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Intro to Newton's 3 Laws of Motion

Updated on May 24, 2014
Contrary to popular belief, Newton's 3 laws were NOT a result of Newton watching an apple fall from a tree.
Contrary to popular belief, Newton's 3 laws were NOT a result of Newton watching an apple fall from a tree. | Source

Who is Isaac Newton?

Isaac Newton (1642-1727), was a (but not limited to) mathematician and physics savant.

He began making important discoveries as young as 23 years. His curiosity and inquisitive nature led him to develop his theories of gravitation, which, in turn, helped him to generate what we call Newton's Laws of Motion.

What is Newton's Principa?

He lays out these laws in a detailed, written study known as "Philosophiæ Naturalis Principia Mathematica". It’s Latin translation-Mathematical Principles of Natural Philosophy, or for short Principa, which was published in 1687.

Newton’s Principa lays out classical mechanics, including his laws of motion.

The common myth is that Newton started pondering gravitation after he watched an apple fall, leading him to form these laws of motion.

Newton's First Law: Law of Inertia

What is inertia? What does inertia mean?

Inertia: The natural tendency for an object to oppose change.

Inertia is a property of matter. The state of motion that some matter is in (whether it be in rest or going along a straight line with a velocity) will remain the same unless an external force interferes. Put simply, inertia is the resistance an object has to a change in its state of motion.

Mass and inertia are't the same thing! Inertia is however, quantified by mass.

  • More inertia --> larger resistance to change in motion
  • More mass --> larger resistance to change in motion

We can come to the conclusion that mass is proportional to inertia. Inertia isn’t a quantity- it’s a property and can be quantified by mass.


Newton's 3 Laws Quiz:

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Newton's Second Law: Law of Motion

Newton’s second law can be put into a simple equation:

Force = (mass)(acceleration)

When a force acts an object, that object will accelerate in the same direction as the force.

The net or total force applied is directly proportional to the mass; however, mass and acceleration are inversely proportional.

How does a force affect an object's motion?

This law explains how an object’s motion will change when a force is applied to it and a force causes motion.

Mass and acceleration each play a role in how a force will affect an object’s motion.


Consider the mass a book to the mass of a truck

  • The book's mass is much smaller than the truck's

Imagine applying equal forces to both objects. Which object will have a smaller acceleration or be accelerating more slowly?

  • The truck accelerates slower than the book

Newton's Third Law: Law of Force Pairs

Newton’s first laws tell you how the forces act and what they do. Newton’s third law however, tells you what forces are.

In classical mechanics, forces are interactions between objects.

What does "forces occur in pairs" mean?

  • Forces always occur in pairs
  • There's an "action force"
  • There's a "reaction force"
  • Each object exerts a force on one another
  • Each force is equal in magnitude
  • Each force is opposite in direction


Imagine yourself punching a wall. Why does punching a wall give you pain?

  • The force you exert on the wall (action force) is of the same magnitude of the force that the wall exerts on you (reaction force). That's why it hurts!
  • Fpunch = - Fwall

Summary of Newton's 3 Laws:

First law (law of inertia)

An object in rest stays in rest and an object in motion stays in motion- unless, of course, an unbalanced force is exerted.

Second law (law of motion)

Force = (mass)(acceleration)

Third law (law of force pairs)

For every action there is an equal and opposite reaction.


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    Shasta Matova 5 years ago from USA

    This is a good introduction to the Newton's laws of motion. Voted up.