Physics 2211: Lecture 2 Mechanics for Scientists and Engineers Agenda for Today

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Physics 2211 Lecture 2Mechanics for Scientists
and EngineersAgenda for Today

• Mastering Physics Introduction (Homework in
  PHYS2211)
• Concepts of Motion (Chap. 1)

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Scope of Physics 2211

• Classical Mechanics
• Mechanics How and why things move.
• Describing motion (KINEMATICS)
• What causes (changes to) motion? (DYNAMICS)
• Classical
• Not too fast (v ltlt c)
• Not too small (d gtgt atom)
• Most everyday situations can be described in
  these terms.
• Path of baseball
• Orbit of planets
• etc...

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Visualising Motion

• Motion is defined as change in an object
  position with time
• Translational, Circular, Projectile, and
  rotational motion.

Trajectory is defined as the path along which an
object moves

• Concepts of motion we need them to describe
  motion
• Motion Diagram imagine making a movie of a
  moving object (frames, filmstrip)
• a composite photo showing objects position at
  equally spaced instants of time

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Motion diagram of a jump shot

• More complex motion decreasing distance means
  that the ball is slowing down increasing
  distance means that the ball is speeding up
• Particle Model
• Math
• Diagrams/Figures/Graphs
• Vocabulary/Technical Terms

• Motion Diagram

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Mechanics Vocabulary

• Coordinate System
• Position and Time

• Changes

Displacement
Time interval
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Vectors...

• 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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Vector addition

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

B
B
A
A
A
C A B
B

• We can arrange the vectors as we want, as long as
  we maintain their length and direction!!

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

C
B
A
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Vector Subtraction
Displacement
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Mechanics Vocabulary

• How fast?
• Speed

distance traveled

• (Average)
• Velocity

Rate of Change of Position
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• Two methods for subtracting vectors
• C B A
• From Tip of A to Tip of B
• Summing with A
• 1 Find -A
• 2 Find B(-A)

C
B
A
-A
B
C
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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)
with Constant speed. What is the direction of
the acceleration at point A?
(Select 9 to indicate the acceleration is zero.)
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4
2
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1
6
8
7
A
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Sprinter Motion Diagram
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Mechanics Vocabulary

• 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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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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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

• This is a very important tool to check your work
• Its also very easy!
• Example
• Doing a problem you get the answer distance
• 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!!

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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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Recap

• Mastering Physics
• Concepts of Motion
• Kinematics (Read Chapter 2)

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