Some Soap Box Derby science tips may help you win at Strawberry Days Saturday


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By Joe Moldovan

With the upcoming Soap Box Derby race quickly approaching – this Saturday, June 16 at Noon during Strawberry Days – I wanted to share some fun science facts for when you are building your soap box racer.

The use of these physics properties may come in handy when you build your design.

Some of the most important behaviors of soapbox racers are:

Angular Momentum, the Moment of Inertia and Angular Velocity:
Angular Momentum is the quantity of rotation of a body, which is the product of its moment of inertia and its angular velocity. In physics, angular momentum (rotational momentum) is the rotational equivalent of linear momentum. It is a conserved quantity, meaning the total angular momentum of a system remains constant unless acted on by an external torque. For example, if your Soapbox race car 12″wheel and 2″ tire is spinning at 25mph.

Then RPM is 525.211 which coverts to 9.166661218 degrees (radians). Multiply that times pi (3.14159265359) in 1 minute time frame, then we can find the wheels angular velocity is 28.797 Gs.

Moment of Inertia is the name given to rotational inertia, the rotational analog of mass for linear motion. It appears in the relationships for the dynamics of rotational motion. The moment of inertia must be specified with respect to a chosen axis of rotation. For a point mass the moment of inertia is just the mass times the square of perpendicular distance to the rotation axis, I = mr2. That point mass relationship becomes the basis for all other moments of inertia since any object can be built up from a collection of point masses.

Angular Velocity is the rate at which the wheel rotates around a chosen center point: that is, the time rate of change of its angular displacement relative to the origin. Therefore; the shape of the wheel enters the problem. A sphere will reach the bottom of an incline before a disk or hollow tube last, independent of the mass and radius! A taller tire will generally have less rolling resistance but footprint will increase and friction with it. Width of tire also increases footprint and mass weight.

Also consider that in regards to total curbside weight including driver, the soap box racer with the highest mass will have superior performance. In drag racing HP/weight ratio is important, but for the soap box racer the potential energy is based on the slope of the racecourse. Just like a full can will roll faster than an empty can, the heavier car has the advantage. It’s just science!

There are also other factors like Wind Resistance, Tire/Hub Friction, Brake Drag, Weather (changing Temp, Wind and Barometric pressures) and other unknown effects from gravitational waves effecting time/space segments and seasonal sun spots that affect out Ionosphere and Magnetosphere. It is a general consideration of all these factors plus others not mentioned that will have the greatest effects of performance of your Soapbox racer.

Good Luck!

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