balancing it while the merry-go-round is spinning very quickly. You can't see it clearly in the photo above, but the surface of the water in Noah's cup really is parallel to the plate. Very cool!