Forces and Newtons Laws of Motion

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Forces and Newtons Laws of Motion
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Newtons First Law of Motion - Inertia

• Aristotle on Motion (4th century BC)
• Natural Motion
• It was thought to be either straight up or
  straight downa rock would fall, smoke would
  rise.
• Circular motion was natural for planets, stars,
  etc.thus planets and stars moved in perfect
  circular motion around the earth
• These motions were NOT thought to be caused by
  FORCES
• Violent Motion
• Imposed motion was the result of forces that
  pushed or pulled
• A cart moved because it was pulled by a horse, a
  ship was pushed by the force of the wind
• Objects in their natural resting places could NOT
  move by themselves

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Newtons First Law of Motion - Inertia

• Copernicus (1473-1543) on Motion
• Theory of the moving earth
• Interpreted astronomical observations by assuming
  that the earth and other planets move around the
  sun
• This idea was extremely controversialpeople
  preferred to think that the earth was the center
  of the universe.
• Copernicus worked on his ideas (De
  Revolutionibus, 1543) in secret to avoid
  persecution
• Can you think of similar controversial scientific
  ideas current to our time??

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Newtons First Law of Motion - Inertia

• Galileo on Motion
• Galileo was outspoken in his support of
  Copernicus
• And was put to trial and house arrest because of
  this
• Great contribution to physics
• Demolishing the notion that a force is necessary
  to keep an object moving.
• A force is any push or pull
• Friction is the force that acts between materials
  that touch as they move past each other
• Examples..???
• Galileo argued that only when friction is present
  (as it usually is) is a force needed to keep an
  object moving
• Galileo was concerned with HOW things move rather
  than WHY

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Forces and Newtons Laws of Motion

• Force and Mass
• Force is a push OR pull
• Force is a Vector quantity F
• Mass is a property of matter that determines how
  difficult it is to accelerate or decelerate an
  object
• MASS-a measure of inertia
• Mass is not volume
• Bag of cotton.bag of nails
• Same volumebut if you get hit by oneone hurts
  and the other doesnt
• Mass is not weight
• Mass is a measure of the amount of material
  (number of and kind of atoms) in an object
• Mass is measured in kilograms (kg)
• Mass is a measure of the inertia

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Forces and Newtons Laws of Motion

• Issac Newton (1642-1727)..WHY things move
• First Law of Motion or Law of Inertia
• Every object continues in a state of rest, or of
  motion in a straight line at constant speed,
  unless it is compelled to change that state by
  forces exerted upon it
• Objects in motion tend to stay in motion (planet
  earth, hockey puck on ice, etc..objects at rest
  tend to stay at rest..(this room, tables, you,
  etc.)

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Forces and Newtons Laws of Motion

• Newtons second law of motion
• When a net force acts on an object of mass m, the
  acceleration a of the object can be obtained from

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Forces and Newtons Laws of Motion

• NET FORCE
• In the absence of a force, objects at rest stay
  at rest and objects in motion continue in motion
• Specifically in the absence of a net force
• Forces can be added and subtracted mathematically
  depending on the direction in which they are
  applied
• Net Force is the vector sum of all forces acting
  on an object
• Equilibrium
• When the net force equals zero, a0
• When an object is at rest with the net force on
  it being zero, the object is in static
  equilibrium

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Forces and Newtons Laws of Motion

• Weight
• Weightmass X acceleration due to gravity
  (weightmg)
• Wmg
• (g -9.8m/s/s)
• Weightmass X acceleration due to gravity
  (weightmg)
• W mg
• Weight is a measure of the gravitational force
  acting on an object (depends on the objects
  location)
• In space an astronaut has the same mass as she/he
  does on earth, but much different weight!
• Weight and force are measured in newtons
  (slightly less than a quarter of a pound)
• What is the weight (W) of the objects at the
  right?

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Forces and Newtons Laws of Motion

• Free Body Diagrams
• Draw a box to symbolize each object in the
  system. Each object of interest should have its
  own free-body diagram. Pretend the object is by
  itself in empty space. (It is a "free body".)
• Include all the forces acting on the body. (Make
  sure you include only those forces acting on the
  body of interest. Don't include the forces the
  body causes on other objects. The net force is
  the sum of other forces not a force itself.
  Remember that accelerations are not forces!)
• Draw an arrow from the box representing each
  force. The direction of the arrow should indicate
  the direction of the force. Label each arrow
  clearly.

