Physics Lesson 7
Newton's First Law of
Motion - Inertia
Eleanor Roosevelt High School
Chin-Sung Lin
History of Motion
Aristotle on Motion
Natural Motion & Violent Motion
 In fourth century B.C.
 Four elements of the world:
earth, water, air, and fire
 Each element had its own
natural place in the hierarchy
of the universe
 Natural motion would be to
return to its natural place
Aristotle on Motion
Natural Motion
 Straight up or down motion
 Heavy things fall (such as
rocks) and light things rise
(such as smoke)
 Circular motion was natural
for the heavens (such as
stars), and they were not
thought to be caused by
forces
Aristotle on Motion
Violent Motion
 Imposed motion with
external cause. It was the
result of forces that pushed
or pulled
 If an object was moving
“against its nature”, then a
force of some kind was
responsible
Aristotle on Motion
Rest is the Proper State
 If there were no force, there
would be no motion
 The proper state of objects
was one of rest (except in the
vertical direction)
 It is a commonly thought for
nearly 2000 years
Galileo on Motion
Break from Aristotle
 Aristotle’s thoughts
dominated the world for nearly
2000 years
 Galileo demolished the
notion that a force is
necessary to keep an object
moving
Galileo on Motion
Friction & Motion
 Friction is the force acting
between materials as they
move past each other
 Galileo argued that only
when friction is present is a
force needed to keep an
object moving
Galileo on Motion
Speed Up
 A ball rolling down an inclined plane picked up speed
Galileo on Motion
Slow Down
 A ball rolling up an inclined plane slowed down
Galileo on Motion
What will happen?
Galileo on Motion
Constant Speed
 A ball rolling on a level surface has almost constant speed
 If there is no friction, the ball moved horizontally would
move forever
Galileo on Motion
Two Inclined Planes Experiment
 A ball released to roll down on one plane would roll up to
the other to reach nearly the same height
 The smoother the planes were, the closer the heights
would be
Galileo on Motion
Two Inclined Planes Experiment
 If the angle of the second plane was smaller, the ball would
roll farther to reach the same height
Galileo on Motion
Two Inclined Planes Experiment
 If the angle of the second inclined plane reduced to zero,
only the friction would keep the ball from rolling forever
Galileo on Motion
Inertia
 Galileo was concerned with how things move rather
than why they move
 He stated that this tendency of a moving body to keep
moving is natural and that every material object resists
change to its state of motion
 We call this property of a body to resist change inertia
Newton’s Law of
Inertia
Newton’s Law of Inertia
Newton’s First Law – Law of Inertia
 Every object continues in a
state of rest, or of motion in a
straight line at constant speed,
unless it is compelled to change
that state by an unbalanced force
exerted upon it
Newton’s Law of Inertia
Newton’s First Law Example
 Place a cardboard on an empty tumbler and a coin on
the cardboard
 Coin drops into the tumbler as the cardboard is flicked
Newton’s Law of Inertia
Newton’s First Law Example
 Snap tablecloth from beneath dishes, and dishes stay
Newton’s Law of Inertia
Newton’s First Law Example
 Tighten the head of a hammer
by banging the handle
Newton’s Law of Inertia
Newton’s First Law Example
 A passenger standing in a moving bus leans forward
when the brakes are applied all of a sudden
Newton’s Law of Inertia
Newton’s First Law Example
 When the string is released, the ball will fly straight
away, not along the curve
Newton’s Law of Inertia
Newton’s First Law Example
 Roller coaster is the driven by inertia
Mass
Newton’s Law of Inertia
Measure of the Inertia
 The amount of inertia an object has depends on its
mass— the amount of material present in the object
 The more mass an object has, the greater its inertia
and the more force it takes to change its state of motion
 Mass is measured in kilograms (kg)
Newton’s Law of Inertia
Mass is not Weight
 We often determine the amount of matter in an object
by measuring its gravitational attraction to Earth
 Mass is more fundamental than weight. Mass is the
quantity of matter in an object and only depends on the
number of and the kind of atoms that compose it
 Weight is the force of gravity on an object. They are
proportional to each other
Newton’s Law of Inertia
Mass is not Weight
 The weight unit in U.S. is pound
 The SI unit of mass is the kilogram (kg)
 At Earth’s surface, an 1-kg object has a weight of 2.2
pounds. The SI unit of force is the newton (N), an 1-kg
object has a weight of 9.8 N
 Weight = mass x acceleration due to gravity
or Fg = mg = 9.8 m
The End