Newton's Laws of Motion: A Survey in Three Acts

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Newton's Laws of MotionA Survey in Three Acts
This survey will cover the following sections in
the Pennsylvania Department of Eduction Academic
Standards for Science and Technology 3.1.10 C,
3.1.10 E and 3.4.10 C
Lesson 1 1st and 2nd Laws
Lesson 2 3rd Law Vectors
Lesson 3 Free Body Diagrams
Christopher Pons King's College ED366 June
27, 2007
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What do these guys have in common, besides the
hair?
Sir Issac Newton
Sir Tony Alva
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They both have intimate relationships with
gravity!

• As you watch this movie, think about how the
  rider is moving and where the board is going,
  before and after the crash.
• Why do the riders fall during their tricks?
  Are there similarities in the way they wipe out?
• Where do you think the motion starts and stops
  for the board and the rider?

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Lesson One Newton's 1st Law of Motion
An object at rest tends to stay at rest and an
object in motion tends to stay in motion with
the same speed and in the same direction unless
acted upon by an unbalanced force.
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Terms to Understand
Velocity the act of motion measured in terms of
change in an object's distance with respect to
the time to travel that distance. Measured in
distance/time (meters, feet, miles.../seconds,
minute, hours...)
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Lesson One Newton's 2st Law of Motion
The acceleration of an object as produced by a
net force is directly proportional to the
magnitude of the net force, in the same
direction as the net force, and inversely
proportional to the mass of the object.
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Terms to Understand
Acceleration the rate at which velocity changes
over time. Measured in velocity/time
(meters/second)/second, (feet/minute)/minute....
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Ideas to Understand

• The 2nd Law is usually restated as Force
  mass acceleration. This can be rewritten as
  acceleration Force/mass.
• Mass is the quantity of molecules in an object,
  measured in grams or slugs. The greater the
  mass the greater the inertia an objects
  resistance to change in motion.
• Force is only possible with acceleration. The
  most common example of a force is your weight
  your mass is being acted upon the acceleration of
  gravity. This is measured in Newtons
  kilograms(meters/second)/second or Pounds
  slugs(feet/second)/second
• Forces are in a state of equilibrium if
  acceleration equals zero, and the direction of
  motion is unchanged.

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Team Activity
Watch the car crash animation, consult your team
members and describe the car and the driver as
follows constant velocity, acceleration, and
equilibrium.
What would prevent the driver from getting
launched out of the car? How is that represented
as a force?
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Lesson One Summary

• All objects have mass, and because they have
  mass they have inertia the property of being
  resistant to changes in their motion.
• Velocity is a quality of motion distance
  traveled over time.
• Acceleration is a quality of change in
  velocity change in velocy over time.

Use the Fma formula to solve these
problems. (Bottom of page)
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Riddle me this Batman...
Which one hits the ground first?
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Lesson Two Multiple Forces on Single Objets
In considering the motion of the two objects,
although the bowling ball was fired horizontally,
it was still under the force of gravity, the same
as the baseball. The bowling ball was under two
forces, gravity and applied force, but they act
independently.
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Contact Forces Action-at-a-Distance Forces

• Applied force
• Friction force
• Air Resistance force
• Tension force
• Normal force
• Spring force

• Gravitational force
• Eletrical force
• Magnetic force

These forces can all act on an object
concurrently. Can you think of some objects that
have multiple forces acting at once? Hint
gravity always counts as one force.
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Newton's 3rd Law of Motion
For every action, there is an equal and opposite
reaction.
Plane towing a glider
Direction of Motion Constant Velocity
Glider and its force
Plane and its force
How do these objects respond to collisions?
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Vectors
Because forces are dependent upon the magnitude
and direction of acceleration, forces are
considered vector qualities. Vector directions
are commonly measured in degrees according to
this notation
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Vectors
Consider the free fall of the Rock of Ages.
Let's quantify the forces in terms of Newton's
2nd Law of Motion Fma
This is the mass
This is the weight
The Rock of Ages
Note the acceleration
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Vectors
Compare this to a rock 1/10th the size
Note the weights of the rocks.
Check the acceleration, it's the same, because
gravity accelerates everything at the same rate!
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Acceleration and Velocity Vectors
In the animation below, the green acceleration
line is also the force vector. What is the force
vector doing in terms of the velocity? The
direction? (Hint check the status.)
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Student Activity
Using the animations on the right, draw diagrams
similar to the Rock of Ages slide to show the
forces on the elephant and the feather in flight
with the air resistance neglected and the air
resistance considered.
AR Neglected AR Considered
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Lesson Two Summary

• Newtons 3rd Law of Motion states for every
  action there is an equal and opposite reaction.
• Forces can be represented in forms of vectors.
• Force vectors act independently on a moving
  object.
• Review the forces on the parachutist. What
  happens when F(air) is greater than F(grav)?

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Can Man Fly?
(Without wings, a plane or a helicopter?)
Technology One
Technology Two
How is this even possible that these guys can
''fly''? What are the differences between the two
technologies? How are they similar?
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Lesson Three The Free Body Diagram
Using our knowledge from Newtons Laws of Motion
and vectors, we can construct diagrams that
represent the states of motion for objects the
Free Body Diagram (FBD). Using Newtons 3rd Law,
we can constuct the most common of all FBD's, the
object at rest...
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Behold the Object at Rest!
This book sits on the table. The force of
gravity holds is pulling it down against the
table surface, the normal force is holding it up.
In turn, the legs of the table are exerting
force against the ground due to the weight of the
book.
The Free Body Diagram of the book. Since the
book is not flying upward , nor falling downward,
the forces are in equilibrium
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Multiple Forces Multiple Vectors
In analyzing an object in motion, often you must
contend with multiple vectors to get a full
understanding of the motion. For example A
book being pushed across a table A
bookbag hanging by two straps
A car coasting to a stop A Japaneese
guy flying through the air
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Vector Addition
In these examples where the forces are at right
angles, the sum of the forces is a matter of
simple addition and subtraction.
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Angles and More Angles
Real life examples of forces often do not occur
at right angles, so we must use vector addition
to consider each of the forces. Three methods
exist to address vector addition, the
parallelogram, the Pythagorean and the
trigonometric. The parallelogram method demands
that an exact scale sketch of the vectors is
made and then one vector is super-imposed on
another. Thanks to modern technology this is
easier to show than explain.
Vector Addition Applet
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More Angles Continued
The Pythagorean method of solving vector addition
is only capable of adding two vectors that
intersect at a right angle. The
trigonometric functions are also quite useful in
this case Sine, Cosine and Tangent. Remember
SOHCAHTOA Sine Opposite/Hypotenuse,
CosineAdjacent/Hypotenuse, TangentOpposite/Adjac
ent.
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Angles Con't. - Simplifying The Problem
Once the multiple vectors are broken down, the
vectors can be easily added, since their order is
independent
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Class Excercise
Gather in groups of three, and each one of you
draw a Free Body Diagram of one of these planes.
Assume the blue vector is the airplane velocity
of 100 miles/hour and the red vector is a wind
velocity of 20 miles/hour. What is your
resultant vector, Victor?
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Remember this?
From the Lesson Two movie, here is a horizontally
fired ball that experiences motion in two
directions. How does the FBD of the cannonball
change as tgt0? Describe the shape of the
cannonball's path.
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Lesson Three Review

• The Free Body Diagram is a versitile tool to
  analyze motion and forces on objects.
• Several methods are available to sum the
  vectors of an object parallelogram, Pythagorean
  and trigonometric.
• Vectors can be added independently of one
  another.
• Let's do some FBD problems together. Here!