Two Speed Shifters

 When I started teaching, the veteran physics teacher had these cars for a particular lab he called "two speed shifters." They are wind up cars that when released, start moving at a slow, nearly constant speed, then shift and accelerate. 

A piece of string was tied and taped to the top of the cars that would run over a smart pulley attached to our Vernier photogates. After the car was wound up and put near the photogate, the string had to be pulled back so that it would move against the pulley while the car pulled the string out. The set-up is specific, and sometimes finicky, but provides a very interesting graph for students to analyze. There are more specific details about the "how" below.  

Students would focus on the velocity-time and position-time graphs. They would learn how to use the tangent line function on the position-time graph to find the spot where the slope started to decrease. They look at the velocity-time graph at that same time to see that the velocity curve decreases at the same moment. Students use the integral function to find the total displacement for the run using the velocity graph and compare it to the final position on the position-time graph. Students have been studying kinematics graphs and how they relate for a few days by this point, even having done the motion matching activity embedded in the Vernier software, but there are still so many "ah hah" moments while they play with the two-speed shifters. 

However, the lab is not without its difficulties. That string does not always want to stay on the pulley, the car sometimes falls off the table, etc. so students have to make a few different runs before they get a "good run" where it all works properly. Here are some more images and information about "how" to get it to work. 

Two pieces of plastic tube (like a thick straw) are put on the string to help guide it and provide tension. One gets taped to the support rod for the photogate and helps thread the string to the pulley. The other is usually taped to the table edge or leg so the string can pull through it; this helps prevent the excess string from getting into a knot and provides just a bit of tension to help it stay on the string.  

The original cars didn't always go straight, so a thin coffee strirrer straw was taped to the top of the car as well. A long piece of fishing through the thin straw acted like a "track" for the cars to help them go straight. In the last few years I haven't added the fishing line track, which does mean students have to be careful about having the car too close to the edge.

Here is a video of this set-up with the straw:

And an example of one more recently, running without the track:

As you can see in the video above, sometimes that string comes off the pulley. It happens more often with the cars attached to the yellow string, less frequently with the "physics string" in the later videos. Some students have held their fingers tips around the point where the string leaves the straw and runs into the pulley to prevent it falling off but we have found the best prevention is using the straw and keeping some tension on the string by pulling it through the second straw. 

Finding the cars themselves was another problem. When I was first introduced to these, we had a finite set of these cars. Although they were older, we were very careful with them and had just enough for running two classes. Two of the older ones are shown above, one with the coffee stirrer for the fishing line track still attached. We were always worried we would eventually run out of working cars and had been on the lookout for more. We caught a lucky break when one of my students spotted one while on a trip to Scotland. We were able to track down the company and increase our inventory. Years later, one of my student teachers found these smaller two speed shifters. The new-in-package shown on the right is the same as the yellow one of the same size on the left, are the ones I've been using for over a decade. Every few years I try to find more of them to no avail. The small version came in the smaller box that looks like a crate was found by my student teacher. 

These aren't normal pull back cars, but every time I see a pull back car in a store I check to see if it's a two-speed car. If anyone finds more, let me know!

Although they are due for a rewrite, here are my in-class instructions, a slidedeck that I use to introduce them to the new-to-them equipment and the student lab sheet.