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Q: Newton's Inertia. ( Answered 3 out of 5 stars,   0 Comments )
Subject: Newton's Inertia.
Category: Science
Asked by: trunks1054-ga
List Price: $3.50
Posted: 03 Dec 2002 17:56 PST
Expires: 02 Jan 2003 17:56 PST
Question ID: 118777
I want information about Newton's discovery of Inertia.

Request for Question Clarification by googlenut-ga on 03 Dec 2002 19:41 PST
Hello trunks1054-ga.

Are you looking for a technical description of Newton's Law of Inertia
or historical information about how he came to dicover inertia or some
combination of the two?

I have come up with some information, but I want to make sure it's
what you are looking for.

Subject: Re: Newton's Inertia.
Answered By: googlenut-ga on 03 Dec 2002 22:21 PST
Rated:3 out of 5 stars
Hello trunks1054-ga.

I decided that I could answer your question without clarification
because any answer would have to include a technical discussion as
well as an historical one.

Actually, Newton did not actually discover inertia.  It was Galileo
who first introduced the concept of inertia.

I found a paper written by a group of students at Penn State
University which summarizes it very well

“Galileo formally introduced the concept of inertia as a physical
truth. Inertia, according to Galileo, is the property which all matter
possesses that resists a change in motion. It is inertia that keeps an
object moving at constant velocity and resisting any changes to that
velocity unless acted upon by an outside force. Thus if a ball is
started rolling, it will continue to roll at the same speed and in the
same direction of the original force unless an outside agent acts upon

They go on to say:

“Although Galileo was credited with noticing the phenomenon, it was
Newton who published and named the concept officially.  Galileo’s idea
allowed Newton to come up with his first law of motion, which
basically states that an object will stay at rest or in uniform motion
unless an outside force acts upon it.  This clearly spurs directly
from Galileo’s concept of inertia.”

The Law of Inertia is the first of Newton’s three Laws of Motion (the
second and third being the laws of “Force” and “Action and Reaction”,

Newton’s First Law of Motion, the “Law of Inertia” is commonly stated
at follows (ref: Inteliquest Media Corporation,

“If a body is at rest or moving at a constant speed in a straight
line, it will remain at rest or keep moving in a straight line at
constant speed unless it's acted upon by a force.”

Newton’s Law of Inertia explains why, if a person is sitting in a
moving car, and the car is caused to suddenly stop by hitting
something, the person will continue to move forward until they are
held back by the seat belt, or until they hit the steering wheel or
windshield if they are not wearing a seatbelt.

It also explains why a person sitting in a car moving in a straight
line, will be forced to lean when the car makes a sharp turn.  The
person’s body will tend to continue in the direction of the original
motion until the friction of the person on the seat forces them to
also turn.

A question may be, “what is really the difference between Galileo’s
Law of Inertia and Newton Law of Inertia.

In a lecture titled “How Newton Built on Galileo’s Ideas”, Michael
Fowler, of the University of Virginia Physics Department

“Galileo's analysis of projectile motion was based on two concepts: 

1. Naturally accelerated motion, describing the vertical component of
motion, in which the body picks up speed at a uniform rate.

2. Natural horizontal motion, which is motion at a steady speed in a
straight line, and happens to a ball rolling across a smooth table,
for example, when frictional forces from surface or air can be

Fowler goes on to say:

“Newton's major breakthrough was to show that these two different
kinds of motion can be thought of as different aspects of the same
thing. He did this by introducing the idea of motion being affected by
a force, then expressing this idea in a quantitative way. Galileo, of
course, had been well aware that motion is affected by external
forces. Indeed, his definition of natural horizontal motion explicitly
states that it applies to the situation where such forces can be
neglected. He knew that friction would ultimately slow the ball down,
and--very important--a force pushing it from behind would cause it to
accelerate. What he didn't say, though, and Newton did, was that just
as a force would cause acceleration in horizontal motion, the natural
acceleration actually observed in vertical motion must be the result
of a vertical force on the body, without which the natural vertical
motion would also be at a constant speed, just like natural horizontal
motion. This vertical force is of course just the force of gravity.”

There is a great deal of information on the Internet about Galileo and
Newton’s Law of Inertia.  In addition to those websites I referenced
above, a few good references are listed below:

The University of Tennessee 
Knoxville, “Galileo: the Telescope & the Laws of Dynamics”

Zona Land, “Galileo and Inertia”

A GUIDE TO PHYSICS, “Physics Timeline”

Workforce Silicon Valley Engineering/Technology Curriculum
“Sir Isaac Newton Biography”

Mark Alford, Massachusetts Institute of Technology, “Isaac Newton: the
first physicist”

Newton's Laws of Motion, Eastern Illinois University

I hope you have found this information helpful.  Please request
clarification if you would like additional information.


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