Newtons First and Second Laws of Motion

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Newtons First and Second Laws of
Motion
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Development of Ideas of Motion and Inertia

• Aristotles Ideas
• Motion was in two forms
• Natural-- straight up or down on earth, circular
  in heavens
• Violent--result of forces making things move
• The natural state of things was to be at rest
• Motion only happens if a continual force makes it
  happen
• Basis of the Geocentric theory

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Copernicus

• Stated that earth was in motion
• Based on observations from astronomy

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Galileo

• Showed that friction causes slowing and
• stop of motion
• Friction is caused by surfaces in contact
• Smoother the surface, less friction
• Without friction, moving object would not stop

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Galileo

• Force is needed only to overcome friction
• Force is a push or pull which changes motion
• Rolling balls between inclines roll up second
• to nearly same level as on first
• Ball on smooth surface rolls without
• deceleration

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Galileo

• Developed idea of inertia
• Every material object has a resistance to
• change in its state of motion
• At rest, stays at rest
• In motion, keeps its velocity

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Galileo

• Earth demonstrates inertia in revolution,
• no force pushes it around the sun and none is
  needed since there is no friction

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Newton

• Restated law of Inertia
• Every object continues in its 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.

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Newton

• Examples
• Magician pulls tablecloth from under dishes
• Air hockey games at arcades
• Voyager and Pioneer spacecraft

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Mass-A Measure of Inertia

• What is mass?
• Not volume
• Volume is space taken up by object--cm3 or L
• Mass is the amount of matter contained by
  object-- kg or g
• Compare pillow with car battery

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Mass-A Measure of Inertia

• Not weight
• Weight depends on gravity--more gravity more
  weight
• Mass is in object no matter the gravity-bowling
  ball in space
• Weight and mass are proportional--
• more mass makes more weight

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Inertia and the Moving Earth

• Bird catching worm-- should miss by 30 km
• Inertia says bird, tree, worm all moving at
• 30 km/hr--no relative motion
• Ancients had no fast motion, thus did not see
• inertia

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Force and Acceleration

• Force Causes Acceleration
• Single force causes a start or change in
  motion-- acceleration
• Multiple forces may act together which add or
• subtract to make net force
• Amount of acceleration is proportional to net
  force

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Mass Resists Acceleration

• Large mass needs more force to move
• e.g. loaded vs. empty grocery cart
• Acceleration is inversely proportional to mass

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Newtons Second Law

• Acceleration depends both on mass and force
  jointly
• The acceleration produced by a net force on an
  object is directly proportional to the magnitude
  of the net force, is in the same direction as the
  net force, and is inversely proportional to the
  mass of the body

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Newtons Second Law

• With consistent units F-- Newton m-- kg
  a-- m/s2
• the equation becomes exact
• acceleration net force or a F
• mass m

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Examples
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Problems
A WWII aircraft carrier with mass of 36,000,000
kg is hit by a bomb with explosive power of 6.912
x 108 newtons. How much acceleration is imparted
to the ship?
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Force in Statics

• 1. Forces can be balanced and thus produce no
  motion
• 2. Book on a table has two forces acting upon it
• a. Support force (of the table) upward
• b. Gravity downward
• 3. Hanging from ropes --gravity downward, support
  upward divided by number of ropes

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Opposing Forces and Application of Force

• Friction
• Friction--a force acting opposite to motion
• Cause--irregularities of surfaces in contact
• Amount depends on the materials involved
• Occurs in fluids as well as in solids
• Viscosity in liquids
• Air resistance in air (similar in other gases)
• Friction causes the need for applied force to
  keep constant velocity

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Problems
Zeke is pushing his sister Zelda in a wagon down
the street. He imparts 100 N on the wagon, but
the wind is blowing against him and the wagon an
amount of 20 N and the wagon wheels are rusty and
impart 10 N of friction. What acceleration is
imparted to Zelda and the wagon if they have a
mass of 70 kg?
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Pressure

• Force per unit area
• Book placed flat or on end has the same
  weight/force
• Book placed on end exerts more pressure
• Pressure force P F
• area A
• Units -- Pascal Newtons
• meters2

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Pressure

• Larger area less pressure
• Stand on one foot-- all weight on that area
• Bed of nails-- force spread over large area-- no
  one nail has enough force to
• penetrate skin

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Problems
Zelda goes out in the snow with high heels on,
which have a surface area of 0.005 m2. If she
weighs 700N, what pressure is she exerting on the
snow?
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Free Fall and Air Resistance

• Acceleration is equal if both masses
• are relatively large
• 1 kg Vs 10 kg masses will fall equally
• over a short distance
• Acceleration depends on weight (force) and mass
• Large weight small weight
• Large mass small mass
• since weight and mass are proportional

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Free Fall and Air Resistance

• Air resistance diminishes net forces
• Amount of decrease depends on surface area
• When air resistance equals weight, object falls
  at constant speed
• If direction is downward the speed is called
  terminal velocity

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Free Fall and Air Resistance

• Terminal velocity for a feather is very small,
  for a coin much larger due to less air resistance
• Skydivers terminal velocity varies from 150-
  200 km/hr
• Heavier person falls faster
• Person spread out falls slower
• Parachute slows speed to 15-25 km/hr

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Free Fall and Air Resistance

• From larger distances, objects of same size
• but different weights are affected differently
• by air resistance
• Baseball falls faster due to its weight than a
  tennis ball
• Galileo found split second differences but not
  the differences Aristotle expected

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Homework

• Chpt 3 RQ-4,8,9,10,12
• TE- 1,3,5,6,10
• Chpt 4 RQ-3,4,5,6,9,11,13,15,16,19,22
• TE- 3,4,6,7,9,12,13,15