# Physics in Action: Newton's Third Law of Motion

Isaac Newton's three laws of motion are the foundation of classical mechanics, one of the main branches of physics. His third law of motion states that if object A exerts a force on object B, object B always exerts a force back, and the two forces are equal in magnitude but opposite in direction to each other.

In other words, for every action, there is an equal and opposite reaction.

For instance, when a bird flies, it pushes the air downward, and in turn the air pushes the bird upward. In addition, the air pushes the bird upward with just as much force as the bird exerts downward with its wings. The third law can also be used when describing how a commercial jet gains loft. (Birds and jets also take advantage of Bernoulli's Principle, which we looked at in our last post.)

To get a better look at how Newton's third law works, we visited Professor Ephraim Fischbach at Purdue University for another of his fantastic demonstrations.

(Amy Bryant, co-founder of Thinkwell, makes a special guest appearance in this one!)