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					Chapter 3
Newton’s            2nd   law
 2nd – an object               A force on an object is
  accelerates in the             equal to the change in
  direction of the net force     momentum of the object
  acting upon it
 A= net force/Mass             Force = mass * accel.
                                F=m*a
 a= Fnet/m
Practice
            Your push your
             friends trike with a
             force of 150N, his
             mass is 35kg the
             trikes is 15kg, what is
             the acceleration?
            150N/50kg=
            3m/s/s
Friction
 Friction is the force
  that opposes motion
  between two
  opposing surfaces
 Friction always acts
  against motion
 Causes of friction –
  unsmooth surface,
  deformations,
  molecular attraction
2 types of friction
 Static friction – force
  of a resting object
 Sliding or Kinetic
  friction - force on a
  moving object
Microwelds-
  deformations that hold
  cause static friction
Ever notice how it takes
  more force to get an
  object moving
Air resistance
 When drag is equal to         This is due to the air
  weight, there is no net        resistance – Friction
  external force on the          force caused when
  object.                        objects fall through air
 The object then falls at a
                                Dependant upon the
  constant velocity as
  described by Newton's          speed, shape, mass, and
  first law of motion.           size of the object, and
 The constant velocity is       the density of the air.
  called the terminal
  velocity.
More massive objects fall faster than less massive
objects because they are acted upon by a larger force
of gravity; for this reason, they accelerate to higher
speeds until the air resistance force equals the gravity
force.
Stuff falls
 http://www.youtube.co        All objects near the
  m/watch?v=5C5_dOEyA           surface of the earth
  fk                            accelerate at a rate of
 http://nssdc.gsfc.nasa.go     9.8m/s/s
  v/planetary/image/feath      Objects of different
  erdrop_sound.mov              masses will fall at the
                                same rate regardless of
                                their mass
Where is he falling fastest?
Where is accelerating the most?
Where is he not accelerating?
Gravitational accelertion
 More force acts upon a
 more massive object, but
 its larger mass requires
 more force to accelerate.

 There is a myth about
 Galileo dropping canon
 balls off the leaning
 tower of Pisa to prove
 this.
Gravitational acceleration
 Velocity of a freefalling object can be found with the
  formula v= g*t
 Where gravities acceleration is 9.8m/s/s


 How can you find falling distance
 D= ½ g *t2
Centripetal
 Centripetal
  acceleration:
  acceleration toward
  the center of a curved
  path
 Centripetal means
  center seeking- it is
  merely a net force
Centripetal ex.

                       As a bucket of
  As a car makes a     water is tied to a   As the moon orbits
  turn, the force of   string and spun      the Earth, the
  friction acting      in a circle, the     force of gravity
  upon the turned      force of tension     acting upon the
  wheels of the car    acting upon the      moon provides the
  provide the          bucket provides      centripetal force
  centripetal force    the centripetal      required for
  required for         force required       circular motion –
  circular motion.     for circular         p.s. the moon is a
                       motion               banana
Weight
 Weight – The
  measure of the force
  of gravity on a body.
 On earth your weight
  in Newtons (N) is
  equal to your mass in
  kg times gravity’s
  acceleration
  (9.8m/s2)
 W= m * g
Weightlessness
 Weightlessness is sensation experienced when
  there are no external objects touching one's
 body and exerting a push or pull
 Astronauts on the orbiting space shuttle are
  weightless because..
 a. there is no gravity in space and they do not weigh
  anything.
 b. space is a vacuum and there is no gravity in a
  vacuum.
 c. space is a vacuum and there is no air resistance
  in a vacuum.
 d. the astronauts are far from earth's surface at a
  location where gravitation has a minimal effect.
Freefallin’
               Orbit is a state of
                constant freefall – the
                Earth’s surface is
                falling away at the
                same rate of orbit
               400 km above the earth's
                surface, the the value of g
                will have been reduced
                from 9.8 m/s/s (at earth's
                surface) to approximately
                8.7 m/s/s. About 10% less
                than the surface
Projectile motion
 Projectile motion:         Projectiles follow a path
  Anything that has           called a trajectory
  horizontal motion is a
  projectile
 Horizontal and vertical
  motion act
  independently of one
  another-
So a bullet dropped and a bullet fired from the same
height will hit the ground at the same time.
Newton’s   3rd   law
                    For every action
                     (force) in nature
                     there is an equal and
                     opposite reaction.
                    Force pairs: Action
                     and reaction forces
                     do not act on the
                     same object.
                    What would happen
                     if they did?
Many reaction forces are not noticed due to other
forces or smaller accelerations.
Introduction to Rocket Performance - Level 3 - Jump
Animation
Momentum
            Momentum: property
             of a moving object
             based on its velocity
             and mass.
            p = mv
            kg x m/s – label
Momentum is conserved
              Law of Conservation
               of Momentum:
               Momentum may be
               exchanged but the
               total amount of
               momentum remains
               the same.
In a game of pool where is the
momentum lost
Practice
 A scooter and a big
  red truck, both
  moving at 20m/s.
  Which has more
  momentum.
 Mass of red truck
  3000kg
 Mass of scooter 8kg
 Momentum of truck:
 of Scooter:
Force and momentum
 Objects with momentum       This is a formula
  can apply force to other     combining the 2nd and
  objects when they strike     3rd laws
  each other                  A baseball strikes your
 F=mvf – mvi/t                glove with a velocity of
                               49m/s, its mass is .145kg.
                               The ball comes to a stop
                               in .05s. What force is
                               applied?
Artificial Satellite vs. Natural Satellite
                       Sputnik: 1957 Russian
                        (October 4th) “Fellow
                        Traveler or Satellite”

                       What do we use satellites
                        for?
                       Weather           TV
                        Communications
                        Science and Research
Satellites
              How many satellites
               currently orbit the earth?
              Geosynchronous
               Satellites: Orbital speed
               matches the earth’s
               rotation
              Where?       35,790 km
               above the equator.
              International Space
               Station: 390 km

				
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