Newton’s Third Law For every action, there is an equal and opposite reaction.
What are “action” and “reaction”? “Action” and “Reaction” are names of forces.
“For every action…reaction” “For every action force, there is … a reaction force” means: Forces ALWAYS occur in pairs. Single forces NEVER happen.
Action & Reaction Forces Since a force is an interaction between objects, two objects are involved in every force. Call the objects A and B: Action force: “A pushes B” Reaction force: “B pushes A”
“Equal” In Newton’s Third Law, “equal” means: Equal in size. The action and reaction forces are EXACTLY the same size. Equal in time. The action and reaction forces occur at EXACTLY the same time.
“opposite” In Newton’s Third Law, “opposite” means: Opposite in direction The action and reaction forces are EXACTLY 180 o apart in direction.
Two Logical Difficulties 1. If Newton’s Third Law action & reaction forces are equal and opposite, how come they don’t always cancel, making net force and acceleration impossible? (This is the question in the “Horse and Wagon” problem.)
The Other Logical Problem 2. If the Newton’s Third Law action and reaction forces are always equal and opposite, how do two objects of different sizes get different accelerations in the same interaction? (When a bug hits a windshield, different things happen to the bug and windshield.)
The Keys to Understanding Only forces pushing or pulling on an object affect the object’s motion. Only forces that act on the same object can cancel. Newton’s Third Law action and reaction forces push/pull on different objects, so they don’t cancel.
Action & Reaction Again The (action) force “A pushes B” affects the motion of object B. The (reaction) force “B pushes A” affects the motion of object A. The action and reaction forces don’t cancel since they push on different objects.
The Second Difficulty Addressed If the action and reaction forces are the same size, how can two objects push on each other and get different accelerations? Newton’s Second Law says that the acceleration of an object depends not only on the force on it, but on the object’s mass.
Don’t Forget Mass! The same force acting on objects of different mass will produce different accelerations! F net m a = m = a Same Force
Internal Forces It is possible for Newton’s Third Law action/reaction forces to cancel - if they act on different parts of the same object. These forces are called “internal forces”.
Systems The object or objects that you are dealing with in a given situation is called a “system”. How you pick your system determines whether a particular action/reaction force pair will cancel or not.
Systems There is no “trick” to picking a “correct” system - Newton’s Laws will work just fine in any system.