Chap.8 Notes Motion

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Chap.8 Notes (Motion)

• Motion - a change in position (relative to a
  point considered fixed)
• Frame of Reference or reference frame (Earth is
  the usual)
• Motion is measured by distance and time in some
  direction from that fixed point.
• Car travels 50 miles/hr, north

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• Relative to the earth, the car moves forward.
• Relative to the plane, the car is dropping
  behind, going backward.

Earth
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Speed

• is distance per unit of time during which the
  motion occurred. (50 miles/hr)
• speed distance/time (see p.291 for ex.)
• Constant speedspeed that doesnt change
• on a distance time graph this is a straight line
• the steeper the slope, the greater the speed
• for a nonconstant speed, average speed isthe
  total distance covered by an object in total
  amount of time (regardless of changes in the
  middle).
• Aver.Speedtotal distance/total time

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Constant Speed Comparisons
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Velocity

• speed in a direction (car goes 10m/s north)
• Navigation requires velocity exactly speed and
  direction are very important to reach home.
• Can indicate as North, South, East, or West of a
  fixed point, or as an angle from a fixed point,
  or positive or negative along a line of motion.
  (We will always use positive velocity here to
  avoid confusion).

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Speed example

• A runner takes 25 seconds to run 150 meters.
  What is his speed in meters per second? What
  would be needed to make this a velocity?
• Speed Dist./Time
• Speed 150m / 25s
• Speed 6 m/s
• (It needs a direction to be a velocity).

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Tailwinds help planes get there faster, b/c...

• Velocities in the same direction
• can be added
• 50km/hr 200km/hr
• 250 km/hr. total
• speed.

200km/hr west (plane)
50km/hr. west (wind)
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Headwinds hinder a plane...

• Velocities in opposite directions should be
  subtracted.

200km/hr west (plane)
80km/hr east
200 km/hr - 80 km/hr 120 km/hr total speed.
(wind)
Both kinds of winds must be taken into
calculations.
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Momentum

• mass of an object X its velocity (kg
  m/s)
• how difficult it is to stop a moving object
• a lighter object may move a heavier object if its
  velocity is greater.
• Stopping distance of a car (Drivers Ed.) is
  directly related to its Momentum (depends on mass
  and velocity).
• Has directionin direction of the velocity in the
  equation.

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• A train has great momentum b/c of its very great
  mass.
• A bullet has great momentum b/c of its very great
  velocity.
• It is the mass times the velocity that makes for
  size of momentumboth are involved.
• Law of Conservation of Momentum- total momentum
  of a group of objects stays the same unless acted
  on by an outside force. (momentum is conserved)
• Ex. momentum of a baseball bat isnt lost in a
  hit--it goes to the ball.

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Momentum Example

• A 200kg object traveling at 10m/s collides
  head-on with a 600kg object traveling at 3m/s.
  What is the momentum of each and which will
  continue in its original path?
• Momentum mass X velocity
• 200kg X 10m/s 2000kg m/s
• 600kg X 3m/s 1800kg m/s
• After they collide they will both continue in the
  path of the 200kg object (b/c its the one with
  more momentum)

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Intermission(end first note session)
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8-2 Acceleration/Forces

• (Mr. Gs most forceful presentation)

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Acceleration (and Forces) (8.2)

• Acceleration is
• rate of change in velocity.
• a change in V/ time
• or a final velocity - original
  velocity time
• deceleration is a decrease in velocity or
  negative acceleration

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Acceleration example

• Jenny is going 30m/s. She brakes to avoid a cat,
  and 4 seconds later is going 10m/s. What is her
  acceleration?
• a final vel. - original vel. time
• 10m/s - 30m/s - 5m/s/s or - 5m/s2 4
  sec.

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• Acceleration is a change in velocity, so graphs
  of acceleration showing distance and time are
  curves. (Graphs of velocity and time are
  straight line increases (p.262).

