Download Notes Ch. 4

Chapter 4
Newton’s First Law: The Law of Inertia
1. ARISTOTLE ON MOTION
Aristotle attempted to
understand motion by
classification.
Two Classes:
Natural and Violent
Ancient Greek philosopher and scientist Aristotle developed the
earliest theory of how things move.
natural motion – motion that could maintain itself without the
aid of an outside agent. (pushing a rock off the
ledge and it falls to the ground, liquids falling or
running downhill, air rising, flames leaping upward)
Aristotle believed everything was made of four elements
Aristotle's
Periodic
Table
Fire
Air
Water
“Natural Motion”
(vertical)
Earth
“Violent Motion” (horizontal)
Aristotelian Universe
Terrestrial Realm
 Composition predicts motion
natural tendencies
Fire and Air tend to rise
Earth and Water tend to sink
 Overall tendency to seek rest
 Objects are corruptible (changing)
How things Move
Aristotle's
Periodic
Table
water’s natural
resting place
is on top of earth
Fire
Air
“Natural Motion”
(vertical)
• Each element has its own
natural motion, and its own
place that it strives to be.
Water
Earth
“Violent Motion” (horizontal)
• Aristotle believed an object’s
natural motion was determined
by how much of each element
the object contained (rocks
sink in water because they
contain mostly earth, wood
floats because it contains
mostly air)
earth moves downward
because Earth’s center
is its natural resting place
• Violent Motion – motion that forced objects to behave contrary to an objects
natural motion, meaning an external push or pull was needed
Aristotle’s Laws of Motion –Summary
1.
Natural Motions are determined by the innate “desire” of the moving object to return
to it’s “natural resting place “. The proper state was “at rest”.
2. Violent Motions occur because outside influences give motion to the object. As soon as
force “runs out”, the object stops. This was the belief for 2000 years.
3. Objects composed of ether travel in gigantic, perfect circles centered on the Earth
which was assumed to be at rest.
Stationary Earth
• Earth is heavy.
– Easier to imagine the sky can move
• It looks like the sky is moving.
– We have no sensation of our motion.
• A rotating Earth would make objects
fly off of the surface.
• It would take a huge force to move the
Earth so the Earth must not be moving.
• The Earth must be the center of the
universe.
Nicholas Copernicus
1473 - 1543
(Niklas Koppernigk)
2. Nicholas Copernicus
developed a mathematical
model for a Sun-centered
solar system based on
astronomical
observations.
Galileo Galilei
1564-1642
• He was among the first to turn a telescope to the
sky.
• He developed the Scientific Method.
• He believed in the popularization of science.
• He defined inertia as resistance to change in
motion.
3. Galileo Galilei helped overthrow Aristotelian physics.
Because of his
radical ideas he was confined to house arrest for the
last 10 years of his life.
Galileo argued that only when friction is present is a
force needed to keep an object in motion. Friction is a
force caused by irregularities in the surfaces of objects
in contact. Friction acts in a way which opposes motion.
I do not feel obliged to believe that the same God who has endowed us with
sense, reason, and intellect has intended us to forgo their use. -Galileo
As a ball rolls down an incline its speed increases. Push the ball up incline, it slows down, stops and rolls
back down. Suppose the incline is nearly horizontal. Obviously, due to friction the ball would probably
come to a stop. Galileo understood that this slowing is due to the roughness of the incline and ball. Let’s
try to visualize the experiment and neglect friction. The ball would then speed up on any downward slope
and slow down on any upward slope. Let’s look at the limiting case when the incline is perfectly horizontal.
In the absence of friction, a ball that got started rolling horizontally would roll forever. This is
contradictory to Aristotle’s view of motion

Aristotle
Galileo
What type of motion?
Celestial
All objects resist a change in their
motion so what are the outside
influences that cause motion?
Natural
Violent
What is
it made of
How much
motion given?
Ether
Circles
Fire
Air
Water
Natural Resting Place?
Earth
Isaac Newton
1642 - 1727
Newton’s Laws
The
st
1
Law
Every object continues
to move as it has been
moving unless acted
upon by an external
force.
