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Chapter 1: Units, Physical Chapter 1: Units, Physical Quantities and VectorsQuantities and Vectors
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About Physics
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What is Physics?
Phys’ics [Gr. Physika, physical or natural things]
Originally, natural sciences or natural philosophy
The science of dealing with properties, changes, interaction, etc., of matter and energy
Physics is subdivided into mechanics, thermodynamics, optics, acoustics, etc.
From Webster's Unabridged Dictionary
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Science
Science [Latin scientia - knowledge]
Originally, state of fact of knowing; knowledge, often as opposed to intuition, belief, etc.
Systematized knowledge derived from observation, study and experimentation carried on in order to determine the nature or principles of what is being studied.
A Science must have PREDICTIVE power
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Physics: Like a Mystery Story
Nature presents the clues Experiments
We devise the hypothesis Theory
A hypothesis predicts other facts that can be checked - is the theory right? Right - keep checking Wrong - develop a new theory
Physics is an experimental science
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The Ancient Greeks
Aristotle (384-322 B.C.) is regarded as the first person to attempt physics, and actually gave physics its name.
On the nature of matter:
Matter was composed of:
Air Earth Water Fire
Every compound was a mixture of these elements
Unfortunately there is no predictive power
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On the Nature of Motion
Natural motion - like a falling body Objects seek their natural place
Heavy objects fall fast Light objects fall slow
Objects fall at a constant speed
Unnatural motion - like a cart being pushed The moving body comes to a stand still when the
force pushing it along no longer acts The natural state of a body is at rest
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Aristotelian Physics
Aristotelian Physics was based on logic
o It provided a framework for understanding nature
o It was logically consistent
It was wrong !!!
Aristotelian physics relied on logic - not experiment
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The Renaissance
Galileo Galilei (1564 -1642) was one of the first to use the scientific method of observation and experimentation. He laid the groundwork for modern science.
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Classical Mechanics
Mechanics: the study of motion
Galileo (1564 -1642) laid the groundwork for Mechanics
Newton (1642-1727) completed its development (~almost~)
Newton’s Laws work fine for
Large Objects - Ball’s, planes, planets, ... Small objects (atoms) Quantum Mechanics
Slow Objects - people, cars, planes, ... Fast objects (near the speed of light) Relativity
Classical Mechanics - essentially complete at the end of the 19th Century
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Why is Physics Important?
Newton’s Laws andClassical Physics
QuantumMechanics
The NextGreat Theory
Planetary motion Steam Engines Radio Cars Television
Microwaves Transistors Computers Lasers
Teleportationo Faster than
light travel(can’t exist today)
"Heavier-than-air flying machines are impossible." Lord Kelvin, president, Royal Society, 1895.
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Mechanics
Physics is science of measurements Mechanics deals with the motion of objects
o What specifies the motion?
o Where is it located?
o When was it there?
o How fast is it moving?
Before we can answer these questions
We must develop a common language
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Units
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Fundamental Units
Length [L]FootMeter - Accepted UnitFurlong
Time [T]Second - Accepted UnitMinuteHourCentury
Mass [M] Kilogram - Accepted UnitSlug
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Derived Units
Single Fundamental Unit Area = Length Length [L]2
Volume = Length Length Length [L]3
Combination of Units Velocity = Length / Time [L/T] Acceleration = Length / (Time Time) [L/T2] Jerk = Length / (Time Time Time) [L/T3] Force = Mass Length / (Time Time) [M L/T2]
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Units
SISI (Système Internationale)(Système Internationale) Units:: mks: L = meters (m), M = kilograms (kg), T =
seconds (s) cgs: L = centimeters (cm), M = grams (g), T
= seconds (s)
British UnitsBritish Units:: Inches, feet, miles, pounds, slugs...
We will switch back and forth in stating problems.
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Unit Conversion
Useful Conversion Factors: 1 inch = 2.54 cm 1 m = 3.28 ft 1 mile = 5280 ft 1 mile = 1.61 km
Example: convert miles per hour to meters per second:
s
m447.0
s
hr
3600
1
ft
m
28.3
1
mi
ft5280
hr
mi1
hr
mi 1
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Orders of Magnitude
Physical quantities span an immense range
Length size of nucleus ~ 10-15 m
size of universe ~ 1030 m
Time nuclear vibration ~ 10-20 s
age of universe ~ 1018 s
Mass electron ~ 10-30 kg
universe ~ 1028 kg
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Physical Scale
Orders of Magnitude Set the Scale Atomic Physics ~ 10-10 m Basketball ~ 10 m Planetary Motion ~ 1010 m
Knowing the scale lets us guess the Result
Q: What is the speed of a 747?
Distance - New York to LA 4000 mi
Flying Time 6 hrs= 660 mph
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Dimensional Analysis
Fundamental Quantities Length - [L] Time - [T] Mass - [M]
Derived Quantities Velocity - [L]/[T] Density - [M]/[L]3 Energy - [M][L]2/[T]2
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Physical Quantities
Must always have dimensions Can only compare quantities with the same
dimensions v = v(0) + a t [L]/[T] = [L]/[T] + [L]/[T]2 [T]
Comparing quantities with different dimensions is nonsense v = a t2
[L]/[T] = [L]/[T]2 [T]2 = [L]
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Vectors
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Scalars & Vectors
A scalar is a physical quantity that has only magnitude (size) and can be represented by a number and a unit.
Examples of scalars?
Time Mass Temperature Density Electric charge
A vector is a physical quantity that has both magnitude (size) and direction.
Examples of vectors?
Velocity Force
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Vectors are
• represented pictorially by an arrow from one point to another. • represented symbolically by a letter with an arrow above it.
Displacement Vector is a change in position. It is calculated as the final position minus the initial position.
