AND chapter 12

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AND chapter 12
FORCES
MOTION
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What is a FORCE?

• A FORCE is a push or pull that acts on an object.
• A force can cause a resting object to move
• OR
• Accelerate a moving object by
• changing the objects speed
• direction

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How do we MEASURE force?

• Forces can be measured on a spring scale
• WEIGHT is a FORCE
• UNITS OF FORCE
• Newton (N)
• A Newton is a force that causes a 1 kg mass to
  accelerate at a rate of 1 meter per second each
  second
• Which is written as 1 N 1 kg ? m/s2

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Representing Forces

• Forces can be represented with an arrow
• The length of the arrow shows strength or
  magnitude
• Direction of the arrow shows the direction of the
  force
• Figure 2 on page 357 shows an example of weight

Normal Force
Applied Force
Friction Force
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Combining Forces

• Net Force the overall force acting on an object
  after all forces are combined

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Combining Forces

• Same Direction

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• Opposite Directions

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Net Force
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Balanced Forces

• Balanced Forces the net force is zero, there is
  no change in the objects motion
• Question what are some examples where the net
  force would equal zero?

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ANSWER

• Net force would be zero when
• You play tug-of-war and neither team moves
• You arm wrestle and neither person wins
• A car using cruise control constant speed!

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Unbalanced Forces

• Unbalanced Forces the net force acting on an
  object does not equal zero

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• Question What happens to the object when the net
  force acting on an object is NOT ZERO, or
  unbalanced? (like the example below)

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Answer

• When an unbalanced force acts on an object, the
  object accelerates

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• 3 examples of a net force causing an object to
  accelerate
• Pushing against the side of a book getting it
  to move
• A team winning a game of tug-of-war pulling the
  other team towards them
• A person in freefall

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Watch video clip The Effects of Forces on Speed

• On video notes page, draw an example of when the
  forces are balanced when they are unbalanced.
  Then describe the result of these forces.

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Try these problems

1. Two tugboats are moving a barge. Tugboat A exerts
   a force of 3000 newtons on the barge. Tugboat B
   exerts a force of 5000 newtons in the same
   direction. What is the combined force on the
   barge? Draw arrows showing the individual and
   combined forces of the tugboats in this problem
   to help you answer the question

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• Same direction so add
• 3000 N 5000 N 8,000 N
• ? ---? ------?

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• 2. Now suppose that Tugboat A exerts a force of
  2000 newtons on the barge and Tugboat B exerts a
  force of 4000 newtons in the opposite direction.
  What is the combined force on the barge? Draw
  arrows showing the individual and combined forces
  of the tugboats in this problem.

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• Opposite directions, so subtract
• 4000 N 2000 N 2000 N
• ?-- ? ?

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• 3. Could there ever be a case when Tugboat A and
  Tugboat B are both exerting a force on the barge
  but the barge doesn't move? Draw arrows showing
  the individual and combined forces in such a
  situation.

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• ? ? 0
• Equal in size opposite in direction

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FRICTION
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FRICTION

• Friction a force that opposes the motion of
  objects that touch as they move past each other
• Without friction, it would be a very different
  world!!!
• Food would not stay on your fork!
• Cars would slide all over the road!
• Walking would be almost impossible!
• Friction acts at the surface where objects are
  in contact

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The 4 Types of Friction

• There are 4 main types of friction
• Static Friction
• Sliding Friction
• Rolling Friction
• Fluid Friction

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STATIC FRICTION

• friction force that acts on objects that are not
  moving
• Static friction always acts in the direction
  opposite to the applied force
• Examples
• every time you take a step and push off
• glass of water sitting stationary on the table

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SLIDING FRICTION

• a force that opposes the direction of motion of
  an object as it slides over a surface
• Sliding friction is a weaker force than static
  friction
• This is why less force is needed to keep an
  object moving than it is to start it moving

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ROLLING FRICTION

• The friction force that acts on rolling objects
• Rolling friction is about 100-1000 times less
  than static or sliding friction
• This is why we use wheeled dollies to move heavy
  objects!
• In machines, ball bearings, are often used to
  reduce friction between two surfaces

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FLUID FRICTION

• Force that opposes the motion of an object
  through a fluid
• Water and a mixture of gases such as air are
  known as fluids
• Example a submarine moving through water
• The motion of the submarine is slowed by fluid
  friction
• Fluid friction increases as the speed of the
  object moving through the fluid increase
• So the faster the sub goes, the greater the
  friction!!!

