Using the

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Using the
FORCE
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Sir Isaac Newton
Newton was an English physicist, mathematician,
astronomer, natural philosopher, alchemist, and
theologian and one of the most influential men in
human history. Among many other things, he is
credited with describing the Universal Law of
Gravitation and the Three Laws of Motion. A 2005
survey of the British Royal Society ranked Newton
ahead of even Einstein as having a greater
influence on the history of science.
January 4th, 1643 March 31st, 1727
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Philosophiæ Naturalis Principia Mathematica
Latin for "mathematical principles of natural
philosophy"
A three-volume work by Isaac Newton published on
5 July 1687 that contains the statement of
Newton's laws of motion forming the foundation of
classical mechanics, as well as his law of
universal gravitation and a derivation of
Kepler's laws for the motion of the planets
(which were first obtained empirically). The
Principia is widely regarded as one of the most
important scientific works ever written.
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Newton's law of universal gravitation

• The physical law describing the gravitational
  attraction
• between bodies with mass
• It states the following
• F is the magnitude of the gravitational force
• between the two point masses,
• G is the gravitational constant,
• m1 is the mass of the first point mass,
• m2 is the mass of the second point mass,
• r is the distance between the two point masses.

Every point mass attracts every other point mass
by a force pointing along the line intersecting
both points. The force is proportional to the
product of the two masses and inversely
proportional to the square of the distance
between the point masses                    
where
Got it?
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Force?
A force is a push or pull upon an object
resulting from the object's interaction with
another object. Forces only exist as a result
of an interaction.
Force has both direction and magnitude
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Inertia
The vis insita, or innate force of matter is a
power of resisting, by which every body, as much
as in it lies, endeavors to preserve in its
present state, whether it be of rest, or of
moving uniformly forward in a straight line.
In English, please! Inertia is the resistance
of an object to a change in its state of motion.
A bowling ball has more inertia than a soccer
ball. Dont believe me? Kick the bowling ball,
then kick the soccer ball. More mass More
inertia
More inertia
Less inertia
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Balanced vs. Unbalanced forces
A soccer ball sits alone on the field, quiet and
still
Ground pushing up
Forces are balanced so ball does not move
Gravity pulling down
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Balanced vs. Unbalanced forces
Suddenly.
An unbalanced force is exerted on the ball in the
direction of the arrow, causing it to move in
that direction until
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Balanced vs. Unbalanced forces
its motion is stopped by an interaction with
another force! Once again, the forces are
balanced and all is well in the universe.
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Balanced Forces
If the forces acting on an object are balanced,
the object is at rest. We say the Net Force
0
Net force sum of all forces acting on an object
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Unbalanced Forces
If the forces acting on an object become
unbalanced, the object moves in the direction
of the greatest net force.
Force applied in this direction
In this case, the Net Force 5 Newtons to the
left
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Mmmm,
Oh, sorry, wrong Newtons
Newtons!
The newton is a unit of force that is defined as
the amount of force required to accelerate a mass
of one kilogram at a rate of one meter per second
per second. Algebraically
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Diagramming Forces
If you were paying attention, you would have
noticed that forces were illustrated using
arrows. The size and direction of the arrow
represents the relative strength and direction of
the force.
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Calculating Net Forces
To calculate net forces, add all forces exerted
on the object. In this example, the net force up
the ramp is greater than gravity, and the piano
is moved forward and up the ramp into the truck.
A
B
For this example Net force (force A force B)
force C
C
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Calculating Net Forces
2
1
For this example Net force force 1 - force
2 The force due to gravity in this example is
negligible
What other forces can you identify in this
picture?
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Calculating Net Forces
2
1
The dogs exert a force on the toy as they bite
down on it
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Feeling the Force
Pow!
Which ram seems to have the advantage? Why?
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Newtons First Law of Motion
Every body perseveres in its state of being at
rest or of moving uniformly straight forward,
except insofar as it is compelled to change its
state by force impressed.
Not again! One more time English, please!
An object at rest will remain at rest unless
acted on by an unbalanced force. An object in
motion continues in motion with the same speed
and in the same direction unless acted upon by an
unbalanced force. AKA - The Law of Inertia
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Newtons First Law of Motion
Said another way, Newtons First Law of Motion
states that a moving object moves in a straight
line with constant speed unless a force acts on
it. ? An object will not start moving unless a
force acts on it ? An object will not stop moving
unless a force acts on it ? An object will not
change speed unless a force acts on it ? An
object will not change direction unless a force
acts on it
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Friction
A force that opposes motion between two surfaces
that are touching
Even surfaces that seem to be extremely smooth
have microscopic hills and valleys, and when two
surfaces are in contact, the hills and valleys of
one stick to the hills and valleys of the other,
causing friction to resist the force of motion
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Friction
Static friction The friction that exists between
two objects in contact
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Friction
Sliding friction When force is applied that is
strong enough to break the bonds of static
friction and movement starts, sliding friction
acts to slow that object down
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Friction
Rolling friction The resistance that occurs when
a round object such as a ball or tire rolls on a
flat surface
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Friction
Fluid friction Fluid friction occurs when a solid
object travels through a liquid or gas.
Wake turbulence and wingtip vortices from jet
airliner passing through a layer of clouds,
showing the fluid nature of air
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Friction
More
Force required to overcome friction
Static friction

