Playing with cars

A few weeks before winter break I wanted to introduce the idea of energy. I usually do this with some boring chalk and talk about how we can use energy to solve more complex problems in a much easier way...


While that works, I'm not sure I get much more buy-in than just filling the chalk board with definitions and equations. Okay maybe that's not quite true, but it feels that way.


This time I decided I wanted to do a little exploration get the kids to figure out something on their own about energy. So as a class we took a short field trip to the maintenance building and got some old gutters and long planks of wood. I let kids (in groups of 3-4) pick their own toy car. I managed to pick up two 5-packs of hotwheels for $10 at Target. I also picked up some $1 slinkies that have yet to find a use...

Finally, I posed 3 questions to each group, the first two just being scaffolding for the last question, and then just got out of the way.

1. Given initial conditions of your choice, what is the speed of the car at the bottom of the ramp (or just off the ramp)?
2. What do you have to do to double the speed of the car at the bottom of the ramp? Triple?
3. What would you have to do so that the car had a final speed of 100 m/s? A theoretical question. 

I was trying to get them to model the idea of velocity squared being proportional to the height above the ground. Thus the length of the ramp and angle are secondary to height. And poof, with a little teacher voodoo the kids would have discovered gravitational potential energy... Well, it was so much better. 

They grabbed stopwatches and meter sticks and went to town. The kids got stuck on number two. One section nailed it during the first period. The other had 3 groups and all 3 got different results. I fought that teacher response of "well the answer really is..."  and said almost nothing. We decided to take one more class period to build a longer (6 meter) gutter ramp, some of the problems were  in measuring the time (we don't have any photogates). The kids spent 45 minutes experimenting and writing down numbers until the whole group saw the pattern! It was hands down the best class period of my career. 

In the end both class periods saw that the height was the determining factor, this required some guiding questions (Jedi mind tricks), and saw that there was more to figuring out the speed of a car than using kinematics and FBD's... There might be something far easier to use.

The handout I gave me students can found as a google doc.