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AIR RESISTANCE

• Air resistance is a type of fluid friction
• Remembergases are considered fluids
• Fluid Friction acting on an object moving through
  the air is called AIR RESISTANCE

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GRAVITY
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GRAVITY

• Gravity a force that acts between any two masses
• Gravity is an attractive force so it pulls
  objects together
• Gravity does not require objects to be in contact
  for it to act on them
• Gravity can act over large distances!!!

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More GRAVITY!

• Earth's gravity acts downward toward the center
  of the Earth.
• There is usually an upward force that acts
  against gravity to balance out the forces and
  allow objects to remain still.

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FALLING OBJECTS

• QUESTION What forces are acting on an object as
  it falls?

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• ANSWER Only two forces acting on a falling
  object are gravity and air resistance

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Forces Acting on Falling Objects

• Gravity causes object to accelerate downward
• Air resistance acts in the opposite direction of
  the motion
• Which means it reduces acceleration

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• Recallwhat happens to the amount of fluid
  friction as an object speeds up?

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Watch video clip The Physics of Skydiving

• On the video notes page, describe what is meant
  by terminal velocity describe the forces on the
  skydiver at this point.

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Falling Objects

• If an object in freefall falls for long enough,
  the upward force of air resistance will become
  equal to the downward force of gravity.
• At this point, the two forces are BALANCED
• Acceleration is zero when forces are balanced
• The object will continue to fall at a constant
  velocity  

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Terminal Velocity

• Terminal Velocity the constant velocity of a
  falling object when the force of air resistance
  equals the force of gravity

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Projectile Motion

• Projectile Motion The motion of a falling object
  (projectile) after it is given an initial forward
  velocity

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Projectile Motion

• Question What are the ONLY 2 FORCES that act on
  a projectile???

All 3 balls are experiencing projectile motion!
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Projectile Motion

• Answer Air resistance and gravity!!!
• Refer to figure 9 on page 362 in textbook
• The combination of an initial forward velocity
  and the downward vertical force of gravity causes
  the ball to follow a curved path

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• If I shoot a bullet horizontally and at the same
  time drop a bullet from the same height as the
  gunwhich will hit the ground first?

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FALLING OBJECTS

• The two bullets WILL hit the ground at the same
  time!

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The two balls fall with the same acceleration and
strike the ground at the same time!!!
Remember the Investigating Freefall Lab with
the marbles? How did that lab compare with the
previous statement???
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12.2 Newtons First Second Laws of Motion
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Its not ALL about Newton

• Aristotle incorrectly proposed that force is
  required to keep an object moving at a constant
  speed
• Galileo studied how gravity produces constant
  acceleration. He concluded that objects not
  subjected friction or any other force would
  continue to move indefinitely
• Newton built off the work of Galileo and later
  published his work in a book entitled Principia 
• Newton summarized his study of force and motion
  in several laws of motion

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1st Law of Motion Law of Inertia
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1st Law of Motion Law of Inertia

• 1st Law The state of motion of an object does
  not change as long as the net force acting on the
  object is zero
• In other words Unless an unbalanced force acts,
  an object at rest remains at rest, and an object
  in motion remains in motion with the same speed
  and direction

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1st Law Law of Inertia

• Example A soccer ball resting on the grass
  remains motionless until a force is applied to it.

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And

• Example A soccer ball in motion remains in
  motion unless a force acts on it.

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Why is it called the Law of Inertia

• INERTIA the tendency of an object to resist a
  change in its motion so
• an object at rest tends to remain at rest
  (resist moving), and an object in motion tends to
  remain in motion (resist stopping)

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• Remembersoccer ball sat motionless (forces were
  balanced) until an unbalanced force acted on it
• The ball has inertia
• Everything with mass has inertia
• The more mass, the more inertia

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InertiaFront-end collision

• Example Front-end collision
• Collision makes car stop suddenly
• Since you have inertia you continue moving
  forward
• Page 364/365 Figure 12

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Newtons Second Law of Motion
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2nd Law of MotionRelates the acceleration of an
object to the force acting on it the objects
mass.