Movement starts
Sliding friction
Friction always acts in the opposite direction of
movement, and always acts to slow object down.
Rolling friction
Fluid friction
Less
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Newtons Second Law of Motion
Force equals mass times acceleration
(F  ma) The net force on an object is equal to
the mass (m) of the object multiplied by its
acceleration (a) Units of Force Mass kilograms
(kg) Acceleration (m/s2) Therefore. F kg
? m/s2 Force is measured in
(Newtons, that is)
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What are the forces acting on this bicycle and
rider, coasting along at 25 km/h on this flat,
wet, Alaskan road?
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Is the net force balanced? Write out the formula
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Is the net force balanced? Write out the formula
Net force (A-B) D - (EC)
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Gravity
Gravity is a force that always attracts or pulls
objects toward each other without direct contact
or impact. Gravitational attraction depends on
the mass of the two objects and the distance they
are apart. Objects on Earth are pulled toward
the center of Earth. The force of gravity, like
all other forces, can cause changes in the speed
of objects. As an object falls, its speed will
continually increase as Earths gravity
continually pulls it downward. When air
resistance is ignored, all objects will speed up
at the same rate as they fall. Gravity can also
cause an object that is thrown into the air to
change its upward motion, slow down, and fall
back toward Earths surface. The pull of Earths
gravity keeps the Moon in orbit the moon is
constantly changing direction because of gravity.
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The acceleration due to gravity is 9.8 m/sec2
When air resistance is ignored, all objects will
speed up at the same rate as they fall. When the
ball is dropped off the cliff, the ball will
accelerate by 9.8 meters/second each second. At
the end of 1 second, the ball is traveling at 9.8
m/sec At the end of 2 seconds, the balls
velocity is 19.6 m/sec (9.8 m/sec X 2) Etc..
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Gravity
Factoring in air resistance (fluid friction) will
cause an object to reach a limit to its
acceleration. This concept is known as terminal
velocity. For an average skydiver, terminal
velocity is approximately 195 km/sec (55 m/sec).
Terminal velocity is reached when the net force
between gravity ( Fg ) and fluid friction, or
drag ( Fd ) reaches zero.
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Gravity
F ma
Acceleration due to gravity 9.8 m/s2 F mass X
(9.8 m/s2) This dude is nuts!
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Newtons Third Law of Motion
Lex III Actioni contrariam semper et æqualem
esse reactionem sive corporum duorum actiones in
se mutuo semper esse æquales et in partes
contrarias dirigi.
Whenever a particle (A) exerts a force on another
particle (B), (B) simultaneously exerts a force
on (A) with the same magnitude in the opposite
direction. This law is often simplified into
the sentence, "To every action there is an
equal and opposite reaction."
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Third Law in Action
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Third Law in Action?
Think about Sabines actions in this photo. Do
they represent Newtons Third Law of Motion? If
yes, explain. If no, which law(s) are
illustrated?
Sabine Lisinki, 2009 Family Circle Cup Champion
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Quick Review
A paratrooper has a mass of 100 kg. He jumps from
his C-17 at an altitude of 10,000 feet and
accelerates toward the ground at 9.8 m/sec2. What
is the force on the paratrooper? Explain how you
got your answer, and which of Newtons laws of
motion is involved.
A roller coaster reaches the top of the big hill
traveling at a speed of 15 m/sec. When it reaches
the bottom of the hill 3 seconds later it is
traveling at 27 m/sec. Calculate the average
acceleration. Explain how you got your answer.
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• Force and Newtons Laws note-taking sheet
• Section 1
• Force
• 1. net
• 2. balanced
• 3. unbalanced
• B. first law
• C. Friction
• 1. slows down
• 2. Static
• 3. Sliding
• 4. Rolling

