Forces and Newton

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• Forces and Newtons Laws of Motion

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• Force
• A push or a pull.
• Force is not a thing in itself, but rather an
  interaction between two objects.

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Examples

• A Flag being blown
• by the wind
• A jet engine propelling an airplane

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Fact or Fiction?

• Motion at a constant velocity requires a force.
• If you apply a constant force to an object, it
  will move with a constant velocity.
• If no forces act on it, a moving object will
  eventually stop.
• Inertia is a force.

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Forces

• Push or a pull
• Vector quantity
• Contact forces- arise from physical contact with
  each other
• Field (Noncontact) forces- force exerted even
  when not in direct contact (ex. Gravity,
  electrical force, magnetic forces)

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Weight VS Mass
http//www.pioneernet.net/curtis/wile_e/inline/wil
e_gravity.gif
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Mass

• the amount of matter in an object
• a constant property
• a measure of the inertia
• measured in kilograms (kg)

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Weight

• the force upon an object due to gravity
• weight mass ? accel. due to gravity
• Fw mg
• measured in Newtons (N)

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• The weight of a 10 kg brick is...
• A) 98 N
• B) 10 kg
• C) 9.8 kg
• D) 10 N
• E) 98 kg

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Mass and Weight

• On the Moon, the force of gravity is only 1/6 as
  strong as on the Earth.
• In space you are practically weightless but your
  mass remains unchanged.
• Your mass does not depend on where your are.
• e.g. Earth, Moon, or space

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Tension

• Tension (T)- force that acts through a solid
  object such as a rope or chain directed along
  the rope and pulls equally on the objects on
  either end.
• .

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Normal Force

• Normal Force (Fn)- contact force that always acts
  perpendicular to the surfaces that are touching

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Free-Body Diagram

• Free-body diagrams are drawings used to show the
  relative magnitude and direction of all forces
  acting upon an object in a given situation.

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Basic Force Diagrams
A Car on a Level Surface All forces on the car
are vertical, so no horizontal force can be
generated.
A Car on a Banked Turn The normal force on the
car due to the road is no longer vertical, so a
component of the normal force acts in the
horizontal direction.
Normal force (N) - contact pressure that pushes
surfaces together. It is easier to drag a light
chair across a room than a heavy table. This is
because the weight of the table exerts a higher
normal force.
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A book is at rest on a table
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A girl is suspended motionless from a bar which
hangs from the ceiling by two ropes.
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An egg is free-falling from a nest in a tree.
Neglect air resistance.
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A flying squirrel is gliding (no wing flaps) from
a tree to the ground at constant velocity.
Consider air resistance.
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A rightward force is applied to a book in order
to move it across a desk with a rightward
acceleration. Consider frictional forces. Neglect
air resistance.
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Real Life Application
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More real life
http//www.ux1.eiu.edu/cfadd/1150/04Nwtn/appl.htm
l
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Another Force Diagram
The chandelier is separated from the chain for
interpretation purposes only!
http//www.ux1.eiu.edu/cfadd/1150/04Nwtn/appl.htm
l
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Fish in an Elevator
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Upward Acceleration

• Fnet FT FW or
• Fnet FT mg
• Fnet FT - mg ma
• FT ma mg
• FT m (g a)

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Downward Acceleration

• Fnet FT - Fw
• or
• Fnet FT mg
• Fnet FT - mg m ( - a )
• FT mg - ma
• FT m (g - a)

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Roller Coaster
http//science.howstuffworks.com/roller-coaster-sa
fety-harness-broke.htm
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Bring in the angles-a bit more complex ?
http//www.ux1.eiu.edu/cfadd/1150/04Nwtn/appl.htm
l
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Forces are Vectors too!

• Consider the tow truck at the right. If the
  tensional force in the cable is 1000 N and if the
  cable makes a 60-degree angle with the
  horizontal, then what is the vertical component
  of force which lifts the car off the ground?
• 866 N.

• and coming soon....

