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We are not rocket scientists at Cat's Science Club, yet ;-) We try to make these activities fun and understandable.
There is a lot of science behind the rocket going up. Here is some basic science to think about.
Newton's Three Laws of Motion come to mind for us. One being that an object at rest will stay at rest unless acted on by an outside force and an object in motion will stay in motion unless acted upon by an outside force (simplified version). The rocket would just sit there if there was no force from the chemical change that is taking place inside the canister. The build up of pressure is released out the bottom of the rocket which causes the rocket to move. The rocket would continue to move up but gravity is a force that stops the rocket from continually going up.
Second law of motion essentially says that the more mass an object has, the more force will be needed to get that object to move or stop moving (simplified). Force = Mass x Acceleration (f=ma). If we put more mass on the rockets the more force will be needed to launch the rockets. So the less paper and tape we put on the rocket the higher it will fly (canisters with low mass on them can be dangerous. Be careful!).
Third law is action / reaction (at least that is the way we remember it). For every action there is an equal but opposite reaction. If you push a wall, the wall pushes back. Try it with skates on and you will see what I mean. Think of a balloon that is filled with air. Let go of the balloon and the balloon moves in the opposite direction the air is coming out of - action / reaction. The rocket has gasses shooting out the bottom of the canister downward (action) and the canister moves upward (reaction).
We thought that kids could have fun adding weights (a jumbo paper clip is about 1 gram) to a rocket that had another canister on top. Put the weight in the top canister without the water and Alka-Seltzer and close the lid. We used a film canister that has an outside lid. This can help with understanding the second law of motion. The more mass that is in the container the more force will be needed to launch the rocket (the rocket will not go as high or as fast).