Rockets - The Law of Action-Reaction

Author's Note: For my next conference I chose to do something on physics, something that has always been a point of interest to me. This is also counting towards one of my products for my Space Exploration merit badge for Scouts.  

The outer shell simplicity of a rocket hides the complexity of the machine itself. The mechanisms inside a rocket engine are incredible, and we take it for granted. It is also extremely hard to harness the power and energy to even get a spaceship off the ground. It is astounding that a simple law of physics drives these machines.

The principle driving a rocket engine is Newton's third law that "to every action is an equal and opposite reaction". A rocket engine forces mass one direction, which pushes the rocket upward, benefiting from the reaction in the other direction. You probably think rocket engines are about flames, pressure, and bursting your eardrums -- not throwing mass.

 A rocket usually throws mass in the form of a high pressure gas. The rocket forces out gas in one direction to move in the opposite direction. The mass comes from the weight of the fuel from the rocket. If there is more mass, you get a bigger reaction and go faster. If you burn a pound of fuel, you'll get a pound of gas coming out of the rocket nozzle. Even though the form changes, the mass does not.

The principle of the propulsion of a rocket is that of expulsion of mass to create a reaction. The principle of equal and opposite reaction that causes rockets to move forward also applies to many situation in everyday life. If you have ever shot a shotgun, you would notice that the gun has a kick that throws you back, a perfect example of how a rocket engine works. The action is the explosion, which both blows you back and the shell forward, as seen in the picture above. If you were sitting on a bike or in rollerblades, the action would cause you to go roll backwards.

Whether you're swinging a baseball bat against a ball, shooting a shell, or firing a rocket, you can know that Newton's third law applies in all aspects of life.