Wednesday, July 13, 2016

Roller Coaster Track

When I teach the law of conservation of energy I talk a lot about roller coasters, as most physics teachers do. Assuming a frictionless roller coaster we can discuss how drops in height reduce the gravitational potential energy and increase the kinetic energy while the total amount of mechanical energy stays the same. Students can use PhET's "Energy Skate Park" simulation to create a roller coaster with two hills, the second lower than the first like pictured:

I like to show students the bar graph and pie charts to illustrate how the energy changes from one kind to another yet the total remains the same. Measurements can still be taken, calculations can still be done but its still not something they can experience and observe directly in my classroom.

I've tried a few different things to create roller coasters in my classrooms: clear tubing with a marble, Hot Wheels track with a small car, paper folded tracks with marbles, etc. Each has their own pros and cons and I'm sure they work great for many teachers. But I want to be able to set up a roller coaster quickly, use it repeatedly and get consistent results. I may or may not take measurements with it but I wanted a visual of this type of problem to use in my classroom that could be set up in moments.

I bought two 8' pieces of wide aluminum U-channel from the local hardware store. I put my blow torch in a pot with brick in it so that it stayed upright and left my hands free. Donning thick leather work gloves I lit the torch and held the U-channel in the flame. It took a minute to get hot enough that it could be bent. I moved from one side of the U-channel to the other because a foot or so on either side of the flame became too hot to hold.

Some bends were difficult because the aluminum was hot or the position made it difficult to get leverage. My leather gloves were not as insulated as I would have liked. This pair now has permanent burned indentations. My hands weren't burned, exactly, but were sore afterwards for sure. My first piece of aluminum ended up in a much different shape because I didn't lay out which way I was bending it ahead of time; I'm still messing with that one.

I also found that while bending it down with the open side up was easy, like an upside down U, sometimes bending it up could cause the channel to buckle. This meant that I had to straighten the sides; I tried doing this two different ways. Some deformation was slight enough that I could bend the side back using pliers. For more serious buckles I inserted a piece of wood that was the same width as the U-channel and hammered it on the floor.

In the end my second piece of channel ended up making the shape I wanted. While I was able to get it to stand up here on the grass I had to use clay to get it to stand upright in my classroom on my front table. I can run a marble from the top of the larger hill through the track and it makes it over the smaller hill, somehow amazing students. A ping pong ball can also be run on top of the U-channel to make it easier to see.

I wanted to make a permanent mount for it so I didn't have to hold it up or use clay to hold it in place. I went through a few different iterations of mounting. First I thought I could just drill a flat head screwed into the track. That wouldn't work on the ends though that are nearly vertical to the horizontal. I also worried that even a flat head screw would affect the marble sent down the track. I cut a few inches off the first track that I had bent incorrectly and tried splaying out the sides with pliers. Splaying the sides out did not hold the track securely.

My next attempt was to create a vertical piece with a cut out that fit the track. I ended up using 4"x4" pieces of wood that were large enough not to need a cut out. The ends are free and not resting on the table which is not required for the ball or marble to roll down it anyway. The pieces I have are about the right size without being cut. The simplest solution has turned out to be the best!

In order to use it, all I have to do is place the two posts out and set the track on it. The aluminum U-channel is light and while it is over 6 feet long it should be easy enough to store on top of some cabinets. Now I have to try and rebend that first track into something more interesting ...

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