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Chapter 4: Newton's Three Laws

Difficulty Level: At Grade Created by: CK-12

Newton's three Laws of Motion are the core of our understanding of force, which is how objects affect each others' motion.  In this chapter, we will cover the definition of force, an object's innate resistance to being moved by force - called inertia, and how forces interact with each other. 

Chapter Outline

Chapter Summary

Newton’s Three Laws

  1. Newton’s First Law of Motion: An object remains at rest or in a state of uniform motion unless acted upon by an unbalanced force.
  2. Newton’s Second Law of Motion: The acceleration, \begin{align*}a\end{align*}, that an object experiences is directly proportional to the net force acting on the object, \begin{align*}\sum F\end{align*} or \begin{align*}(F_{net})\end{align*}, upon it and inversely proportional to its mass, \begin{align*}m\end{align*}. \begin{align*}\sum F = F_{net} = Ma\end{align*}
  3. Newton’s Third Law of Motion: Whenever two objects interact they must necessarily place equal and opposite forces upon each other. \begin{align*}F_{ab} = - F_{ba}\end{align*}

Solving problems using Newton’s Laws


  1. Read the problem carefully and draw a rough sketch of what is happening
  2. Draw a careful Free-Body-Diagram for each object in the problem.
  3. Write an equation associated with each Free-Body-Diagram using Newton’s Second Law.
  4. Add other equations if necessary.
  5. Solve the system of simultaneous equations.
  6. Check your results to see if they are physically reasonable.

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Difficulty Level:
At Grade
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Date Created:
Jun 27, 2013
Last Modified:
Jan 14, 2016
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