Forces Come in Pairs
Newton’s Third Law of Motion states that for every action there is an equal and opposite reaction. Another way to say this is that all forces come in pairs. A very clear way to demonstrate this is to think about touching. You can’t touch without being touched. If you touch someone’s hand, his hand touches you. If you lean against a wall, the wall holds you up by pushing against you. More examples that illustrate Newton’s third law are sitting in a chair, and letting go of an inflated balloon. Rockets are also prime examples of Newton’s third law.
When you sit in a chair, the chair is pressing up on your body, holding you above the ground. Your body in pressing down on the chair with the same amount of force that the chair is using to hold you up. The pair of forces in this example is the chair pushing your body up and your body pushing the chair down.
A rocket moves upward by expelling fuel downward. The rocket pushes the fuel in one direction, and the fuel pushes the rocket in the opposite direction. The pair of forces in this example is the rocket pushing the fuel down and the fuel pushing the rocket up. Rocket image from http://www.esteseducator.com/Pdf_files/Science.Model.Rocket.pdf
A balloon is an example of a rocket. When you blow a balloon up and then let it go, it zips around the room. What makes it move forward? Recalling that forces come in pairs, we see that the air that the balloon is pushing back is in fact pushing the balloon forward. In this case, the balloon rocket’s fuel is the air you blew into the balloon. Balloon image from http://exploration.grc.nasa.gov/education/rocket/TRCRocket/IMAGES/balloon.gif
The paper covering a plastic drinking straw is another example of a rocket that uses air blown into it as a fuel. If you curl up the end of a straw’s paper covering and blow it off the straw, you have created a rocket. The air fills the cavity of the paper straw covering and is pushed backwards, resulting in an opposite force pushing the straw forwards.