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Some Vector Properties
Two vectors that have the same direction are said to be parallel.
Two vectors that have opposite directions are said to be anti-parallel.
Two vectors that have the same length and the same direction are said to be equal no matter where they are located.
The negative of a vector is a vector with the same magnitude (size) but opposite direction
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Magnitude of a Vector
The magnitude of a vector is a positive number (with
units!) that describes its size.
Example: magnitude of a displacement vector is its length.
The magnitude of a velocity vector is often called speed.
The magnitude of a vector is expressed using the same letter
as the vector but without the arrow on top of it.
AAAofMagnitude
)(
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Vector Addition
Vector C of a vector sum of vectors A and C.
Example: double displacement of particle.
Vector addition is commutative (the order of vector
addition doesn’t matter).
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Vector Addition C A U T I O N
Common error: to conclude that if C = A + B the
magnitude C should be equal the magnitude A plus
magnitude B. Wrong !
Example: C < A + B.
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Vector Addition
Add more than two vectors:
CDCBAR
EACBAR
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Vector Subtraction
Subtract vectors:
)( BABA
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Vector Components
There are two methods of vector additionGraphical represent vectors as scaled-
directed line segments; attach tail to headAnalytical resolve vectors into x and y
components; add components
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Vector Components
If R A B
Then and x x x y y yR A B R A B
Where cos and cosx A y AA A A A
cos and sinx B y BB B B B
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Vector Components
If Rx< 0 and Ry > 0 or if Rx< 0 and Ry < 0 then + 180o
2 2x yR R R
1tan y
x
R
R
R
xR
yR
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Vector Components C A U T I O N
The components Ax and Ay of a vector A are numbers; they
are not vectors !
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Vector Components
)( BABA
BAR
xxx BAR
yyy BAR
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Vector Components
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Problem Solving Strategy
IDENTIFY the relevant concepts and SET UP the problem: Decide what your target variable is. It may be the magnitude of the
vector sum, the direction, or both. Then draw the individual vectors being summed and the coordinate
axes being used. In your drawing, place the tail of the first vector at the origin of coordinates; place the tail of the second vector at the head of the first vector; and so on.
Draw the vector sum R from the tail of the first vector to the head of the last vector.
By examining your drawing, make a rough estimate of the magnitude and direction of R you’ll use these estimates later to check your calculations.
VECTOR ADDITION
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Vector Components
There are two methods of vector addition Graphical represent vectors as scaled-directed
line segments; attach tail to head Analytical resolve vectors into x and y
components; add components
yx AAA
xx AA
yy AA
Component vectors
Co
mp
on
ents
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Vector Components
You can calculate components if its magnitude and direction are known
Direction of a vector described by its angle relative to reference direction
Reference direction positive x-axis
Angle the angle between vector A and positive x-axis
Θ = 0
Θ = 90
Θ = 180
Θ = 270
x
y
90 < Θ < 180cos (-) sin (+)
180 < Θ < 270cos (-) sin (-)
0 < Θ < 90cos (+) sin (+)
270 < Θ < 360cos (+) sin (-)
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Vector Components
cosA
Axsin
A
Ay
cosAAxsinAAy
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Vector Components C A U T I O N
The components Ax and Ay of a vector A are numbers; they
are not vectors !
The components of vectors can be negative or positive
numbers.
90 < Θ < 180cos (-) sin (+)
180 < Θ < 270cos (-) sin (-)
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Finding Vector Components
What are x and y components of vector D? Magnitude of D=3.00m, angle is =45.
IDENTIFY AND SET UP Vector Components Trig EquationsEXECUTE Angle here is measured toward
negative y-axis. But we need angle measured from positive x-axis toward positive y-axis. Thus, θ=-=-45.
mmDDx 1.2))45)(cos(00.3(cos
mmDDy 1.2))45)(sin(00.3(sin
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Finding Vector Components
What are x and y components of vector E? Magnitude of E=4.50m, angle is =37.0.
IDENTIFY AND SET UP Vector Components Trig
EquationsEXECUTE Any orientation of axes is
permissible, but X- and Y-axes must be perpendicular.
E is the hypotenuse of a right triangle! Thus:
mmEEx 71.2))0.37)(sin(50.4(sin
mmEEy 59.3))0.37)(cos(50.4(cos
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Vector Components
Reverse the process: We know the components. How to find the vector magnitude and its direction?
Magnitude: Pythagorean theorem
Direction: angle between x-axis and vector
22yx AAA
x
y
A
Atan
x
y
A
Aarctan
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Vector Addition, Components
xxx BAR yyy BAR
Ax AA cos
Ay AA sin
Bx BB cos
By BB sin
BAR
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Problem Solving Strategy
IDENTIFY AND SET UP Target variable: vector
magnitude, its direction or both
Draw individual vectors and coordinate axes
Tail of 1st vector in origin, tail of 2nd vector at the head of 1st vector, and so on…
Draw the vector sum from the tail of 1st vector to the head of the last vector.
Make a rough estimate of magnitudes and direction.
EXECUTE Find x- and y-components of each
individual vector
Check quadrant sign! Add individual components
algebraically to find components of the sum vector
Magnitude
Direction
Bx BB cos
By BB sin
yx RRR
... xxxx CBAR... yyyy CBAR
x
y
RR
arctanEVALUATE Check your results comparing them with the rough estimates!
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Vector Components
θA=90.0-32.0=58.0 θB=180.0+36.0=216.0 θC=270.0 Ax=A cos θA
Ay=A sin θA
Distance Angle X-comp Y-comp
A=72.4m 58.0 38.37m 61.40m
B=57.3m 216.0 -46.36m -33.68m
C=17.8m 270.0 0.00m -17.80m
-7.99m 9.92m
12999.792.9arctan
7.12)92.9()99.7( 22
mm
mmmR