Forces and Newton

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• Forces and Newtons Laws of Motion

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Isaac Newton (1642-1727)
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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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Newtons First Law

• 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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Net Force

• 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.

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Balanced Forces (zero net)
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Unbalanced Forces (non-zero net)
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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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Weight
Location
Mass
Earth
18.4 kg
180 N
Moon
18.4 kg
30 N
Space
18.4 kg
0 N
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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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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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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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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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When the acceleration is g we have Free Fall
m
2m
F
2F
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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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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 (se D below)
  terminal velocity has been reached.

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• Example
• Light and heavy parachutists

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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're very rough and jagged.

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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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Pressure

• The amount of force applied over a
  given area.
• Pressure Force ? Area
• P FA ? A
• Measured in pascals (Pa)

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Your Force vs Your Pressure

• How could you determine the amount of pressure
  you exert on the ground while standing?
• How would this change if you stood on on only one
  foot?

How do the pressures compare? All the acrobats
weigh the same.
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Atmospheric Pressure

• Air pressure is the force exerted on you by the
  weight of tiny particles of air
• Although air molecules are invisible, they still
  have mass and take up space.

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How much pressure are you under?

• Earth's atmosphere is pressing against each
  square inch of you with a force of 1 kilogram per
  square centimeter (14.7 pounds per square inch).
• The force on 1,000 square centimeters (a little
  larger than a square foot) is about a ton!

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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.

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Newtons Third Law of Motion
For every action, there is always an equal
(magnitude) and opposite (direction) reaction.
By action or reaction, we mean a 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 earth pulls on kid
Reaction kid pulls on earth

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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 their
  pulls compare?
• If each dog pulls with 50 N of force, what is the
  tension force in the middle of the rope (between
  the dogs)?