Transcript Dynamics-cause of motion

Warm Up 5
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10.25.11
What actually causes motion? In
other words, how do objects start,
stop, or change direction?
Dynamics
- the study of what causes motion
Newton’s First Law of
Motion – “Inertia”
Galileo
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For 2000 years, people believed objects
were naturally at rest.
Galileo used experimentation to disprove
2000 years of accepted belief in motion
Galileo (15641642)
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“An object, once
placed in motion, will
move further until
another force brings
it to a stop”.
Implied that the
natural state of an
object is constant
motion.
What does this actually mean?
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“Implied that the natural state of
objects is continuous motion.”
What does this actually mean?
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“Implied that the natural state of
objects is continuous motion.”
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Things stay in motion unless
something stops it.
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Galileo figured out the identity of that
something and called it “Resistance”
Galileo’s Breakthrough #1
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He realized that
things slowed down
due to friction.
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Friction opposes all
motion.
Galileo’s Breakthrough #2
In the real world, we have friction.
 What if there was no friction?
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What would happen to motion?
Galileo hypothesized and
experimented discovered something
entirely new.
In a frictionless world …
Objects will never stop, will go forever
 Objects needs no force to keep it
moving
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Why don’t things move on their own on
a frictionless surface?
Why don’t things move on their own on
a frictionless surface?
Something keeps them from moving
 That “something” must be universal
Photograph by Beverly Joubert
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In a frictionless world …
 All
objects have “it” even on a
frictionless surface.
“INERTIA”
Newton incorporated this into his work
and called the property “inertia”.
Newton’s First Law of Motion
The Law of Inertia
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When no external , unbalanced force acts on an object
its velocity remains constant. or
If no net force acts on an object, it maintains its state of
rest or its constant speed in a straight line or
An object at rest will remain at rest,
an object in motion will remain in
motion in a straight line unless they
are acted upon by an external force.
What is Inertia?
Inertia is a property of matter like
color and mass.
 Inertia is directly related to mass
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More mass, more inertia
Mass is a measurement of the
amount of inertia and matter.
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mass = matter = inertia
But, what is inertia?
Inertia is the “laziness” of matter
 Matter does not like to change its
motion
 The more matter, the lazier it gets
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More matter = harder to change
Example
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An elephant at
rest has a lot of
“laziness” so it
has a greater
tendency to
remain at rest.
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(hard to move)
Example
A runaway train at constant velocity
has a lot of “laziness” so it has a
greater tendency to stay in motion.
 (hard to stop)
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Implications
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External force is required to
accelerate an object.
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Remember what accelerates mean?
External force must be unbalanced.
NET Force – resultant force.
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Which object will remain at rest, according
to Newton’s First Law?
5N
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5N
5N
8N
The cat will move
Which object will continue moving with
constant velocity, according to Newton’s
First Law?
5N
5N
5N
8N
This car will accelerate.
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Look
True or False?
6 Notions of Force and Motion
1. If there is motion, there is a force acting to
keep an object in motion.
2. There cannot be a force without motion- if
there is no motion then there is no force
acting.
3. When an object moving there is a force in
the direction of motion.
6 Notions of Force and Motion
4. A moving object has a force within it which
keeps it moving.
5. A moving object stops when its force is
used up.
6. Motion is proportional to force acting.
Therefore a constant speed result from a
constant force.
Answers?
NOT one of these notions are
correct.
These are 6 commonly held
misconceptions of force and
motion, and they all originate
from Aristotle’s views.
Difference between Mass
and Weight
Differentiating Mass and Weight
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Mass measures the amount of matter
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Mass is a scalar
Does the amount of matter change
when an object is moved from the
earth into space?
Differentiating Mass and Weight
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Weight depends on the mass of an
object TIMES the acceleration due to
gravity.
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Weight is a FORCE and a vector.
Compare your mass and your weight
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Divide your weight in pounds by 2.2.
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This is your mass in kg
What is your weight on the moon
where gravity is 1/6th the earth’s?
 What is your mass on the moon?
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Forces
Force
SI unit – Newton; N
 A force needed to accelerate a 1 kg
mass 1 m/s every second.
 1kg*m/s2 = 1N
 A vector quantity
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A force is a push or a pull
 A force can cause
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1. a stationary object to move
 2. a moving object to stop
 3. an object to accelerate (change speed
or direction)
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Net Force - Resultant Force
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Net force – the combination of all the
forces acting on an object.
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The net force changes an object’s
state of motion.
Net Force
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Balanced force – Net force is equal to
zero
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Object remains
at rest
5N
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Object maintains its motion
5N
5N
5N
Net Force
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Unbalanced force – Net force is NOT
equal to zero
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The cat will move
5N
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8N
The car will accelerate
5N
8N
Net Force
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Objects move in the same direction
as the Net Force
What is the Net Force on the cat?
 Which way does
5N
8N
it move?
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And the car?
8N
3N
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What are the
forces acting on
this book?
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1. Gravitational Force
(Weight)
2. Normal Force
(Support Force)
The net force on the
book is 0N. The forces
acting on the book are
balanced and the book
is in a state of
equilibrium.
Types of Forces
Contact Force
Field Force
•Normal Force
•Frictional Force - Ff
•Magnetic Force
•Tension Force - Ft
•Electrical Force
•Normal Force - Fn
•Air Resistance Force - Fair • Gravitational Force
•Applied Force - Fapp
•Spring Force - Fspring
•Gravitational Force (Weight) – Fg
•Resultant or Net Force Fnet
Free body Diagrams
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Determine the resultant vector (net
force) for these objects.
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400 N, Up
200 N, Down
20 N, Left
Determine the net force for each
situation.www.physicsclassroom.com
0N–
balanced
force
0N–
object in
equilibrium
5 N, Left
15 N, Up
Exit Ticket
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Determine the magnitude of the
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unknown forces.
Light Bulb Moment
 No
matter how many applied
forces are acting on an object
AND the net force is equal to
zero, the object will maintain its
inertia.
 Balanced
forces on an object,
maintains the state of inertia of an
object.
If the forces acting on an object are cancelled, the object is
said to be in a state of equilibrium. Meaning the net force
is equal to zero and the object maintains its inertia.
Weight
Weight
Weight
Air Friction
Normal Force
by the ground
The object is at rest
Air Friction
Falling objects will experience
terminal velocity
Example
If you are standing,
at rest, the
balanced forces
acting on you are:
a.) your weight and
b.) the upward
support force of
the ground called
the
normal force.
Questions
1. Why do professional
photographers use heavy cameras for
their shots?
 2. While making a turn to the right,
why is your body “moving towards the
left side of the car”?
 3. Why do we use safety seat belts?
 4. In which location will it be difficult to
shake a 50 kg rock, on earth or in
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Questions
5. Which object has more inertia, a 15
kilogram bowling ball or a 15 kilogram
bag of pure cotton?
 6. While standing still what is your inertia
with
respect to the sun? with respect to the
ground?
 7. What is the value of the normal force if
you are standing on the table?
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Helps explain the behavior of objects
in motion- they resist change in
motion due to their inertia.