•  2nd Law The acceleration of an object is equal
  to the net force acting on it divided by the
  objects mass. A F/M
• In other words, the amount of acceleration an
  object has depends on
• How hard it is pushed (force)
• How heavy it is (mass)

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• Net Force
• Acceleration --------------
  Mass

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AccelerationForce Mass Relationship What is
the acceleration?
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ANSWER

• Acceleration is calculated by dividing the FORCE
  by the MASS
• Acceleration 100 N / 50 kg
• A 2 m/s2

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2nd Law

• Example You apply force to a ball when you throw
  it
• The harder you throw, the more the ball
  accelerates
• If you double the force, the acceleration of the
  ball doubles as well
• If you double the mass of the ball the
  acceleration is cut in half
• More Examples

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Important NotesRegarding Newtons 2nd Law of
Motion

• The acceleration of an object is always in the
  same direction as the net force.
• In using Newtons second law, it is helpful to
  realize that the units N/kg and m/s2 are
  equivalent
• Newtons second law also applies when a net force
  acts in the direction opposite to the objects
  motion
• producing deceleration (See figure 13, page 368)
• This is the principle used by automobile seat
  belts
• See Math Skills page 367 (in textbook) for extra
  help

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Weight MassWhat really is the difference?
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• We often talk about weight and mass as if they
  were the same thing
• Weight is the force of gravity acting on an
  object.
• An objects weight is the product of the objects
  mass and acceleration due to gravity acting on
  it.
• Written mathematically as w m x g

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Weight lessens as
Weight will DECREASE if gravity DECREASES!!!
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Mass Versus Weight

• Mass is a measure of the inertia of an object,
• Measure with a balance
• Weight is a measure of the force of gravity
  acting on an object
• Measure with a spring scale
• Weight Mass x Acceleration due to gravity
• W mg
• Value of g 9.8 m/s2 (Earth)

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Important Note Regarding Units

• Be sure when you use the weight formula or
  Newtons second-law formula that you use the
  CORRECT units!!!
• Force (F or W) in N (newtons)
• Acceleration (a or g) in m/s2 (meters per second
  per second)
• Mass (m) in kg (kilograms)

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Practice Problem

• If an astronaut has a mass of 112 kilograms, what
  is his weight on Earth where the acceleration due
  to gravity is 9.8 m/s2?
• What would his weight be on the moon (g1.62
  m/s2)?

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Answer to Practice Problem

• ON EARTH
• Weight Mass ? Acceleration due to gravity
• W 112 kg x 9.8 m/s2
• W 1097 kg?m/s2 1097 N
• ON MOON
• W 112 kg x 1.62 m/s2
• W 181 kg?m/s2 181 N

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Mass Weight are Related

• Doubling the mass of an object also doubles the
  objects weight (if gravity remains the same)
• If Gravity changes then
• Example Think about the astronaut above
• On the moon, the acceleration due to gravity is
  only about 1/6 that on Earth
• Thus, the astronaut weighs only about 1/6 as much
  on the moon as on Earth
• In both locations, the mass of the astronaut is
  the same!

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12.3 Newtons 3rd Law of Motion
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Forces CANT Exist Alone

• Forces always exist in pairs. 
• According to Newtons 3rd law of motion, whenever
  one object exerts a force on a second object, the
  second object exerts an equal and opposite force
  on the first object. 
• These two forces are called action and reaction
  forces 

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Action Reaction Forces

• Example The book lying on the table is exerting
  a downward force on the table, while the table is
  exerting an upward reaction force on the book.

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Newtons Third Law Ex A rocket
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• More Examples
• Jumping off of a raft
• Action force you push on the raft
• Reaction force raft pushes back
• A swimmer in a pool
• Action force swimmers arm pushes on water
• Reaction force water pushes back on swimmer

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Action Reaction Forces

• Question Since these action/reaction forces are
  equal in size and opposite in direction then why
  dont they cancel each other out and produce a
  net force of zero?

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Action Reaction Forces

• Because the action/reaction forces do not act on
  the same object!
• Swimmer Example
• The action force acts on the water
• The reaction force acts on the swimmer

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Momentum

• Momentum the product of an objects mass and
  its velocity
• An object with a large momentum is hard to stop

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• Momentum mass X velocity
• Practice Problem What is the momentum of a rock
  with a mass of 0.5kg that is moving at a velocity
  of 5m/s?

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• Momentum 0.5kg X 5m/s 2.5kg x m/s
• The units do NOT cancel!

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• Momentum can be transferred from one object to
  another during a collision

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12.4 Universal Forces
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Four Universal Forces

• Electromagnetic forces- only forces that both
  attract and repel.
• Strong Nuclear forces- holds neutrons protons
  together
• Weak Nuclear forces- attractive force between
  particles in nucleus
• Gravitational forces- attractive force between
  any two objects 

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GRAVITY

• Gravity is the weakest universal force 
• On a daily basis, you dont notice the force of
  gravity that you exert on objectsthis is because
  your mass is sooooooo small. 
• It takes a huge mass like the Earths to exert a
  large gravitational force.  

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GRAVITY

• Gravitational force depends on two things the
  mass of an object and the distance between two
  objects.
• A greater mass will exert a greater gravitational
  force on an object.
• The greater the distance between two masses
  significantly decreases the gravitational force.

