and applying graphite as a lubricant. Competition The track usually has two to eight lanes and slopes down to the ground, since the cars are powered by gravity. Tracks may be owned by the pack or rented. The race is run in heats, giving every car the chance to run on each lane. The racers can be grouped with others from the same rank (Tiger Cubs, Wolf Cubs, Bear Cubs, etc.), or can compete against the pack as a whole. First, second, and third-place winners usually receive ribbons, medals, or trophies. Some packs also award on the basis of car design or styling. The first place race winners get to advance to the district level, then each of the district-wide race winners get to race each other from across the entire council. Car modifications The force accelerating a pinewood derby car is gravity; the opposing forces are friction and air drag. Therefore, car modifications are aimed at maximizing the potential energy in the car design and minimizing the air drag and the friction that occurs when the wheel spins on the axle, contacts the axle head or car body, or contacts the track guide rail. Friction due to air drag is a minor, although not insignificant, factor. The wheel tread can be sanded or turned on a lathe and the inner surface of the hub can be tapered to minimize the contact area between the hub and body. Polishing the wheel, especially the inner hub, with a plastic polish can also reduce friction. Often one front wheel is raised slightly so that it does not contact the track and add to the rolling resistance. Axles are filed or turned on a lathe to remove the burr and crimp marks and polished smooth. More extensive modifications involve tapering the axle head and cutting a notch to minimize the wheel-to-axle contact area. Packs can establish additional rules for what, if any, modifications are allowed. In some areas, no changes can be made to the axles or wheels. A second consideration is the rotational energy stored in the wheels. The pinewood derby car converts gravitational potential energy into translational kinetic energy (speed) plus rotational energy. Heavier wheels have a greater moment of inertia and their spinning takes away energy that would otherwise contribute to the speed of the car. A standard wheel has a mass of 2.6 g, but this can be reduced