Chance and Luck

A picture of the apparatus for an experiment about bell curves.

In this section we run a car down a track 100 times to measure the spread in the car’s run times. We plot these run times in a “Histogram” plot. We give instructions on how to perform a simple experiment at home to introduce the concept of a normal distribution (or a “Bell Curve”), a key concept in a branch of mathematics called “Statistics.” We also introduce the use of decimal notation in expressing a car’s run time.

Introducing the Gaussian or Normal distribution (also known as a bell curve).

Introducing the concept of decimal notation by relating to counting money.

Introducing a histogram as a way to visualize many test runs of a car.

Weight of the Car

In this section we look at how the weight of the car affects its runtime. We conduct an experiment to see how light a car can be without slowing down unacceptably. We introduce a new type of plot (a line plot) to show the results of the experiment.

A car on a scale.

A plot used to visualize car weight versus run time.

Wind Resistance

A picture of the apparatus for an experiment about bell curves.

We’ve all seen cars that have been built with an aerodynamic taper, presumably to reduce wind resistance. But how important is this REALLY? After all, a pinewood derby race car hardly moves at racetrack speeds. In this section, we perform an experiment where we vary the aerodynamic properties of a pinewood derby race car, and measure the effects on the car’s speed. We show you how to conduct a simple experiment at home to illustrate the effects of aerodynamics on a car’s speed.

A picture of two cars with very different aerodynamics.

A plot showing run time versus cross-sectional area.

A picture of our test car made aerodynamic.

Center of Mass

Setting the center of mass using a putty knife.

The center of mass is perhaps the most overlooked -- and the most important -- aspect of any pinewood derby race car. In this section, we examine this aspect of a car from every angle, and introduce the viewers to the concept of a computer model, using Lagrangian Mechanics (don’t worry--it’s at a level that boys will understand!) We show you how you can perform a trivial experiment at home to illustrate how scientists calculate the center of mass. We also look at different types of energy important for any pinewood derby race car.

A introductory illustration of the importance of the location of the center of mass.

An experiment about calculating the center of mass.

Using computer simulations to test hypotheses.

A plot of results showing run time versus the center of mass.

Moment of Inertia

An experiment about the car's moment of inertia.

The moment of inertia is also an often-overlooked feature of a pinewood derby car. We conduct a series of experiments to investigate the effects of moment of inertia on speed. These experiments suggest a dramatic departure from the typical car design is necessary to optimize speed. We show a simple experiment that you can do at home to illustrate how the Moment of Inertia affects a car’s run time.

A slide about calculating the moment of inertia.

Cutting the car body to minimize the moment of inertia.

Plots showing how the moment of inertia affects run times.

The Scientific Method

The Scientific Method is an objective means of “figuring things out.” Here’s how it works:

You begin by making observations. We do this throughout much of the video.

Next, you make a guess (or a “hypothesis”) as to what’s going on.

Then, you conduct an experiment to test your hypothesis.

You look at the results of the experiment and see if it agrees with the predictions implied in your hypothesis.

Finally, you form conclusions about the validity (or lack thereof) of your hypothesis.

Often people arrive at incorrect conclusions because they don’t follow the Scientific Method. Imagine what would happen, for example, if you formed your conclusions at the very beginning, before doing experiments. This may seem silly, but people do this all the time!That’s why we feature the Scientific Method in the video. Specifically, we apply the Scientific Method to the problem of the finding the best “Moment of Inertia” for a pinewood derby race car.

Light-weighting the Wheels

An experiment about the moment of inertia of the wheels.

Taking material off of your car’s wheels is perhaps the most controversial aspect of the pinewood derby. Most competitions don’t allow you to modify the wheels at all -- but each pack and district is responsible for its own rules. In this section, we use a computer model to analyze the effects of light-weighting the wheels on a car’s run time. We conduct a very simple experiment -- using soup cans -- to show how the placement of mass in the wheels can make a big difference in rolling speed.

The moment of inertia of standard wheels.

The moment of inertia of wheels with some material removed.

The moment of inertia of wheels with a lot of material removed.

Running on Three Wheels

All pinewood derby competitions require a car to have 4 wheels...but often they don’t require that these wheels actually touch the race track. Is there any advantage to lifting a wheel off the track? If lifting one wheel off the track, how about lifting two wheels off the track? We look at this issue in detail, using Moment of Inertia calculations, through computer simulations.

An experiment about the moment of inertia of the wheels.

Running a car with three wheels.

Tradeoff between three and two wheels

Running a car with two wheels.

Lifetime of the Wheels

You may have heard that a car will actually get faster after each race. In fact, I’ve actually known people to “spin up” their cars with an electrical tool prior to the race competition, to “break in” the wheels. In this section, we run a car down the track 100 times to see how the car’s run time changes after use. We also look at the effects of periodically adding graphite to the wheel axles. I think you will be very surprised by the results, and the implications for your racing strategy!

A plot showing the run time versus the number of times the car has gone down the track.

Running the car lifetime experiments.

Nail Preparation

People often spend a lot of time polishing the nails supplied with a pinewood derby car kit to a mirror-like finish. But how important is this? It’s a lot of work...is it really worth it? In this section we use an indirect experimental method to measure the affects of different kinds of nail preparation on the car’s run time. (Actually, this experiment uses an ancient device known as a “record player” to spin the wheels!) We also introduce the “Coefficient of Friction” and discuss its affects on the speed of a car. I think you’ll be surprised by these results as well.

A comparison a nails with a variety of stages of polishing.

An experiment using a turntable to test nail finishes.

Using a stopwatch to time how long a wheel will spin with different nail finishes.

Wheel Gap

I’ve seen cars with the wheels attached so tightly to the car body that the wheels barely turn. Needless to say, these cars don’t do very well in the race competition. I’ve also seen cars with the wheels so loose that they “wobble” all over the race track -- also slowing them down. Well, somewhere between “too tight” and “too loose” there has to be a “just right” and that’s what we determine -- experimentally -- in this section.

An illustration of the appropriate wheel gap.

A plot of experimental results showing run times versus the wheel gap.

Using calipers to measure wheel gap for our experiment.

Making Multiple Cars

Enjoy building your car? Well, why not make several and pick the fastest car to enter into your race competition? Of course, this begs the question as to how you judge the speed of each candidate car. Later in the video, we show you how to build a very simple race track just for this purpose.

A picture of multiple cars.

Testing multiple cars to find your entry car.

The results of using physics to build a pinewood derby car.

Building a Pinewood Derby Car

After learning the physics of pinewood derby cars, we will head over to the shop and show you steps we use to build a really fast car. Hopefully your own experiments will help you build and even faster car.

Building a Test Track

You will need a track of some sort to test your cars. If your pack's track isn't available, we show you how to build a simple two-lane track at home.

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Video Details

Chance and Luck
Weight of the Car
Wind Resistance
Center of Mass
Moment of Inertia
Scientific Method
Light-weighting the Wheels
Running on 3 Wheels
Lifetime of the Wheels
Nail Preparation
Wheel Gap
Multiple Cars
Building the Car
Building a Test Track

Extras

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