Newton's Third Law of Motion

   Written in 1686, Mathematical Principles of Natural Philosophy was the work of Sir Isaac Newton's three basic laws of motion.
   In his first law, Newton states that every object remains at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force, which is commonly known as inertia. In other words, if there is no net force acting on an object, or all the external forces cancel each other out, then the object maintains a constant velocity and if that velocity is zero, then the object remains at rest. However, if the velocity is not zero, then the object maintains the same velocity and travels in a straight path. 
   While in motion, if a net external force is applied on the rocket, its velocity changes because of this force. For example, the liftoff of the rocket from the launch pad represents this principle because before its launch, the velocity of the rocket is zero and the rocket remains at rest, but as pressure builds up from the external force, the rocket begins to rise. As a result, the velocity (speed) of rocket increases under the acceleration produced by the external force. 

   Through the principles of Newton's third law of motion, the bottle rocket gets its lift from the built-up air pressure and the water pushing out of its tail (chamber), and the action force pushes the rocket up in the opposite direction (against the force of gravity), creating the reaction force and sending it into flight.