Download Aristotle`s Universe Terrestrial Realm

Chapter 4
Newton’s Laws of Motion
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
Rotating Earth would make objects fly off
of the surface
 Stellar Parallax

Stellar Parallax

Hipparchus 150 BC
June
Sun
January
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)
1. ARISTOTLE ON MOTION
Aristotle attempted to
understand motion by
classification.
Two Classes:
Natural and Violent
Aristotelian Universe
Celestial Realm
Celestial Objects composed of Aether
Self luminous but does not consume
 Motion is constant, circular
 Objects are incorruptible (not changing)
Meteors and comets were phenomena of
the Earth’s atmosphere

The falling speed of an object was
supposed to be proportional to its
weight.
 Galileo’s Experiment

Nicholas Copernicus
1473 - 1543
(Niklas Koppernigk)
Developed a
mathematical model
for a Sun-centered
solar system
Galileo Galilei
1564-1642
 Among
the first to turn a telescope to the
sky
 Developed the Scientific Method
 Believed in the popularization of science
 Developed the Law of Inertia
He tested with planes.
Demo - Ball and incline plane
 The change in speed depended on
the slope of the incline.
 Seemed that the ball was trying to
achieve the same vertical height.


Isaac Newton
1642 - 1727
Newton’s Laws
The
st
1
Law
A body continues to
move as it has been
moving unless acted
upon by an external
force.
No mention of chemical composition.
 No mention of terrestrial or celestial
realms
 Force required when object changes
motion.
 Acceleration is the observable
consequence of force acting.

Net Force

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
Equilibrium

When the net force is zero (SF = 0)
Acceleration is zero
Velocity may not be zero
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
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 = 32 ft/s2 = 9.8 m/s2
Inertia Demonstrations
Demo - Table setting
 Demo - Bottle, hoop, and chalk

Newton’s Laws
The
nd
2
Law
The Sum of the Forces acting on a
body is proportional to the acceleration
that the body experiences
SFa
S F = (mass) a
2. NEWTON'S 2nd LAW OF
MOTION
F
a
m
F
M
a
m
F
a
m
F
M
a
m
m
F
m
1
a
m
a
F
 
aF
M
or
a


F  ma
If the net force is perpendicular to the velocity,
the direction of the velocity changes.
• Acceleration is always in the direction of
the net force.


F  ma
When Acceleration Is Zero - Equilibrium
Static Equilibrium
Scales pushing up
Normal up
Velocity is zero
Examples:
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
Velocity is nonzero and constant
Examples:
Driving at constant velocity
Force from road
Normal up
Friction
Weight down
Newton’s Laws
The 3rd Law
For every action force there is an
equal and opposite reaction force
(You cannot touch without being
touched)
Dissipative Forces
Oppose motion or the tendency of motion


Friction
Air Resistance

Depends upon...
◦ Materials that are in contact
◦ Forces that press surfaces together (Normal
forces)

Static friction (no motion)
◦ Proportional to force used until maximum is
reached

Kinetic friction (motion)
◦ Nearly constant for normal speeds
Friction
Static
F
Kinetic
f
f
F
Air Resistance

Depends on
◦ Surface Area
◦ Velocity

When weight = air resistance we have
SF= 0  a = 0
Terminal Velocity!
Weight
Air
Resistance