Physics 1
Dynamics Notes
Newton's Third Law

Introduction

Newton's First Law tells you what happens to an object if the net force on it is zero. Newton's Second Law tells you what happens to an object if the net force on it is not zero. What could Newton's Third Law possibly tell you?

In a sense, Newton's First and Second Laws tell you what forces do. Newton's Third Law tells you what forces are.

Newton's Third Law

Newton's Third Law is often stated:

This statement is correct, but terse and confusing. You need to understand that it means:

• "action...reaction" means that forces always occur in pairs. (Forces are interactions between objects, like conversations are interactions between people. See "Thinking About Forces... .") Single, isolated forces never happen. The two forces involved are called the "action force" and the "reaction force." These names are unfortunate for a couple of reasons :
  
  
  • Either force in an interaction can be the "action" force or the "reaction" force. See Identifying Action and Reaction Forces.
    
    
  • People associate "action" and "reaction" with "first an action, then a reaction" - as in, first Suzie smacks Johnnie (action) then Johnny says "Mommy! Suzie hit me!" (reaction). That is NOT what is going on here! The action and reaction forces exist at the same time.

  
  
  

• "equal" means two things:
  
  
  • Both forces are exactly the same size. They are equal in magnitude.
    
    
  • Both forces exist at exactly the same time. They both start at exactly the same instant, and they both stop at exactly the same instant. They are equal in time.

  
  
  

• "opposite" means that the two forces always act in opposite directions - exactly 180^o apart.

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