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# Intro to Newton's 3 Laws of Motion

## 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-

**or for short**

*Mathematical Principles of Natural Philosophy*,*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.

## Schaum's Outline: College Physics:

## 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, __t__hat 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.

**Example)**

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*

## Example)

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!**F*_{punch}= - F_{wall}

## 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)*

*(law of force pairs)*

* *For every action there is an __equal and opposite reaction__.