ok, so i have a physics project. we were assigned to use a mouse trap tied to the axle by a string to propel a homemade car of sorts. but he says we can use and propulsion system as long as it doesn't use combustion or electricity. i don't wanna use a mouse trap and i'm looking for other ideas. maybe magnetic? any ideas? we also are to design a braking system so that the car will stop at a self-set distance such as 40m. any ideas?