• Section 2
• second law
• Gravity weight
• calculate
• centripetal
• terminal velocity
• unbalanced

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• Force and Newtons Laws note-taking sheet
• Section 2
• second law
• Gravity weight
• calculate
• centripetal
• terminal velocity
• unbalanced

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Self-check questions

1. Think about our happy cyclist from earlier in the
   discussion
2. Your weight will decrease as you get farther from
   Earth because the gravitational pull of Earth is
   decreasing
3. Greater the speed, greater the air resistance, up
   to the point of terminal velocity
4. Net force will push diagonally at some angle on
   the car and to the right
5. The motion of the box will not change until the
   forces become unbalanced

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Self-check questions
6) F ma F 1500 kg X 2.0 m/s2 3,000 N 7)
F ma m F/a 300 N / 1500 m/s2 0.2 kg
or (300 kg ? m/s2) / 1500 m/s2 0.2 kg
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Quick Review
A paratrooper has a mass of 100 kg. He jumps from
his C-17 at an altitude of 10,000 feet and
accelerates toward the ground at 9.8 m/sec2. What
is the force on the paratrooper? Explain how you
got your answer, and which of Newtons laws of
motion is involved.
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Quick Review
A roller coaster reaches the top of the big hill
traveling at a speed of 15 m/sec. When it reaches
the bottom of the hill 3 seconds later it is
traveling at 27 m/sec. Calculate the average
acceleration. Explain how you got your answer.
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Quick Review
A paratrooper has a mass of 100 kg. He jumps from
his C-17 at an altitude of 10,000 feet and
accelerates toward the ground at 9.8 m/sec2. What
is the force on the paratrooper? Explain how you
got your answer, and which of Newtons laws of
motion is involved.
F ma F 100 kg ? 9.8 m/sec2 F 980 kg ?
m/sec2 (1 Newton 1 kg ? m/sec2) Answer 980
Newtons (or 980 N) This is Newtons Second Law of
Motion
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Quick Review
A roller coaster reaches the top of the big hill
traveling at a speed of 15 m/sec. When it reaches
the bottom of the hill 3 seconds later
it is traveling at 27 m/sec. Calculate the
average acceleration. Explain how you got your
answer.
Average acceleration (Final velocity Initial
velocity) / Time (27 m/sec
15 m/sec) / 3 seconds (12
m/sec) / 3 seconds 4 m/sec2
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Third Law the Boat moves back when the boy
moves forward.
First Law the car stops but passengers Inertia
keeps them moving forward.
Second Law the golf ball moves with more force
with larger acceleration.
Second Law same force on large ball produces
less acceleration than small ball.
First Law bike continues to coast at constant
speed w/o unbalanced force.