Balloon Propulsion

• An empty two litre plastic bottle
• A foot pump or bicycle pump with long tube and preferably an inflation needle
• A rubber cork that fits reasonably tightly in the neck of the bottle
• A strong cardboard box or tripod to act as a launch pad
• Some water
• Paper and paint to make a nose cone and fins to decorate your rocket

1. Push the inflation needle through the centre of the rubber cork, or make a hole in the cork big enough to take the tubing from the pump. Washing up liquid might help when pushing the tubing through.
2. Prepare the cardboard box launcher by cutting a hole in the top and in the side of the box while retaining its structure. The bottle will go upside down in the top with the pump tube coming out the side.
3. Fill the bottle a third full of water and, while the bottle is upright and open, place the cardboard box (or tripod) upside down on top. Put the cork in quite firmly and turn the entire structure so that the bottle is upside down and balanced on the box (or tripod). The rubber tube connected to the foot pump should come out the side of the box.
4. Start pumping and launch your rocket
5. You can now retrieve and decorate your rocket, then relaunch it!

By pumping air into the bottle you are increasing the pressure inside. This pressure forces the water down which in turn forces the rubber cork out of the bottle.

This force, pushing the cork, water and air out the bottom of the bottle, is the action, and it is coupled to a reaction, the bottle flying into the air. This was what Newton was referring to in his third law where he stated that for every action there is an equal and opposite reaction.

To find out why water is best to use, you can look at conservation of momentum for the entire rocket. The whole system of water and rocket has zero momentum before launch and zero momentum after launch. The heavier the water and the faster it exits the bottle, the more downward momentum it will carry. To balance this, the lighter the bottle the faster it will go, and the more spectacular the launch.

• A toy car that has no propelling mechanism (e.g. you don't pull it back and let go). It needs to be fairly small.
• A balloon
• Some sticky tape
• A bendy straw

1. Inflate the balloon a few times to stretch it. Slip the end of the balloon over the end of the straw (nearest its bend.)
2. Secure the end of the balloon to the straw with tape and seal it tight so that the balloon can be inflated by blowing through the straw.
3. Tape the straw along the length of the car - so the balloon is at the front and the straw pokes out at the back - it needs to poke out by enough so you can blow the balloon up.
4. To launch the car blow through the straw to inflate the balloon, then pinch the straw or put your finger over the end.
5. Put the car on the floor and let go!

Air escapes from the balloon through the straw. The force with which the air is funnelled down the straw backwards forces the car forwards, not because the air is pushing on the outside air but because all forces have an equal and opposite force. This is Newton's Third Law.

This is exactly the same principle that rockets use to launch: the fuel is burned and the exhaust, under pressure, is funnelled out the back, and the rocket is forced forwards. This works in space where this nothing for the rocket to push on, the simple act of expelling the exhaust pushes the rocket forward.

Another example is in a swimming pool. If you are holding a float and push it away from you, you will find that you also move away from it - in fact both you and the float move away from a central point. The float will move more than you because it has less mass; it can do more with the small force exerted.

1. Blow up the balloon, fold the neck and put a peg or clip on it to stop the air coming out. Thread the string through the drinking straw.
2. Tie each end of the string to two chairs placed a distance apart. Make sure the string is at least 30 cm off the ground
3. Pull the straw to one end of the tied string and stick the straw lengthways to the balloon using sticky tape
4. Remove the peg or clip and watch your rocket zoom away!
5. Measure how far it goes and then repeat the activity but this time rub a little washing up liquid on the string first. Notice any difference?

When the air from the balloon rushes out backwards it propels the rocket forwards. If the friction between the string and the straw is high then the rocket will only go a short distance. However, if the friction is low then the rocket will go much further. The washing up liquid acts as a lubricant on the string, which lowers the friction and makes the rocket go further.

Don’t inflate the balloon yourself if you are under 8 years – ask an adult to do it for you.