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Newtons First Law of Motion
The Law of Inertia
Conceptual Physical ScienceChapter 2
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Aristotles Classification of Motion
Greek scientist (Born384 BCE)
Classified motion into
two categories Natural motion Unnatural motion
Natural motion occurs
without force. Unnatural motion
required a force.
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Aristotles Classification of Motion
Aristotle believed that All objects have resting places that they naturally seek.
All motion on the Earth is linear.
All motion in the heavens (outer space) is curved.
The speed at which an object falls is directly related to the massof an object.
Motion continues so long as there is only an applied motion(force) to an object. Removing the motion (force) stops theobject.
Aristotles ideas lasted almost 2000 years It would take a man named Galileo to start the wheels of
change in this field of knowledge called physics.
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Concept Check:
According to Aristotle, what tendency of
moving objects governed their motions?
According to Aristotle, what kinds of
motion required no forces?
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Galileos Concept of Inertia
Galileo Galilei was an
Italian scientist (1564-
1642 CE).
Believed inexperimentation
Blew away Aristotles
notions of motion
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Galileos Concept of Inertia
Galileo found that
Except for the effects of friction, objects fall at
the same rate regardless of size.
Force is required to start an object moving,
but not to keep it moving.
Tested with this experimental design.
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Galileos Concept of Inertia
The tendency of things is to remain as theyare If moving, they tend to keep moving.
If at rest, they tend to stay at rest.
is called inertia.
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Concept CheckA ball rolling on a pool table
slowly comes to a stop.
How would Aristotle explainthis behavior?
How would Galileo explain it?
How would you explain it?
What two main ideas of Aristotledid Galileo discredit?
What is the name of theproperty of objects tomaintain their states ofmotion?
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Galileo Formulates
Speed & Velocity
Until Galileo motion was described as
eitherfastorslow.
Galileo measured speed by considering
distance and the time it took to cover it.
Distance
Speed = Time
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Galileo Formulates
Speed & Velocity
Approximate speeds in different units
12 mph = 20 km/h = 6 m/s (bowling ball)
25 mph = 40 km/h = 11 m/s (super sprinter)
37 mph = 60 km/h = 17 m/s (sprinting rabbit)
50 mph = 80 km/h = 22 m/s (tsunami)
62 mph = 100 km/h = 28 m/s (cheetah)
75 mph = 120 km/h = 33 m/s (batted softball)
100 mph = 160 km/h = 44 m/s (batted baseball)
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Galileo Formulates
Speed & Velocity
Velocity differs from speed in that we also
know the direction of the moving object.
Velocity is both speed and direction.
Velocity is a vector quantity.
Speed is a scalar quantity.
Constant speed doesnt mean constantvelocity the opposite is true though.
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Concept Check
What is the average speed ofcheetah that sprints 100 m in 4 s?
The speedometer on a bicyclemoving east reads 50 km/h. Itpasses another bicycle movingwest at 50 km/h. Do both bikeshave the same speed? Do theyhave the same velocity?
She moves at a constant speed
in a constant direction. Say thesame sentence in fewer words.
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Motion is Relative
Everythingis alwaysmoving!
Our speed relative to the sun is 100,000
km/h. Faster relative to center of our
galaxy!
Discussing motion
always involves a
reference point.
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And then there was Newton!
Aristotle valued logic to figure things out.
Galileo used experiments.
Galileo showed that experiments arebetter than logic in testing knowledge.
The path was clear then for Isaac
Newton
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Newton
Sir Isaac Newton (1642-1727)
Mathematician and physicist
Discovered many things: Laws of motion
Optics
Gravity
Calculus
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Newtons 1st Law of Motion
An object tends to remain in a constant
state (at rest or in motion) until an
unbalanced force acts upon it.
The larger the mass, the larger the inertia.
Mass and inertia are similar, but not
equivalent.
Inertia depends on the distributionof
mass.
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Concept Check
How does the law of inertia account for
removing dirt from your shoes by stamping
on the porch before entering a house or
removing dust from a coat by shaking it?
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Net Force
The Net Force (Fnet) is the sum of all the
forces acting on an object.
Force is measured in Newtons (N).
5 N
5 N
10 N
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Equilibrium for Objects at Rest
Objects at rest are said to be in staticequilibrium.
Static objects have Fnet = 0 N. Even
though there may be many forces actingon that object they all cancel each otherout.
5 N 5 N
Fnet = 0 N
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Equilibrium rules
Forces are vector so they have direction
and magnitude.
Upward forces +, downward forces -,
forward forces +, backward forces -.
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Equilibrium
Weight is a force (-) that acts in a
downward direction (towards the center of
the Earth).
For an object at rest, there must be an
opposite but equal force acting on it to
have a net force = 0 N.
That force is called the Normal force (Fn).
Its always perpendicular to the surface.
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Concept Check
As you stand at rest
on a floor, does the
floor exert an upward
force on your feet? Ifso, what exactly is
this force?
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Equilibrium for moving objects
Moving objects are in dynamic
equilibrium, not static.
Sum of forces must be = 0 N if
its velocity is constant.
Remember: Newtons First Law!
The balancing a deck of cards
in your hand while on a moving
train, example cards dont
know the difference.
Am I
moving or
standing
still?
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Earths Rotation
If the Earth came to a sudden stop, and I
jumped up into the air at that exact
moment, wouldnt I land safely back on the
ground?
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Earths Rotation & Inertia
The Earth is rotating very rapidly (even as youwrite this down). Do you feel it?
Could you travel from San Francisco to New
York by staying stationary in the air for 3 hours?
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Hard Work!
Humankind struggled
for nearly 2000 years
in developing the
ideas of this section.You should be patient
with yourself if it takes
a few days or weeks
to achieve as much!
Thinking by Broken Persona