Law of Inertia – An object will move in a straight line at a
constant speed
UNLESS acted upon by an external force
- objects in motion stay in motion
- objects at rest stay at rest
UNLESS influenced in some way
YOU
you start turning here
•outside influence – tires on car
friction on tires
•outside influence – door on you
PASSENGER
Car begins turning here
Another example: “Canis Insanis”
you fastened to you seat belt
you stop too!
Jeep
Miss Pickles standing on the
console
apply brakes at
red light
trip to the
vet needed
Mass and Inertia
• Inertia is a property of a body that resists
changes in motion.
• Mass is a measure of the amount of matter in a
body.
• Inertia and Mass are the same concept.
– Units – Kg or slugs
• Mass and weight are NOT the same concept but
they are proportional.
• Mass and volume are NOT the same concept but
they are proportional.
Weight
• Force of gravity pulling on the mass of
the body.
– Units – Newtons or Pounds
• A newton is about the weight of a small
apple.
– One kg weighs 2.2 lbs.
• W = mg
– g = acceleration due to gravity at the
surface of the Earth
• = 32 ft/s2 = 9.8 m/s2
6. Net Force
• A force is a push or a pull.
• Force is a vector
– We must add all the forces acting on the body
5N
5N
5N
5N
10 N
5N
0N
10 N
5N
7. Static Equilibrium
Velocity is zero
Examples:
Scales pushing up
Normal up
Weight down
Computer setting on a table
Weighing yourself on a set of scales
Hanging from a tree
Tree
Car parked on an incline pulling up
Friction
Weight down
Normal
Weight down
Weight down
Dynamic Equilibrium
• When the net force
is zero (SF = 0).
This refers to all of
the forces acting on
an object, including
friction.
Acceleration is
zero.
Velocity may not
be zero.
How things Move
• Gravity is the “outside influence”
that causes the ball to drop
• In fact all falling objects fall at the
same rate, called the acceleration of
gravity (neglecting air resistance)
• Drop different objects their speed
will increase at the same rate!
• Their speed will increase by ~ 10 m/s
(32 ft/s) every second
Free Fall and Friction
Air friction slows the
ping pong ball down
compared to the
billiard ball.
In the absence of air,
Galileo predicted
that they would fall
at the same rate, like
a wooden and iron
ball.
How things Move
Two things Galileo figured out as a result of the law of inertia
1.
Assuming that the only “outside influence” has the same effect on all objects,
all should fall at the same rate. [Galileo’s Law Falling]
Galileo knew not of force or the law of gravity, these concepts
came along later. Rather he took this “same effect” idea from his
observations and idealizations
2.
There is an outside influence called friction present in the “real world”.
Friction acts differently on different objects.
Galileo’s reasoning for “why moving things stop”
Uniform Circular Motion – acceleration by virtue of a change in direction
“centripetal acceleration” – center pointing
Imagine that a car travels with a constant speed around a circular track
instantaneous velocity at
this point on the track
In this type of motion, the
acceleration is always
directed toward the center
of the circular path
C
Quantitative:
travel at speed = s
around circle of radius = R
your “centripetal” acceleration is
s2
ac 
r
C = center of the circular path
R = radius of the circular path
R
where the car
ends up a moment
later if there
is no
acceleration
the difference in velocity
points in the direction of
the acceleration…….
toward the center
instantaneous velocity of the
car at a new point on the
track a moment later….same
speed – different direction
• Motion not on earth followed a different set of rules. Motion in the heavens
had no beginning or end. Circular paths were considered natural for the stars
and the planets.
• 5th element – ether (from the Greek word for to kindle or blaze) – had no
weight and was unchangeable, and perfect in every way
• Moon, sun, planets and stars were made of ether
• Ether’s natural place was in the “heavens”
and it moved in perfect circles.
celestial
motion
“perfect circles”
• Objects on earth could not move the way the
stars did because they did not contain ether.
• Aristotle's physics governed science until about
the mid 16th century.
• It was popular because it reinforced
•religious beliefs.