I had thought, for many years, that adding gears to the output of NXT motors would be a big hassle. It turns out to be quite easy - all you need is to use a "lift arm" (bent beam) attached so that it gives you holes in horizontal or vertical alignment with the motor axle. The very simple framework pictured here lets you add three different pairs of gear sizes: 24:24, 8:40, or 12:36 (thick "knob" gears). Of course, the latter two can be applied to either reduce or increase speed.
So, we told the students to add the 24:24 gears to the motor, as pictured above, and measure how far the car goes in 5 seconds and see how well it climbs up a 30-degree ramp. (We have 350 cm (12 foot) tape measures for each group and are having them do all measurements in metric.) We then asked them to replace the gears with the 8:40 gears and repeat the experiments. They could choose whether to have a "race car" or "tow truck". I had expected that this would be easy for them - just pop the wheels and gears off, put on the other gears, and put the wheels back on. Nevertheless, most of them still had a lot of trouble. A few of them tried to put the wheels on the motor output, ignoring the gear. I'm not reporting this to chastise them - I think that they just weren't quite ready for this activity. After all, most of them had never seen a gear before this, much less tried to build anything with them.
After the last gears class, we had a meeting with the teachers to figure out what we could learn from the experience. We realized several things:
- Most of the students are not yet comfortable building with the parts. We tried to deal with this by giving them detailed instructions for building things.
- In trying to make it easy for the students to experiment with the gears, we'd reduced the activity to direct instruction - "build this, add that, do it this way" etc. They didn't have any opportunity to be creative, which is what had made the first classes so much fun and energizing.
- There had been no chance for them to present their work - another part that they'd enjoyed - since they were all building the same thing.
- Although they'd definitely learned about gears, it just wasn't fun - and we believe that there's no reason that these activities can't be fun.
There were some good things that we did which engaged them.
- They liked measuring how far their cars would go. We collected the distance measurements and they showed a great distribution. If we'd had more time, those numbers would have been great fodder for discussing why apparently identical experiments give different results.
- We used a ramp to demonstrate the speed vs. power trade-off. They all wanted to try their cars on it. Few of them could believe that their race car wouldn't go up the ramp even the slightest bit, no matter how many other race cars they saw stall.