PHYS2211N Lecture

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PHYS2211N Lecture 2

• Jan12, 2005

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Jan 12, 2005

• Todays Agenda
• More on vectors
• Displacement
• Velocity and speed
• Acceleration
• UNC vs GT (ESPN)

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PRS DEMONSTRATION

• Prior to this class, have you taken physics at
  either the high school or college level? (Note
  Do not count physical science courses as
  physics.)
• 1. Yes 2. No

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Vectors...Review

• An arrow is commonly used to represent a vector
  quantity moreover, there are two common ways to
  symbolize a vector quantity
• Boldface notation A
• Arrow notation

A
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Coordinate Systems

• Before we can talk about out things move we need
  to decide how to concretely define where an
  object is.
• In 2-D, or 3-D there are two basic types of
  coordinate systems
• Cartesian coordinates (x,y,z) use units
• three mutually perpendicular directions.
• Conventions require right-handed system
• 2) Polar (or spherical) coordinates (r,q,f) use
  a distance and one or more angles

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Vector additiongraphical method

• Consider the vectors A and B. Find A B.

B
B
A
A
A
C A B
B

• The Head-to-Tail Rule
• We can pick up and move vectors at will. Just
  dont change their length or their direction!!

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• Which of the following corresponds to the
  diagram?
• 1. C B A
• 2. C A - B B
• 3. C B - A
• 4. B C - A

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Vector Subtraction
Displacement
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Displacement

• The change in the position vector is a vector
  called displacement.

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A vector divided (or multiplied) by a scalar is
another vector. We define a vector average
velocity as We define average speed as Note
average speed is NOT the magnitude of the average
velocity vector.
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Example 1 My GPS in my
Land Rover says that the Phys building is 5.77
miles from my house in a direction of 195 degrees
(wsw). The speedometer on my car say I drive 8.0
miles to work. If the trip takes 25 min, what is
my average velocity, and average speed.
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My average speed is My average velocity is
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Useful trivia

• Navigation directions are measured from north
  being direction 0 degrees. This is what is used
  for wind, ships, and airplanes.
• The numbers on the end of runways are the
  direction/10. If you land at an airport and see
  27 on the end of the runway, you are flying
  directly towards the west.

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If you run once around a circular track of radius
R in time T, what is your average speed and
average velocity? Your average speed is 2pR/T
and your average velocity is
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Vocabulary of Mechanics

• Speeding Up vs Slowing Down

• Accelerate vs Decelerate

• Positive acceleration vs Negative
  acceleration

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Mechanics Vocabulary

• How fast?
• Speed

distance traveled

• Average Velocity

Rate of Change of Position
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Mechanics Vocabulary

• Average Acceleration

Rate of Change of Velocity
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Example 1.4 (Skier)
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The motion diagram for a sprinter running around
a circular track (displacement vectors shown).
What is the direction of the acceleration at
point A?
(Select 9 to indicate the acceleration is zero.)
3
4
2
5
1
6
8
7
A
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Circular Motion
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Vocabulary of Mechanics

• Speeding Up vs Slowing Down

• Accelerate vs Decelerate

• Positive acceleration vs Negative
  acceleration