Download Inertia - SFA Physics and Astronomy

Chapter 2
Newton’s First Law of
Motion - Inertia
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
 We
is a vector
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
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
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
 One
W
is about the weight of a small apple
kg weighs 2.2 lbs.
= mg
g
= acceleration due to gravity at the surface of the
Earth
• = 32 ft/s2 = 9.8 m/s2
Inertia Demonstrations
 Demo
- Table setting
 Demo - Bottle, hoop, and chalk