Physics 2211: Lecture 21 Notes

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Physics 2211 Lecture 3Todays Agenda
Motion Diagrams Uniform motion vs.
Non-uniform Motion Instantaneous) Velocity
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Trajectory Recap

Origin of Coordinates
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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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Example 1.7 (Projectile Motion)
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A
Green arrows are vectors of for a runner circling
a track.
Which red vector best represents the average
acceleration at point 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

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Kinematic Signs (Tactics Box 2.1)
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Problem Solving Strategy (Example 1.10)
A rocket is launched vertically with an
acceleration of 30 m/s2. It runs out of
fuel after 30 seconds. What is its
maximum altitude?
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Dimensions and Units...

• SI (Système International) Units
• mks L meters (m), M kilograms (kg), T
  seconds (s)
• cgs L centimeters (cm), M grams (gm), T
  seconds (s)
• British Units
• Inches, feet, miles, pounds, slugs...
• We will use mostly SI units, but you may run
  across some problems using British units. You
  should know how to convert back forth.

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Units of Kinematic (Motion) Quantities
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Converting between different systems of units
Useful Conversion factors 1 inch 2.54 cm 1
m 3.28 ft
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Converting between different systems of units
Useful Conversion factors 1 inch 2.54 cm 1
m 3.28 ft
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Dimensional Analysis

• An important tool to check your work. EASY TO
  USE!
• Example
• You do a problem and find for a distance d
• d vt 2 (velocity x time2)
• Units on left side L
• Units on right side (L / T) x T2 L x T
• Left units and right units dont match, so answer
  must be wrong!!
• Tip Dont do algebra with numbers.

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More Tips

• Read !
• Before you start work on a problem, read the
  problem statement thoroughly. Make sure you
  understand what information is given, what is
  asked for, and the meaning of all the terms used
  in stating the problem.
• Watch your units !
• Always check the units of your answer, and carry
  the units along with your numbers during the
  calculation.
• Understand the limits !
• Many equations we use are special cases of more
  general laws. Understanding how they are derived
  will help you recognize their limitations (for
  example, constant acceleration).

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Next Lecture
Kinematics Read Chapter 2