When we push air or water backwards, we move forwards. When we push air or water downwards we move upwards. Thrust is the force in the opposite direction to the push. Ask a student to push hard against a wall, and he will move backwards. The backward movement is the "thrust". A swimmer will push water backwards, and so thrust forwards. A bird will push air downwards with its wings and so thrust upwards.
Four forces act on a moving thing (see the plane on the left). The thrust (in the direction it is going), the drag (trying to hold it back), the weight trying to pull it down, and the lift (in the upward direction)
A rocket burns fuel so that it pushes out gas and moves upwards away from the force of gravity, which is trying to pull it back to earth.
WHAT YOU NEED
1 balloon (round ones will work, but the longer shaped balloons work best)
1 long piece of kite string (about 10-15 feet long, or more if you are outside)
1 plastic straw
tape
WHAT TO DO
1. Tie one end of the string to a chair, door handle, or other support.
2. Put the other end of the string through the straw.
3. Pull the string tight and tie it to another support in the room.
4. Blow up the balloon (but don't tie it.) Pinch the end of the balloon and tape the balloon to the straw as shown above. You're ready for launch.
5. Let go and watch the rocket fly!
So how does it work? It's all about the air...and thrust. As the air rushes out of the balloon, it creates a forward motion called THRUST. Thrust is a pushing force created by energy. In the balloon experiment, our thrust comes from the energy of the balloon forcing the air out. Different sizes and shapes of balloon will create more or less thrust. In a real rocket, thrust is created by the force of burning rocket fuel as it blasts from the rockets engine - as the engines blast down, the rocket goes up!
Balloon rocket experiment can be done in the class room, or outside.
