Newton's Laws of Motion

  In the year 1686, at the age of twenty three, Isaac Newton presented his three laws of motion in the “Principalia Mathematica Philosophae Naturalis.” These three laws can be used the explain the motion of a rocket. The first law is known as the law of inertia and the second law relates force to mass and acceleration. The third law explains actions and reactions, which create the thrust that accelerates the rocket.

Newton’s first law states that “Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed upon it.” This is known as inertia. If all external forces cancel each other out (a net force of zero), the object will maintain a constant velocity. If this velocity is zero, the object will stay at rest. If it is greater that zero, the object will maintain its velocity and travel in a straight line. 

If an external force is applied, the velocity will change. Velocity, the speed of an object in a certain direction, is a vector quantity. A vector quantity has both magnitude and direction. Therefore, a change in velocity can involve magnitude, direction, or a combination of both.

One application of Newton’s first law of motion is to the flight of a rocket. When the rocket is sitting on its fins, its weight is balanced by the reaction (from Newton’s third law) of the Earth on the rocket. Because there is no net force and the velocity is zero, the rocket is at constant velocity and would remain at rest indefinitely. When the engine is fired, the rocket does not immediately lift off because the weight is greater than the thrust. As the thrust increases, it eventually equals the force of the rocket’s weight. This creates a net force of zero and the rocket does not lift off. When the force of the thrust is greater than the weight, a net force (thrust-weight), lifts the rocket. As it rises, it encounters another force: aerodynamic drag. Drag opposes motion. Drag increases as the square of velocity. However, with full size rockets, drag eventually decreases as it rises higher into the atmosphere because of the decrease in air density. For the rocket to continue accelerating, the thrust must be greater than weight, which is constantly changing as the fuel is used.

  Newton’s second law of motion states that “A force is equal to the change in momentum per change in time. For constant mass, force equals mass times acceleration."
  For objects with constant mass, the equation is F=ma.  The external force, F, is the combination all four aerodynamic forces. Therefore, and increase in force would create an increase in acceleration. The assumption of constant mass may work for stomp rockets and solid model rockets, but for bottle and full-scale rockets, it is not usable.   
       Newton’s third law states that for every action, there is an equal and opposite reaction. This law is very important for rockets because it explains the generation of thrust by the rocket engine. In a rocket engine, hot exhaust gas is created in the combustion chamber using fuel and an oxidizer. This gas is accelerated through the rocket nozzle. The reaction to this force is thrust . According to newton’s second law, this accelerates the rocket. 
        Even though these fundamental laws were created long ago, they still influence current physics and engineering. All of these laws have more applications that just to rocketry. The laws of inertia, force equals mass times acceleration, and actions and reactions are basic physics principles that influence our entire universe.