Rubber Band Car* Rubber Band Car* by Shawn Beightol, Michael Krop Sr. High School, Miami, FL September 08, 2006 *Adapted from Brad Nunn’s (Purdue University) Feb 25, 2006 Presentation

The Mantra: Smaller! Smaller! Lighter! Lighter! Farther! Farther! Say it again! Do the Math…

Today’s Activities Typical rubber band cars Typical rubber band cars Propeller based designs Propeller based designs Review competition rules Review competition rules Component design Component design Build! Build!

Typical Rubber Band Cars

Dixie Roller

Propeller Rubber Band Kit

Propeller Rubber Band Cars

Propeller Rubber Band Kit

Review Rules Refer to Go Cart Competition Construction and Operating Rules Refer to Go Cart Competition Construction and Operating Rules

Review Scoring F = 1 D 2 * W L W L Consider tradeoffs - (D+L squared)Consider tradeoffs - (D+L squared) Smaller! Lighter! Farther!

Terminology Potential to kinetic energy transfer Potential to kinetic energy transfer Torque Torque Acceleration Acceleration Speed Speed Momentum Momentum Friction Friction

Desired Outcomes A small car that travels 2500 cm and doesn’t weigh much A small car that travels 2500 cm and doesn’t weigh much Smaller!Smaller! Lighter!Lighter! Farther!Farther! A gradual transfer of energy that has just enough torque to establish motion A gradual transfer of energy that has just enough torque to establish motion A sustained transfer of energy that delivers sufficient momentum to cover the distance A sustained transfer of energy that delivers sufficient momentum to cover the distance

Competitive Benchmarks :2004-5: :2003-4:

Score as a function of Length

Score as a function of Mass

Thoughts from analysis of competition data: Competition formula suggests Competition formula suggests F  1/L 2 – this should be most significant Data supports this with a power function of 2.5 Data supports this with a power function of 2.5 Correlation is low! (0.3) Correlation is low! (0.3) F W –mass should be less a factor F W –mass should be less a factor Data suggests in practice this is more significant with a power function of 2.0 Data suggests in practice this is more significant with a power function of 2.0 Correlation is almost twice as high (0.5) Correlation is almost twice as high (0.5) Interpretation? Students assemble long vehicles from kits without thinking? Interpretation? Students assemble long vehicles from kits without thinking? Students who modify for mass acquire and apply design skills? Students who modify for mass acquire and apply design skills?

The Mantra: Smaller! Smaller! Lighter! Lighter! Farther! Farther! Say it again! Do the Math…

Rubber Bands All shapes and sizes All shapes and sizes Experiment! Experiment!

Component Design Low inertia and light weight Low inertia and light weight

Wheel and Axle Design Minimize friction loss Minimize friction loss

Lubrication Graphite powder Graphite powder Silicone spray (WD-40 is not recommended) Silicone spray (WD-40 is not recommended)

Creative thinking What advantage is there to winding a rubber band compared to stretching it? What advantage is there to winding a rubber band compared to stretching it? Are there other ways to use a rubber band that is wound up? Are there other ways to use a rubber band that is wound up?

Iterative Design Prototype Prototype Measure and record performance Measure and record performance Tweak the design (smaller, lighter, farther) Tweak the design (smaller, lighter, farther) Iterate Iterate Replicate Replicate Calculate Calculate Celebrate Celebrate

Design optimization Frictionless bearing Frictionless bearing Frictionless unwinding of the rubber band Frictionless unwinding of the rubber band Thick or thin rubber band Thick or thin rubber band Length of the lever arm Length of the lever arm How many wheels How many wheels Wheel materials for low inertia Wheel materials for low inertia

The Mantra: Smaller! Smaller! Lighter! Lighter! Farther! Farther! Say it again! Do the Math…

Most important Have Fun! Have Fun!