# Lesson study: Engineering a photo finish

To wrap up the Constant Acceleration Model, I decided to go with the “photo finish challenge.” But first, a bit of history. I’ve been quasi-modeling for a while now, and really liked the idea of having kids study constant velocity carts, before they get to acceleration. About three years ago, I thought about having them create a race between a constant velocity cart and a constant acceleration cart on a ramp. I basically added this as a question to the end of the lab report, and though I found the question interesting, none of the kids really dug into it to think about it.

When I came to this new school, my colleague, K, had this awesome idea that when engineering the race, we should get the kids to make it a photo finish—that both objects reach the end at exactly the same time. Back at that time, I was a worksheet generating fiend, so I ran off and put together the following.

Yes, imagine taking an awesome idea like this and squeezing out all the joy you possibly can, and you get these worksheets. It shouldn’t surprise you that it too FOREVER for kids to complete this. But wait, there’s more! And so I write another set of worksheets to squeeze those last few drops of joy I saw while the kids were working on this.

Ok, so how do I approach this now that I’m embracing the modeling approach? Well, we started discussion with a single homework problem. One of the most thought provoking homework problems of the year. This comes from Kelly O’Shea’s great revision to the modeling materials and the fantastic Ranking Task Exercises in Physics by O’Kuma and Maloney. (click the link on the embedded scribd document to jump to the question).

Breaking down this problem took a good half hour of kids talking to one another, us coming together to ask question, and then more discussion until everyone was like “Whoa! I get it!.”

Then on the next day, I showed the kids these two slides:

I asked them what information the photos showed. “We know they were at the same position at the end of the race, the average speeds are the same!”, they replied.

And then I set them out to work, with a pretty limited amount of time, thanks to me spending way too long on exploring the wonders of kinematic equations and PB&J.

Kids were setting up photogates, motion sensors, stopwatches, and video analysis on the iphone. Within minutes everyone was taking data from a photo finish. Then they started creating whiteboards, and correcting each other when the velocity graph they drew didn’t show that the displacements were the same. Once or twice, I asked the question, what can you tell about the final speed of the two objects? And they nailed it—the ball bearing is moving twice as fast as the car.

In this short amount of time, were my kids able to answer, on their own, 80% of the questions I was asking on those joy-killing worksheets from last year? Absolutely yes, and they did it in 1/4 the time or less. Will we go even deeper when we have our whiteboarding discussion on Monday? You becha.

So, again, the message couldn’t be clearer. Your kids can learn more, have more fun, and do it all in less time and with less stress if you just free them of the yoke of the worksheet or step-by-step lab procedure. Now if I could just go the next step like Shawn, and bring in the horse skeleton…