Picture of Situation
Free Body Diagram
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Forces and Newtons Laws of Motion
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Forces and Newtons Laws of Motion

• Normal Force
• The force supporting the block (since the block
  is not accelerating downward) is the normal force
• Its the force pushing back on the block by the
  table normal to the surface
• This is why it is called the Normal Force

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Forces and Newtons Laws of Motion
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Forces and Newtons Laws of Motion

• Static and Kinetic Frictional Forces
• The normal force is one component of the contact
  force between two objects, acting perpendicular
  to their interface. The frictional force is the
  other component it is in a direction parallel to
  the plane of the interface between objects.
  Friction always acts to oppose any relative
  motion between surfaces and is parallel to the
  surface
• Static Frictional Force
• The magnitude of the static frictional force
  depends on the magnitude of the applied force and
  can assume any value up to a maximum of

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Forces and Newtons Laws of Motion

• Static and Kinetic Frictional Forces
• Kinetic Frictional Force
• The magnitude of the static frictional force
  depends on the magnitude of the applied force and
  can assume any value up to a maximum of

Now were moving We have a kinetic frictional
force
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Forces and Newtons Laws of Motion

• Newtons Third law of motion
• Action-Reaction Law
• Whenever one objects exerts a force on a second
  object the second object exerts an equal and
  opposite directed force on the first object.
• CJ6 Example 4 page 87, note the difference in
  acceleration of the astronaut and the
  spacecraft..why?

Action? Reaction?
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Forces and Newtons Laws of Motion

• Tension Force
• A special description for a force acting through
  a rope, cable or beam
• Because of tension, a rope or cable transmits a
  force from one end to the other
• T-T . T is the reaction force to T (equal
  and opposite direction)

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Forces and Newtons Laws of Motion

• Equilibrium Revisited
• Zero acceleration therefore
• Static equilibrium is when a0 and v0
• This is a special branch of engineering called
  Statics

Static Equilibrium can be used to analyze the
forces on a bridge truss design
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Forces and Newtons Laws of Motion

• Non-Equilibrium Revisited
• there is acceleration therefore

Atwoods Machine is a good example of equilibrium
or non-equilibrium. Why?
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Forces and Newtons Laws of Motion

• Newtons Law of Universal Gravitation
• The force between any 2 masses is
• represented by the equation
• Its why you are being pulled towards the ground
• Because the earth is so massive
• Is there also a force between you and
• the person sitting next to you?
• Is the earth also accelerating towards you?
• Why is g9.8 m/s/s for any size mass????

consequences
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Forces and Newtons Laws of Motion

• The Real World of Newtons Laws
• Newton's first law states, an object stays in
  motion unless is is acted upon by an unbalanced
  force. Another reason why we do not notice it is
  because there are other forces which act against
  it to lessen the effects. One way in which we
  could see the effects of Newton's first law would
  be if we were moving at a high rate of speed then
  hits a solid object and comes to a stop abruptly.
  Even though the car comes to a stop, the 
  passenger inside the vehicle is still moving at
  the same speed that car was originally traveling.
  To help counteract these immediate effects of
  Newton's first law, things like the air bag and
  crumple zone were invented. By slowing down the
  time it takes for the passengers in the car to
  come to a stop, it will lessen the injuries that
  would normally be fatal or serious to people.
• Newton's second law can describe the exact
  effects of what happens  when things like the air
  bag and crumple zone are put to use in an
  automobile. Newton's second law can be easily
  expressed by one equation alone. Acceleration -
  Force / Mass. This law governs acceleration and
  is simple to understand. You can lower the
  acceleration which is produced by your body in a
  collision by using safety features like crumple
  zones and air bags. "The acceleration a of a mass
  m by an unbalanced force F is directly
  proportional to the force and inversely
  proportional to the mass, or a- F / m."  This is
  the second law of motion.
• "Every action has an equal and opposite
  re-action." This is Newton's third law and it
  also has plays a role in exactly how both airbags
  and crumple zones work. Newton's third law would
  probably be the most important of all of them.
  When you have an impact with your car, not only
  does your car hit another object, but the object
  pushes back on your car. Since the crumple zone
  on your car is made soft to absorb impacts, it
  will take the brunt of the collision so that the
  time it takes the car to come to a complete stop
  is spread out over a longer period of time. Not
  only may this save other people's lives it may
  also save money in low speed collisions.

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Forces and Newtons Laws of Motion

• The Real World of Newtons Laws
• To study an auto accident, you would have to
  study the changes in momentum that a car faces
  during the collision. According to Newton's laws,
  "A body in motion will stay in motion until it is
  acted up by an outside force."
• This is the same idea when it comes to an
  accident. An accident is the same as if you were
  to stop immediately and go from 50 miles per hour
  to 0 in less than a second. The amount of
  momentum your body has is still 50 miles per hour
  where as the car has now stopped. This is where
  airbags help in the event of an accident. Before
  your head gets the chance to smash into the
  steering wheel, the air bag deploys and cushions
  the impact that is about to happen. The fact that
  your body when in motion, wants to resist changes
  in their state of motion, like when coming in
  contact with an air bag, is called it's inertia.
• Air bags do not just cushion your body in the
  even of an impact, but they also spread the
  impact over a larger area. By doing this, the
  force is not all concentrated in one small area
  of your body. This in turn will cause the
  seriousness your injuries to be reduced because
  the force you feel is spread out. As seen in the
  illustration below.

Airbag deploys in about 15msec