Greetings!

A mousetrap car is a toy car that is powered by a mousetrap, using the energy stored in a wound-up trap to drive the wheels. Cars can be designed to any of several goals, but my favorite is the distance competition, in which the goal is to build a mousetrap car that travels the greatest distance between a standing start and an eventual rolling stop. There are two reasons that I prefer this one, I suppose.

First, it's open-ended -- there is no theoretical upper limit on how far a car can travel. While the average "first attempt" might travel 8-10 metres, it's easy to find video of a variety of designs travelling 34 metres, 42 metres, 49 metres, even approaching 60 metres and steering to fit that run into a gym. The Doc Fizzix website keeps a leaderboard that includes cars that apparently travelled more than 100 metres -- alas, without video.

Second, the distance competition is loaded with physics. A thorough analysis of any mousetrap car requires an understanding of of static friction, rolling resistance, potential and kinetic energy, efficiency, work by conservative and nonconservative forces, inertia, rotational inertia, simple machines and other physics concepts. Doc Fizzix publishes a beautifully illustrated and very comprehensive guide to building and testing mousetrap cars that discusses many of these concepts in that context (see this sample for some idea of what they're about).

Design at the longer distances can be fickle, with very similar designs differing by a factor of two in distance and seemingly small tweaks resulting in large increases or decreases in distance. This sort of uncertainty is fertile grounds for superstition. So what are the most important parameters for long distances? Should wheels be large or small? Heavy or light? Rubbery or hard? How can energy best be transferred from the mousetrap to the wheels? Where are the "bottlenecks" -- the bits that most urgently need to be improved in a basic design?

You'll find a lot of answers to these questions on the web, but remarkably few good scientific approaches to the problem, even from science teachers. Good science must include two things: experimental work that establishes what is true in the world, and theoretical work that makes an attempt to explain why it's true (or, from another viewpoint, to extrapolate to what else might be true). It's probably not surprising that many student designs are thin on theoretical consideration, and take a trial and error approach in any improvements. Also common, though, and no more scientific, is an approach that is strictly theoretical, discussing only what "should work".

I'd like to take a more balanced approach to exploring these problems here, beginning with some experiments to lay the groundwork and hopefully leading to a better mousetrap car. If you have your own work to share, or comments on what I post here, I'd love to hear from you!