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Attractive Force of Gravity
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GRAVITYhow can it be so weak?

• Even though gravity is the weakest universal
  force, it is the most effective over large
  distances.
• Gravity holds you on the ground
• It holds the moon in orbit around the Earth
• It holds the planets in orbit around the Sun
• It holds the stars in orbit around their galaxies

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Centripetal Force

• The force of gravity from the Earth continuously
  pulls the moon in a nearly circular orbit around
  the Earth.
• Centripetal force center-directed force that
  continuously changes the direction of an object
  to make it move in a circle.
• As an object moves in a continuous (constant)
  circular motion, it is accelerating.
• How is it accelerating if it does not change
  speed?
• Change in direction!!! 

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Centripetal Force

• Its a lot like a string tied to an eraser
• The force from the center of the string allows
  the eraser to twirl in a circle over your head
• As you twirl the eraser, the string exerts a
  centripetal force on the eraser.

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Centripetal force Orbital Motion

• Objects need only a centripetal force and their
  own inertia to maintain an orbit.
• Orbital motion is a balance between the
  centripetal force and inertia.
• If Centripetal force is stronger it will crash,
  if inertia is stronger it will fly off into space

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• Examples of objects in orbit
• Satellites
• Our moon is a natural satellite of our planet

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Orbital Motion
http//upload.wikimedia.org/wikipedia/commons/4/4e
/Orbital_motion.gif
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How is this possible?!?!
Same PlaceSame Day??? HOW!!!???
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How is that possible?

• Ever been to the seashore? What do you notice
  about the level of the water throughout the day? 
  Why does it change? 
• The gravitational pull from the Moon produces two
  tides in the Earths oceans as it moves around
  the Earth.
• One bulge where the moon is closest to the Earth
• One bulge where the moon is farthest from the
  Earth

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Tides

• Since the Earth rotates once per day, it results
  in two high tides and two low tides per day on
  Earth!

http//home.hiwaay.net/krcool/Astro/moon/moontide
s/  
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Rotation versus Revolution

• The Earth moves in two major ways
• Rotation- the spinning of the Earth on its axis
• This rotation causes day and night
• Revolution- the movement of one body in space
  around another
• When does the Earth show revolution? Revolves
  around the Sun
• How long does it take for the Earth to revolve?
• 365 Days

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Ch 13 Forces in Fluids
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13.1 Fluid Pressure

• Why is an Aeron chair so much more comfortable
  than a bicycle seat?

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• Pressure the result of a force distributed over
  an area

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• Ex the tip of a pencil easily pokes through a
  sheet of paper while the eraser end does not
• Ex finger poke vs needle poke

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• Pressure Force/Area
• Units of Pressure N/m2 Pascal (Pa)

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• Practice problem A box weighing 2700 N is
  resting on the ground. If the box is covering
  1.5 square meters of area, what is the pressure?

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• Pressure 2700 N/1.5 m2 1800 Pa 1.8 kPa

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Pressure in Fluids

• Recall a fluid is a substance that takes the
  shape of its container, both liquids and gases
  are fluids

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• What happens to the pressure exerted on your body
  as you dive deeper in a pool?

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• Water pressure increases as depth increases
• The relationship between pressure and the depth
  of a fluid can be shown with this formula P d
  x h x g
• d density
• h depth
• g accel due to gravity

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• Fluid pressure is determined by the type of fluid
  and its depth
• Which would exert more fluid pressure at a depth
  of 20 m, Lake MI or the Gulf of Mexico?

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Air Pressure the Atmosphere

• We live at the bottom of a vast ocean of air.
• The weight of the Earths atmosphere exerts
  pressure. At sea level 101 kPa
• We refer to a certain altitude instead of a depth
• Air pressure decreases as the altitude increases

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• Why do your ears pop while traveling up a
  mountain?

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• The difference in pressure between the inside of
  your ear and the air outside causes air to pass
  through a small tube within your ear to equalize
  the pressure.
• This is the popping sensation you feel

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• The atmosphere is exerting more than 1000 N on
  the top of your head. Why arent we crushed by
  this?

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• The inside of your body exerts pressure against
  the air pressure outside which balances and
  results in a net force of zero.

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Bernoullis Principle

• Bernoullis Principle as the speed of a fluid
  increases, the pressure within the fluid
  decreases
• Demo take a piece of paper and blow across the
  top of the paper

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Wings Lift

• The ability of birds and airplanes to fly is
  explained by Bernoullis principle

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• The air traveling over the top of the curved wing
  travels faster so the pressure is less which
  create the lift

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