I need to build a vehicle for a contest in my physics class and need some ideas. I am going to build a vehicle that is designed to travel a distance between 5 and 10 meters and come to a complete stop without outside assistance. The distance chosen by the judge will not be announced until the day of the competition after all vehicles have been impounded, so that means the vehicle needs to know where to stop. All energy used to propel the vehicle must be stored in a non-metalic elastic solid. The elastic device may be left unattached until just prior to the run, but must be impounded with the vehicle. The transfer of the energy from the non-metallic elastic solid to metallic storage elements is permissible, as long as the metal storage element is at its lowest energy state at the start of the run. The vehicle may not use electrical energy for any purpose. The over all size of the vehicle may not exceed 60cm in length and 30 cm in width. After the vehicle is impounded no parts may be added to or removed from it. The vehicle must start in motion by actuating some sort of release mechanism with a pencil, pen, dowel or similar item. When released, all parts of the vehicle must travel down the track. No launch ramps or stationary wheel chocks. The item used to start the vehicle need not travel with it. Contestants may not hold or restrain the vehicle to prevent it form moving. The track will be 1.5 meters wide, and the distanced traveled will be announced on the day of the race. I was thinking about a spring that is contracted by rubber bands. Once the rubber band is somehow cut, the spring will repel a wheeled vehicle. Of course I need to test how compact the string needs to be for every meter. Also, I need to test the angle of the spring, because I don’t want the vehicle to curve outside the 1.5 meter track. Any ideas?