Newton’s Third Law of motion is also known as the Law of Opposite and Equal Forces. According to this law, for every force, there is an equal and opposite force. This explains why we don’t fall through the floor. Our weight on the floor pushing down is balanced by an opposite force pushing up.
Picture a fish swimming forward in the water. It is moving forward because it is forcing water backward. The motion forward and the motion backward are the same – they are opposite and equal. This action and reaction is what makes it possible for a fish to swim. It’s also what makes it possible for a bird to fly.
This law is useful when calculating the effect of two opposing forces. It is one of the basic assumptions of physics.
More on this topic: Modeling Ships and Space Craft: The Science and Art of Mastering the Oceans and Sky by Gina Hagler — Part II – Evolution of Theory, Chapter Four – Hydrodynamic Theorists and Part III – Scale Model Testing Begins, Chapter Ten – It is Rocket Science