Create a bottle rocket car using a 500ml cool drink bottle, a balloon, and 4 bottle tops. Full video here: http://www.hometrainingtools.com/balloon-rocket-car-project/a/1346/
16-20 oz. plastic water bottle
Wooden shish-kabob skewers
4 plastic bottle caps
Duct tape or masking tape
Nail, hammer, knife, scissors
The water bottle forms the chassis, or body, of your balloon car. You can start by mounting the wheels on this body.
1. Cut a drinking straw into two pieces as long as the water bottle is wide. Use strips of tape to attach them to the bottle - one near the front and one near the back. The axles for the wheels will run through these straws, so line them up carefully so the wheels won't be crooked.
2. Use a hammer and a small nail to poke holes through the center of four bottle caps. Cut two pieces of a wooden skewer about an inch-and-a-half longer than the pieces of straw you taped to the bottle. Push one end of each skewer through the hole in the center of a bottle cap. If the cap doesn't fit snugly on the skewer, use some modeling clay to hold it in place. Next, thread the skewers through the straws on the bottle and attach the other wheels to the other ends. Make sure your car rolls smoothly.
3. Stretch out a large balloon by blowing it up and then letting the air out of it a few times. Next, make a nozzle. The size of the nozzle is very important. If it is too small, the air can't escape with enough force to propel the car forward. If it is too big, the air will escape too fast and the car won't go very far. Create the nozzle by taping four drinking straws together. Insert the straws into the mouth of the balloon and seal the opening by wrapping a strip of duct tape around it several times.
4. To mount the balloon/nozzle on the car, use a knife to cut two perpendicular slits (to make an X) in the top of the car about 4" back from the mouth of the bottle. Thread the nozzle through this opening and out through the mouth of the bottle. Leave about an inch of the nozzle sticking out of the mouth.
5. Find a hard surface, like a long table, linoleum floor, or sidewalk. Blow up the balloon through the straws at the mouth of the bottle. Pinch the base of the balloon to prevent the air from escaping too soon. Set the car down, let go of the balloon, and watch it go!
The air in the balloon is gas under pressure. The air pushes against the balloon, causing it to expand, but the balloon is also pushing back on the air. The pressure of the balloon pushes the air right out through the nozzle, which creates thrust that propels the car forward.
Keep track of how long the car rolls and how far it goes. Try it several times, then try changing the design to see if you can get it to go farther or faster. How will it work if you only use three straws for the nozzle? What if you use a bigger or smaller balloon? Does the car go farther on linoleum or the sidewalk? Why do you think this might be? Will the car go farther if you start it at the top of a ramp?
Decorate your car and have races with siblings or friends. Try to figure out why one car goes faster or farther than another, and keep experimenting to make your design better!