Transcript Review Ch.1-4 Quiz 1 Wednesday 1

Review Ch.1-4
Quiz 1 Wednesday
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Chapter 1
About Science
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theory
a synthesis of a large body of
information.
The criterion of a theory is not whether
it is true or untrue, but rather whether it
is useful or nonuseful.
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hypothesis
1. a possible explanation, a “small”
theory.
2. something taken to be true for the
purposes of argument or investigation.
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scientific hypothesis
the criterion is
that it is testable
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scientific facts
are not immutable and absolute, but are generally in
close agreement as measured by competent
observers of the same phenomena.
The observations must be testable.
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Chapter 2
Newton’s 1st Law of Motion
Inertia
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Aristotle on Motion
• Two Categories:
• natural motion - every object has its
natural place and strives to get there.
• violent motion - pushing or pulling
• Objects need force to maintain motion.
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heavier objects
• Aristotle reasoned that heavier objects
would fall faster than lighter objects.
• Galileo disproved this in his famous
“Leaning Tower” ball drop.
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Galileo’s Inclined Planes
• Galileo believed that an object would
move endlessly in a straight line if it were
not interfered with.
• Aristotle argued that you must continue to
push on something to keep it moving.
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Newton’s 1st Law of Motion
• Every object continues in its state of rest,
or of uniform motion in a straight line,
unless it is compelled to change that state
by forces impressed on it.
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Inertia
• the property of objects to resist changes in
motion.
• measurement is called “mass”
• Scientific unit for mass is kilogram (kg)
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Net Force
• the summative action of forces acting on
an object.
• forces have size and direction
• Scientific unit for force is newton (N)
• US customary unit is pound (lb)
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Examples of Calculating the Net Force
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1 lb = 4.45 N
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Mechanical Equilibrium
• achieved when Fnet = 0, which is the case
of Newton’s 1st Law:
• object at rest and remains so, or,
• object moves without change in speed or
direction.
• symbol for sum of forces is “S ”.
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Examples of Mechanical Equilibrium:
Note how similar these two situations
are!
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You try it. What is the unknown tension?
What is the upper limit on the man’s
weight?
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Example of Mechanical Equilibrium and
Newton’s 1st Law.
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Does the mass of the books affect the
amount of protection she gets? Would a stiff
but light box of the same size protect her as
much?
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Chapter 3
Linear Motion
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Speed
• scalar
• average speed = distance traveled divided
by elapsed time
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Velocity
• Vector
• Velocity is speed in a given direction (velocity is
a vector, speed is a scalar)
• vavg = displacement divided by time
vavg
x

t
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Acceleration
•
Measures how quickly velocity changes:
Acceleration = change of velocity
time interval
aavg
v

t
• Note acceleration refers to : decreases in speed, increases in speed,
and/or changes in direction i.e. to changes in the state of motion --- from
Newton’s law, lurches…
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Velocity and Displacement in Free
Fall
• v = gt
• d = ½gt2
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Chapter 4
Newton’s 2nd Law of Motion
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Newton’s Second Law: When a Net External Force acts
on an object with mass m, the resulting acceleration of
the object is parallel to the net external force and has
magnitude of

 F
a
m
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When a Net External Force acts on an object at rest,
the object will begin moving.
If the object is already moving, its motion will change.
The direction of the change of motion is in the
direction of the net force.
Example Motion Diagrams when the Net Force is not zero.
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Friction
• Objects in contact “like each other”, i.e.
they form a chemical “bond”.
• They resist being moved when in contact.
• These resistance forces are called
“frictional forces”.
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Categories of Friction
• Sliding Friction: exists when one object
slides against a second object, e.g. box
along floor.
• Static Friction: exists when a force is
applied to an object, but that force is not
large enough to break the frictional bond.
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Mass and Weight
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•
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Mass is the quantity of matter.
Mass measures “inertia”.
Mass is measured in kilograms (kg)
Weight is the force on mass due to gravity.
Weight is measured in newtons (N) or
pounds (lb).
• Weight ~ Mass
• 1 kg of mass has a weight of 2.2 lbs
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Free-Fall
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only force acting is weight
no other forces act
air drag is negligible in size
Ex. A solid steel ball falling a short
distance is in free-fall.
• Ex. A feather falling to floor is not in freefall since air drag is very significant.
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Comparing Accelerations of
Objects in Free Fall.
• Downward force is weight.
• a = weight/mass
• but an object with twice the mass will have
twice the weight…
• so the accelerations are the same…
• We call this acceleration “g”.
• g is about 10m/s/s downward.
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Non Free Fall
• Whenever air drag is significant compared
to weight the object will fall with
acceleration less than 10m/s/s.
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Terminal Speed/Velocity
• As objects speed up the air drag increases
until it is equal in size to the objects
weight.
• In this case the net force is zero and
therefore a = 0.
• a = 0 means velocity no longer changes.
• This velocity is the “terminal velocity”.
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End
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