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Box on an Incline
http//img.sparknotes.com/figures/1/174f9bfb530794
0e1bb1c2ddac88fd8d/problem1_5.gif
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Net Force
Balanced No Net Force Fnet 0 N

• Determined by combining ALL forces acting on an
  object.
• Zero net force zero acceleration
• If there is a net force, there will be an
  acceleration.

Unbalanced Net Force Fnet ? 0 N
http//4.bp.blogspot.com/_fDHmeCjB-bk/SS4hRNbLwaI/
AAAAAAAAAHw/gx0p9HIDS1o/s1600-h/12.gif
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Balanced Forces (zero net)
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Unbalanced Forces (non-zero net)
Imagine pushing your text book so that it would
slide across the desk
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Equilibrium

• Balance
• In regards to motion an object has constant
  velocity if it is in equilibrium.
• NO acceleration.
• Types
• Static Equilibrium
• Dynamic Equilibrium

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Equilibrium- net force is equal to zero
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Static Equilibrium

• Velocity is zero
• Examples

Scales pushing up
Weighing yourself on a set of scales
Weight down
Car parked on an incline
Normal
Friction

Weight down
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• Dynamic Equilibrium
• Velocity is nonzero and constant
• Examples
• Driving at constant velocity

Normal up
Friction
Force from road
Air resistance
Weight down
Terminal velocity when parachuting

Weight down
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Friction

• The force that opposes the motion between two
  surfaces that are in contact.
• Friction is the "evil monster" of all motion.
  Regardless of which direction something moves in,
  friction pulls it the other way.
• Move something left, friction pulls right. Move
  something up, friction pulls down.
• It appears as if nature has given us friction to
  stop us from moving anything.
• Friction is actually a force that appears when
  there is relative motion between two objects.
• Although two objects might look smooth,
  microscopically, they are very rough and jagged.

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Friction

• A force that opposes motion. Friction acts in a
  direction opposite to the object's direction in
  motion. Without friction, the object would
  continue to move at a constant speed forever
• Static Friction when object is at rest
• Kinetic Friction when object is moving
• Sliding Friction when two surfaces
  slide one over the other
• Rolling Friction when two surfaces
  slide one over the other

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Science Friction

• Friction (Ff)- contact force between two surfaces
  that always opposes motion
• 2 types
• Static friction (fs) keeps an object from
  moving (must be overcome to move an object at
  rest
• Kinetic friction (fk) acts when an
  object is moving
• Kinetic friction is weaker than static friction
• Fklt Fs

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Friction is not always a bad thing!

• Walking! Your foot pushes backwards and friction
  opposes it by pushing it forward

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Static (starting) Friction

• The force that opposes the start of the motion.
• Static means stationary ( not moving).

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Kinetic (sliding) Friction

• The force between surfaces in relative motion
• For the same object, why is the force of kinetic
  friction less than the force of starting friction?

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Coefficient of Friction

• A constant that depends on the two surfaces in
  contact
• Ff ?FN
• ?(mu).coefficient of friction

Coefficient of Friction - Friction on an Incline
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Newtons First Law (1642-1727)

• The Law of Inertia
• A body remains at rest or moves in a straight
  line at a constant speed unless acted upon by a
  net force.
• Objects do not accelerate unless a net force is
  applied.

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Objects at Rest Remain at Rest
http//talesfromechocanyon.blogspot.com/2007_03_02
_archive.html
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Objects in Motion Remain Motion, unless
http//talesfromechocanyon.blogspot.com/2007_03_02
_archive.html
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Acted Upon by a NET Force
http//talesfromechocanyon.blogspot.com/2007_03_02
_archive.html
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Newtons 1st Law of Motion
http//quest.nasa.gov/space/teachers/rockets/image
s/hand_and_ball.gif
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Newtons Second Law

• The accel. of an object is directly proportional
  to the net force acting on the object, and
  inversely proportional to the mass of the object.

