Transcript Newton’s First Law of Motion – The Law of Inertia

Newton’s First Law of Motion –
The Law of Inertia
Conceptual Physical Science
Chapter 2
Aristotle’s Classification of Motion
• Greek scientist (Born
384 BCE)
• Classified motion into
two categories
– Natural motion
– Unnatural motion
• Natural motion occurs
without force.
• Unnatural motion
required a force.
Aristotle’s 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 mass
of an object.
– Motion continues so long as there is only an applied motion
(force) to an object. Removing the motion (force) stops the
object.
• Aristotle’s 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.
Concept Check:
• According to Aristotle, what tendency of
moving objects governed their motions?
• According to Aristotle, what kinds of
motion required no forces?
Galileo’s Concept of Inertia
• Galileo Galilei was an
Italian scientist (15641642 CE).
• Believed in
experimentation
• Blew away Aristotle’s
notions of motion
Galileo’s 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.
Galileo’s Concept of Inertia
• The tendency of things is to remain as they
are…
– If moving, they tend to keep moving.
– If at rest, they tend to stay at rest.
• is called inertia.
Concept Check
A ball rolling on a pool table
slowly comes to a stop.
• How would Aristotle explain
this behavior?
• How would Galileo explain it?
• How would you explain it?
What two main ideas of Aristotle
did Galileo discredit?
What is the name of the
property of objects to
maintain their states of
motion?
Galileo Formulates
Speed & Velocity
• Until Galileo motion was described as
either fast or slow.
• Galileo measured speed by considering
distance and the time it took to cover it.
Distance
• Speed = Time
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)
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 doesn’t mean constant
velocity… the opposite is true though.
Concept Check
• What is the average speed of
cheetah that sprints 100 m in 4 s?
• The speedometer on a bicycle
moving east reads 50 km/h. It
passes another bicycle moving
west at 50 km/h. Do both bikes
have the same speed? Do they
have the same velocity?
• “She moves at a constant speed
in a constant direction.” Say the
same sentence in fewer words.
Motion is Relative
• Everything is always moving!
• 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.
And then there was Newton!
• Aristotle valued logic to figure things out.
• Galileo used experiments.
• Galileo showed that experiments are
better than logic in testing knowledge.
• The path was clear then for Isaac
Newton…
Newton
• Sir Isaac Newton (1642-1727)
• Mathematician and physicist
• Discovered many things:
– Laws of motion
– Optics
– Gravity
– Calculus
Newton’s 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 distribution of
mass.
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?
Net Force
• The Net Force (Fnet) is the sum of all the
forces acting on an object.
• Force is measured in Newtons (N).
5N
5N
10 N
Equilibrium for Objects at Rest
• Objects at rest are said to be in static
equilibrium.
• Static objects have Fnet = 0 N. Even
though there may be many forces acting
on that object they all cancel each other
out.
Fnet = 0 N
5N
5N
Equilibrium rules
• Forces are vector so they have direction
and magnitude.
• Upward forces +, downward forces -,
forward forces +, backward forces -.
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).
• It’s always perpendicular to the surface.
Concept Check
• As you stand at rest
on a floor, does the
floor exert an upward
force on your feet? If
so, what exactly is
this force?
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: Newton’s First Law!
• The “balancing a deck of cards
in your hand while on a moving
train,” example… cards don’t
know the difference.
Am I
moving or
standing
still?
Earth’s Rotation
• If the Earth came to a sudden stop, and I
jumped up into the air at that exact
moment, wouldn’t I land safely back on the
ground?
Earth’s Rotation & Inertia
• The Earth is rotating very rapidly (even as you
write this down). Do you feel it?
• Could you travel from San Francisco to New
York by staying stationary in the air for 3 hours?
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