Distance
Acceleration
Deceleration
Time
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• Acceleration is a change in velocity.
• Velocity is direction and speed.
• So, an object moving in a circle is accelerating,
  even though its moving about the same speed, b/c
  its changing its direction
• It is accelerating towards the center of the
  circlethis is called centripetal acceleration

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• Satellite stays in orbit b/c
• it is always falling toward earth (gravity)
• it has enough velocity to continue to miss
  falling on the earth
• Astronauts in orbit arent weightless b/c there
  is no gravity (if there wasnt they wouldnt stay
  in orbit!). Its b/c they are always falling
  (accelerating!)
• This is called apparent weightlessness.
• Earths gravity acceleration is 9.8 m/s2

Velocity
Gravity
Earth
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Force a push or a pull causing acceleration
(makes a change in objects velocity).

• Force mass times acceleration
• F m X a
• If several forces, the net force is the resulting
  sum of all forces acting on object. The object
  will accelerate in direction of net force (unless
  they sum to zero).

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Balanced Forces are forces opposite in direction
and equal.

• Balanced forces zero
• no change in motion (or no change in velocity)
• Example Book on a desk,pushing against a wall
  (no movement), or a satellite in orbit (no change
  in motion).

Force of gravity
Equal and balanced forces
Force of table
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Do you think these are balanced forces?
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Unbalanced forces are not equal/balanced cause
change in motion

• When forces are not balanced, what is left over
  exerts a push or pull.
• When two unbalanced forces are opposite in
  direction, combined forces are equal to
  difference between em. (May subtract them from
  each other).

30 Newtons
80 Newtons
Net result 50 Newtons w/dog.
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• Force gives energy to an object - it can move the
  object, stop its motion or change the direction
  of the object.
• An object set in motion would tend to stay in
  motion, except for another force.
• Friction - a force between two objects in contact
  that opposes motion of either object.
• Friction is often helpful! Ex.w/o friction,
  walking would not be possible, and skydiving
  would be fatal.
• Friction depends on surfaces in contact.
• It may be in the form of air resistance.

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What makes this activity so low in friction?
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Gravity- an acceleration, due to the force of
gravity between two particles of matter, due to
their mass.

• Gravity is an acceleration. An object only being
  affected by gravity is said to be in free
  fallan acceler. Of --gt g 9.8 m/s/s
• a falling object is also, however, slowed by air
  resistance (a form of fluid friction).
• Eventually air resistance gravity and the
  object falls at a constant velocity.
• This velocity is called Terminal velocity.
• For humans it is about 320 km/hr (200mph).

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• Law of Universal Gravitation - all objects with
  mass attract each other by force of gravity. How
  much depends on mass and distance between them.
  (p.267, fig.8-11)
• Force of gravity is very small except for very
  large objects (like the earth).
• Weight is a force. F m x a, therefore w
  m x g
• Your weight on the moon would be less, and
• Your weight away from the earth is less.
• Mass (amount of matter) is constant, weight
  (depends on location gravity) is not constant.
• Example What is Mr. Gs weight if mass is 89Kg?
• W m x g 89Kg x 9.8 m/s/s 872 Kg m/s/s
• 872 N (Anyone figure that
  out in lbs?)

Gravity, 9.8m/s/s
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Falling object illustration...
0sec.
9.8 m/s
1sec.
Ball accelerates 9.8 m in each second.
19.6 m/s
2 sec.
29.4 m/s
3 sec.
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8.3 Force/MotionNewtons 3 laws of Motion

• Newtons 3 laws explain rest, constant motion,
  acceleration, balanced and unbalanced forces.
• 1st Law Inertia. An object at rest will stay at
  rest, an object in motion maintains its velocity
  (constant velocity), unless acted on by an
  unbalanced force.

Kick continues until something (ground, catch)
exerts a force on it to change its motion.
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• 2nd law of motion- F m x a
• The unbalanced force acting on an object equals
  the objects mass times its acceleration.
• 1 N 1 Kg X 1 m/sec/sec
• If you see a change in motion, you are seeing an
  unbalanced force.
• Example problem A 750 Kg car accelerates
  2m/s/s. At this moment, what force is being
  exerted?
• F m x a
• F 750Kg X 2m/s/s 1500 N

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Newtons 3rd Law for every action force, there
is an equal and opposite reaction force.

• All forces tend to come in pairs
• These oppose one another and push against each
  other.
• An action you make is called an action force the
  force equal and opposite is the reaction force.

Burning fuel pushes hot gases down towards
earth, reaction force pushes rocket in an
upward direction.
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The End of Chap. 8 NotesMay the chinchilla be
with you...
My brother Peter with chinchilla