Duct-tape a balloon to a plastic straw, duct-tape the straw to a plastic car, blow up the balloon and let it go! The air escaping the balloon will propel the car in the opposite direction. This is an example of Newton’s Third Law (for every action there is an equal and opposite reaction) and demonstrates how rockets work. With balloons, escaping air molecules are essentially “thrown” from the balloon in one direction, pushing the balloon in the opposite direction. Since the balloon is attached to the car, the car moves along with it. These balloon racers work best on a very smooth surface with very light cars!
Similar technology is used to launch vehicles, like space shuttles, which are attached to a number of powerful rockets, burning rocket fuel. The rockets carry the shuttle into space as they’re pushed by the gases being “thrown” from them. Once outside the atmosphere, the space shuttles, or other space vehicles can maneuver through space, where there’s no air, by firing smaller rockets which push them in the opposite direction. Jet (turbine) engines and propellers don’t work in space, since they must push against air to work!
Don’t get frustrated if your rocket racer doesn’t work on the first try- it may take some work to get them going. Make sure the duct tape is tightly sealed against the straw, but isn’t mashing the straw, since the air must escape to push the car in the opposite direction! Engineers and scientists often have to work on designs for years to perfect them!