Chapter 8: Motion and Forces

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Chapter 8 Motion and Forces
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Speed

• How fast an object moves.
• 2 quantities needed to measure speed
• Distance
• Time
• SI units for speed meters per second (m/s) or
  (km/h) or (mi/hr).

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Constant Speed

• Object covers equal distances in equal amounts of
  time.
• Ex If a race car has a constant speed of 96
  m/s, then the race car travels a distance of 96 m
  every second. CRUISE CONTROL!

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Distance-Time Graph

• Speed can be determined from this graph.

Check this out!
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Which object is moving slower?
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At Rest

• Speed is 0 m/s.
• The slope of a line at rest would be

MOREPRACTICE!
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Speed Calculation

• Speed distance
• time
• Or ? d
• t
• Example Racer finishes 132 m in 18 s. Whats
  the speed?
• ? 132m 7.3 m/s
• 18s

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Velocity

• An objects speed and direction.
• Example A lions velocity was 4.5 m/s to the
  north or 2.0 km/hr toward the highway.
• Direction of motion can be described in many
  waysnorth, south, east, west, or specify angle,
  etc.

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Momentum

• Depends on both velocity and mass.
• Can determine how difficult an object would be to
  stop.
• Example Train vs. Car
• SI units kilograms times meters per second
  
  (kgm/s)

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Momentum Calculation

• Momentum mass x velocity
• Or p m ?
• Example Calculate momentum of a 6.00 kg bowling
  ball moving at 10.0 m/s down the alley.
• p 6.00 kg x 10.0 m/s 60.0 kgm/s down
  alley

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Law of Conservation of Momentum

• The total amount of momentum in a system is
  conserved.
• Example Total momentum of two cars before a
  collision is the same as the total momentum after
  a collision.

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Acceleration and Force

• Any change in velocity.
• Small valuespeeding up gradually.
• Great valuespeeding up rapidly.
• Positive valuespeeds up.
• Negative valueslows down.

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Acceleration Calculation

• Acceleration change in velocity
• time
• OR acceleration final velocity initial
  velocity
• 
  time
• OR a ?? ?2 ?1
• t t
• SI Units meters per second squared (m/s2)

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Acceleration Example

• Find the average acceleration of a northbound
  subway train that slows down from 12 m/s to 9.6
  m/s in 0.8 s.
• a ?? ?2 ?1
• t t
• (9.6 m/s 12 m.s) -3 m/s2 north or 3m/s2 south
• 0.8 s

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Velocity-Time Graph

• Acceleration can be determined using velocity and
  time graphed.

CHECK IT OUT!
PRACTICE
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Acceleration and Velocity

• Any change in VELOCITY (including any change in
  DIRECTION) is ACCELERATION.
• Example Press the gas pedalspeed up.
• Press the brakeslow down.
• Turn the steering wheel
• 
  direction changes.
• ALL show how a change in velocity.
• SO acceleration is a part of many types of
  motion.

PRACTICE!
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Force

• The cause of acceleration or change in an
  objects velocity.
• Net Force combination of all the forces acting
  on an object.
• Balanced Force combined force equals zero.
• Unbalanced Force do not cancel each other out
  completelynet force will cause the object to
  accelerate.

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Types of Forces

• Contact Forces
• Friction
• Tension
• Normal
• Air resistance
• Applied
• Spring

• Action-at-a-Distance Forces
• Gravity
• Electrical
• Magnetic

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Force Examples

• Friction
• Unbalanced acceleration
• Balanced constant speed or no motion
• Depends on type of surface
• Air resistance (increases with
• object size and speed)
• Its everywhere!

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Force Examples cont.

• Gravity
• Acts even when objects do not touch.
• Force of gravity depends on the mass and distance
  between two objects.
• Free fall acceleration due to
  gravity 9.81 m/s2

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Balanced or Unbalanced?
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Newtons First Law

• An object at rest remains at rest and an object
  in motion stays in motion unless it experiences
  an unbalanced force.
• Inertia tendency of object to stay at rest or
  at constant velocity
• Objects resist changes in motion

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Newtons Second Law

• The unbalanced force acting on an object equals
  the objects mass time its acceleration.
• Force mass x acceleration
• F ma
• SI Units for force Newton or kgm/s2
• 1 Newton 0.225 lb

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2nd Law Example

• What is the net force necessary for a 1.6 x 103
  kg automobile to accelerate forward at 2.0 m/s2?
• Fma (1.6 x 103 kg )(2.0 m/s2) 3.2 x 103 N
• Little mass, accelerated at small force
• Large mass, accelerated at larger force
• Same force, different mass, different acceleration

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Free Fall and Weight

• Free fall the motion of an object with gravity
  being the only acting force
• Earths free fall 9.81 m/s2
• Weight force on an object due to gravity
• Weight mass x free-fall acceleration
• w mg

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Weightlessness

• Weightlessness is due to everything accelerating
  at the same rate.
• No reaction force pushing on you.
• Examples astronauts and roller coasters
• When standing on earth, people normally
  experience the sensation of 1 g of acceleration
  vertically, which equals an acceleration of 9.8
  m/s2. A g force of 3 gives a person the sensation
  of being three times heavier than normal weight.
• Weightlessness equals 0 gs.

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Weight

• Different from mass (measure of amount of
  object).
• Earth 9.8 m/s2 Mars 3.7 m/s2
• Venus 8.8 m/s2 Neptune 11.8 m/s2
• Influences the shape of living things.
• Skeletons or Woody tree trunks
• Terminal velocity when air resistance and
  gravity are equalstops acceleration and reaches
  max. velocity.

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Newtons Third Law

• For every action force, there is an equal and
  opposite reaction force.
• Example
• Kicking soccer ball (the ball and the foot)
• Rockets
• Egg Drop
• Others?

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References

• Got Gravity? Pic www.pioneernet.net/curtis/wile_
  e/, http//camprrm.typepad.com/photos/uncategorize
  d/gravity.jpg
• Velocity-time graph http//www.bbc.co.uk/schools
  /gcsebitesize/physics/images/ph_forces02.gif
• Roller Coaster Pics http//www.glenbrook.k12.il.
  us/gbssci/Phys/mmedia/circmot/rcd.html
• Elements of Physics Motion, Force, and Gravity.
  United Learning. 2006.unitedstreaming. 1
  September 2006. http//www.unitedstreaming.com/
• Physics of Motion, The. Discovery Channel School.
  2004.unitedstreaming. 1 September 2006.
  http//www.unitedstreaming.com/
• Physical Science Structures. Discovery Channel
  School(2002). Retrieved September 14, 2006,
  fromunitedstreaming http//www.unitedstreaming.c
  om/