1.0 Physics of the Soap Box Derby.

Goals/I can…  maximize potential energy in the car  maximize acceleration  demonstrate the most efficient conversion of Potential to Kinetic Energy.  minimize aerodynamic drag  design a car with the lowest center of gravity possible.  make correct weight adjustments for the track and ramp combination.  Minimize movement of Inertia.  Minimize vibration of car and parts.  Minimize friction.  place weights in the most effective position so that the car does not exceed the weight of 240 pounds with driver.

1.1 Primary Influences  Force the propels the SBD car and driver down the track is…  gravity  The amount of force available is called  Potential Energy (PE)  What determines the amount of PE?  Total weight of the car and amount of drop from start to finish.  What is Kinetic Energy?  When the car begins accelerating, PE is converted to KE.  The more efficiently this happens, the faster the car goes.

What impedes acceleration?  Aerodynamic drag  Vibration  Friction  Moment of inertia  Driving habits  Minimizing these scenarios will increase acceleration and ultimately increase speed.

1.1.1 Potential Energy  To calculate Potential Energy (PE): Multiply total weight of car and driver times the car’s Center of Mass vertical drop.  Vertical drop at Akron is 48 ft.

Examples

Acceleration= vf-vi/tf-ti

Acceleration is impeded by:  Debris on wheel tread (small rocks)  “Flat” spot on wheel tread due to sitting in starting gate for long period of time.  Imperfections in ramp surface.  Wheel bearing breakaway torque  Wheel tread “squirm” (side to side movement) rolling down hill.  Rough track surface  Aerodynamic drag  Friction  Vibration  Moment of Inertia  Steering of car by driver.

Aerodynamics, cont.

Center of mass (gravity): point at which weight of an object is centered.

Center of mass, cont.  1 D  2 D  Lead and Styrofoam square  Applying force to center of mass  Applying force to different area other than center of mass. \  High jumping…  processes/newtons-laws-and-equilibrium/v/center-of-mass processes/newtons-laws-and-equilibrium/v/center-of-mass

Balance: Weight of car and driver measured at axles.  Balance is determined when car and driver are at racing weight and the driver is in driving position.

1.1.7 Moment of Inertia  An object’s resistance to rotating about its Center of Mass. 1)All weight (car body, wheels, axles, added weight, driver, etc.) not located at the Center of Mass increases the car’s Moment of Inertia. 2)The farther a weight is from the CM, the larger the car’s Moment of Inertia. 3)Following current rules, only driver position and weight added to the car can be adjusted to minimize a car’s moment of Inertia.

1.1.8 Vibration: A cyclic back and forth motion of an object.  Track surfaces are irregular and cause the car to vibrate up/down, front to rear, corner to corner, and side to side.  A car that “rattles” rolling down the hill will be slower than a car that does not rattle- all other things being equal.

1.1.9 Friction: a Force that resists the relative motion of two surfaces in contact.

Driver: individual steering down the hill.

1.2 Weight Distribution

1.2.1 Track and Ramp Types.

1.2.4 Potential Energy on Track  Constant (inclined plane) slope: It does not matter if the Center of Mass is high or low. Locating CM toward tail will increase PE, but also increase rear wheel friction.  Concave (varying) slope: A low center of Mass produces greater vertical drop of weight from top of hill to bottom than a high Center of Mass. A larger vertical drop means more PE. Example  Convex slope: A high CM produces greater vertical drop of weight increasing potential energy.

Weight Distribution  Constant Slope Track  Flat ramp : racing tail heavy will increase PE and may be an advantage but, increasing weight on rear wheels increases wheel friction. Increased rear wheel friction may slow the car’s speed more than the speed gained by having more PE.  Standard Ramp : Racing tail heavy is such a significant advantage due to the ramp to track transition, that it overrides other considerations.  Drop-off Ramp : A longer wheel base is the most effective method of increasing speed. Racing nose heavy may increase speed depending upon ramp shape, track slope and track length.

Weight Distribution  Concave Slope Track  Flat Ramp: Racing with low weight will produce maximum advantage.  Standard Ramp: Racing tail heavy is such a significant advantage, due to the ramp to track transition that it overrides all other considerations.  Drop-Off Ramp: A longer wheel base is the most effective method of increasing speed. Racing nose heavy may increase speed depending upon ramp shape, track slope and track length.

Weight Distribution  Convex Slope Track.  Flat Ramp: A longer wheel base is the most effective method of increasing speed. Racing nose heavy may increase speed depending upon track slope and length.  Standard Ramp: Racing tail heavy is such a significant advantage, due to the ramp to track transition that it overrides other considerations.  Drop-Off Ramp: A longer wheel base is the most effective method of increasing speed. Racing nose heavy may increase speed depending upon ramp shape, track slope and track length.