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Newtons Second law of Motion mathematically
Net Force (mass)(accel) Fnet ma
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NEWTON'S 2nd LAW OF MOTION
a
F
F
a
F
a
F
a
F
a
F
a

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When the acceleration is g we have Free Fall
m
2m
F
2F
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Terminal Velocity
Net Force
Acceleration g
Velocity 0 but motion is about to begin
mg
F
Acceleration lt g
v increasing downward
mg
F
Acceleration ltlt g
v still increasing downward just not as rapidly
as before
mg
F
Acceleration 0
Terminal velocity

mg
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Falling with Air Resistance

• Air resistance increases with speed and increased
  cross-sectional area.

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Terminal Velocity

• When falling the force of air resistance becomes
  large enough to balance the force of gravity.
• At this instant in time, there is no net force
  the object stops accelerating (see D below)
  terminal velocity has been reached.

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• Example
• Light and heavy parachutists

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Practice
http//www.freewebs.com/mathphyedu/14.bmp

• A 0.02 kg bullet shot horizontally with a speed
  of 340 m/s enters a 1.2 kg apple that is 8.5 cm
  in diameter. If the bullet exits the apple with a
  perceived speed of 320 m/s, how much force was
  applied to the bullet by the apple? What was the
  acceleration of the apple?

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Fact or Fiction??

• When you kick a can, the can kicks you back with
  the same force?
• When we jump the floor pushes us upward.
• It is possible to get yourself off the floor or
  chair without an outside force being exerted on
  your body

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Newtons Third Law

• Action-Reaction
• When one object exerts a force on another object,
  the second object exerts a force of equal
  strength in the opposite direction on the first
  object.

Example of Newtons 3rd Law http//bp3.blogger.co
m/_vQA3QRUnk3M/RrgyfrRniPI/AAAAAAAAAMs/DYlhM7pDeI8
/s1600-h/newton
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Newtons Third Law of Motion
For every action, there is always an equal
(magnitude) and opposite (direction) reaction.
action or reaction refers to
force.
Action/Reaction forces do NOT act on the SAME
object!
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Reaction road pushes on tire
Action tire pushes on road

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Reaction gases push on rocket
Action rocket pushes on gases

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Action- Reaction Forces

• Do Action-Reaction forces cancel each other?
• No, they are acting on different objects. Forces
  can only be added together when they are acting
  on the same object.

http//quest.nasa.gov/space/teachers/rockets/princ
iples.html
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Action earth pulls on kid
Reaction kid pulls on earth

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Newtons 3rd Law of Motion
Action Force A guy is pushing a ball leftwards
Reaction Force the ball is pushing the guy
rightwards
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• Horse pulls on cartcart pulls on horse. How
  does the cart ever move forward?
• Use Newtons 2nd and 3rd laws of motion to
  explain.

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Identify at least six pairs of action-reaction
force pairs in the following diagram
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Tug-a-war

• If Fido and Rover play tug-a war, how do the
  pulls of the dogs compare?
• If each dog pulls with 50 N of force, what is the
  tension force in the middle of the rope (between
  the dogs)?

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• While driving, Mrs. Ingle observed a bug striking
  the windshield of her car. The bug hits the
  windshield and the windshield hits the bug. Which
  of the two forces is greater the force on the
  bug or the force on the windshield? EXPLAIN!

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Newtons Cradle
http//www.celsum.com/newton.htm
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Putting Newton's Laws of Motion Together

• An unbalanced force must be exerted for a rocket
  to lift off from a launch pad or for a craft in
  space to change speed or direction (First Law).
• The amount of thrust (force) produced by a rocket
  engine will be determined by the rate at which
  the mass of the rocket fuel burns and the speed
  of the gas escaping the rocket (Second Law).
• The reaction, or motion, of the rocket is equal
  to and in the opposite direction of the action,
  or thrust, from the engine (Third Law).

http//www.spacetoday.org/images/Rockets/ArianeRoc
kets/Ariane5LaunchArianespace.jpg
http//quest.nasa.gov/space/teachers/rockets/princ
iples.html