Chapter 2 Linear Motion

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Chapter 2 Linear Motion

• Motion Displacement
• Speed Velocity
• Acceleration
• Formulas!!!!!!!

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Dynamics

• The branch of physics involving the motion of an
  object and the relationship between that motion
  and other physics concepts
• Kinematics is a part of dynamics
• In kinematics, you are interested in the
  description of motion
• Not concerned with the cause of the motion

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Motion

• Change in the position of an object
• Motion is relative
• It is observed by using a frame of reference

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Position and Displacement
A

• Position is defined in terms of a frame of
  reference
• Frame A xigt0 and xfgt0
• Frame B xilt0 but xfgt0
• One dimensional, so generally the x- or y-axis

y
B
x
O
xi
xf
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Position and Displacement

• Position is defined in terms of a frame of
  reference
• One dimensional, so generally the x- or y-axis
• Displacement measures the change in position
• Represented as ?x (if horizontal) or ?y (if
  vertical)
• Vector quantity (i.e. needs directional
  information)
• or - is generally sufficient to indicate
  direction for one-dimensional motion

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Displacement

• Actual change in position of an object, NOT
  always distance covered.
• Straight-line distance from point A to point B

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Speed

• The rate of change in position of an object
• only has magnitude
• Calculated by dividing distance by time
• m/s, km/hr

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Instantaneous Speed

• Speed at any given point in time
• Not used as much as average speed
• Speedometer in your car shows instantaneous speed

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Variations in speed as you drive to school
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Velocity

• Speed in a given direction.
• m/s, South km/h up
• contains magnitude and direction

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Positive Velocity VS. Negative Velocity
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(No Transcript)
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Speeding up VS. Slowing Down
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Acceleration

• A change in velocity
• recall velocity has both speed in direction..so
  any change speed or direction is acceleration

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Free Fall

• All objects moving under the influence of only
  gravity are said to be in free fall
• All objects falling near the earths surface fall
  with a constant acceleration
• This acceleration is called the acceleration due
  to gravity, and indicated by g

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Free Fall, Acceleration due to Gravity

• The position of the free-falling object at
  regular time intervals, every 0.1 second, is
  shown.
• The distance which the ball travels every
  interval of time is increasing.

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Acceleration due to Gravity

• Symbolized by g
• g 9.8 m/s² (can use g 10 m/s² for estimates)
• g is always directed downward
• toward the center of the earth

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Free Fall -- an Object Dropped
y

• Initial velocity is zero
• Frame let up be positive
• Use the kinematic equations
• Generally use y instead
• of x since vertical

x
vo 0 a g
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Free Fall -- an Object Thrown Downward

• a g
• With upward being positive, acceleration will be
  negative, g -9.8 m/s²
• Initial velocity ? 0
• With upward being positive, initial velocity will
  be negative

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Free Fall -- object thrown upward

• Initial velocity is upward, so positive
• The instantaneous velocity at the maximum height
  is zero
• a g everywhere in the motion
• g is always downward, negative

v 0
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Thrown upward

• The motion may be symmetrical
• then tup tdown
• then vf -vo
• The motion may not be symmetrical
• Break the motion into various parts
• generally up and down

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Non-symmetrical Free Fall

• Need to divide the motion into segments
• Possibilities include
• Upward and downward portions
• The symmetrical portion back to the release point
  and then the non-symmetrical portion

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Combination Motions
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The direction of the velocity changes as the car
moves around the curve, so that the velocity v2
is not the same as the velocity v1 even though
the speed has not changed.
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Acceleration continued...

• Change in velocity divided by time
• m/s2, km/h/s
• speed up, slow down or change directions

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The direction of the velocity changes whena ball
bounces from a wall. The wall exerts a force on
the ball in order to produce this change.
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Falling Objects

• Near the earths surface, all objects accelerate
  at a rate of 9.81 m/s2 in the absence of air
  resistance.
• Use 9.81 m/s2 as the value of g on Earth to
  represent the downward direction of the
  acceleration.
• Therefore the speed of a falling object increases
  by 9.81 m/s every second and the speed of an
  object tossed upward decreases by 9.81 m/s each
  second.

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Problem

• A ball is thrown into the air and rises for 3
  seconds before it begins to fall. Draw this
  situation and describe the velocity and
  acceleration of the ball.