Physics 121C Mechanics Lecture 1 Motion Concepts September 29, 2004

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Physics 121C - MechanicsLecture 1Motion
ConceptsSeptember 29, 2004

• John G. Cramer
• Professor of Physics
• B451 PAB
• cramer_at_phys.washington.edu

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Lecture Schedule (Part 1)
You are here!
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Stop-Motion Photography
We can superimpose all the frames to create
astop-motion picture that shows the
progressionof positions as equal intervals of
time pass.
The equal spacing of the images indicates that
the carhas a constant velocity.
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Change and Motion
A stationary ball on the ground.Same position at
all times. Velocity0.
A skateboarder rolling on a sidewalk.Images are
equally spaced. Velocityconstant.
A sprinter starting the 100 meter dash. Image
spacing grows. Velocity increasing.
A car stopping for a red light. Image spacing
shrinks. Velocity decreasing.
The actual images can be replaced by dots
showing successive positions.
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Images to Dots to Motion
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Motion Diagrams
Velocity Constant
Accelerating
Decelerating
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Motion in Two Dimensions
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Adding Vectors
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Subtracting Vectors
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Finding Dr r1-r2
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Sams Motion
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Dr in Motion Diagrams
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Velocity
Average speed Distance/Time Example 15
miles/½ hour 30 mph
Generalization velocity
Speed is a scalar (1D) quantity. Velocity is a
vector (3D) quantity.
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Motion and Velocity Vectors (1)
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Motion and Velocity Vectors (2)
Velocity is changing in both Magnitude and
direction.
Velocity is changing in magnitude.
Jake throws a ball to Jim.
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Acceleration
Definition
Acceleration Changing
Acceleration Constant
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Changing Words to Symbols

• ? Sketch the situation. Not just any sketch. Show
  the object at thebeginning of the motion, at the
  end, and at any point where the character ofthe
  motion changes. Very simple drawings are
  adequate.
• ?Establish a coordinate system. Select your axes
  and origin tomatch the motion.
• ? Define symbols. Use the sketch to define
  symbols representingquantities such as position,
  velocity, acceleration, and time. Every
  variableused later in the mathematical solution
  should be defined on the sketch.Some will have
  known values, others are initially unknown, but
  all should begiven symbolic names.
• ? List known information. Make a table of the
  quantities whosevalues you can determine from
  the problem statement or that can be
  foundquickly with simple geometry or unit
  conversions. Some quantities are impliedby the
  problem, rather than explicitly given. Others are
  determined by yourchoice of coordinate system.
• ? Identify the desired unknowns. What quantity or
  quantities will allowyou to answer the question?
  These should have been defined as symbols in step
  3.Dont list every unknown only the one or two
  needed to answer the question.

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End of Lecture 1

• Before the next lecture, read Knight,Chapters
  1.1 through 2.1
• Lecture Homework 1 is submittedon the Tycho
  system and is due at900 PM next Wednesday, Oct.
  6.