PH 215 Physics I

1
PH 215(Physics I)

• Instructor Craig PutnamOffice DWH 109-IOffice
  Hours M-F 1100am 1200pmor by appt.Phone /
  Voicemail 6064Email putnam_at_dwc.edu

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Textbook

• Fundamentals of Physics Extended,7th edition,
  Halliday / Resnick / Walker, Wiley Publishers

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Course Outline

• Kinematics Chapters 1 4
• Newtons Laws Chapters 5 6
• Conservation Laws Chapters 7 9
• Rotation Equilibrium Chapters 10 12
• Oscillations Waves Chapters 15 16

4
Kinematics

• Measurement
• Motion in 1 Dimension
• Vectors
• Motion in 2 3 Dimensions

5
Newtons Laws

• Force Motion I
• Force Motion II

6
Conservation Laws

• Kinetic Energy Work
• Potential Energy Conservation of Energy
• Systems of Particles
• Collisions

7
Rotation Equilibrium

• Rotation
• Rolling, Torque Angular Momentum
• Equilibrium Elasticity

8
Oscillations Waves

• Periodic Motion (Oscillations)
• Waves

9
Evaluation

• 45 - Three exams (3 _at_ 15)
• 10 - Class Participation
• 10 - Recitation
• 15 - Homework Assignments
• 20 - Final Exam (cumulative)

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Evaluation

• Grade Cutoffs
• A/A- gt88
• B/B/B- 80-87
• C/C 70-79
• D 63-69
• F lt63

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Policies

• No late work will be accepted (unless
  arrangements have been made in advance)
• Ask questions participate actively in class
• You are responsible for what is covered in class
  even if you dont show up
• Recitation is a required part of this class
  attendance is taken and factored into your grade

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Policies

• You are encouraged to help each other with your
  homework assignments but you must turn in your
  own work
• If you are found to be cheating, you will fail at
  least the assignment / test and perhaps the
  entire class
• If you have any learning disabilities or special
  needs, please let me know in advance
• Check your email regularly for messages

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Homework

• A group of problems will be assigned for each
  chapter
• The problems for Chapter X are due at the lecture
  that begins Chapter Y (typically Monday)
• Feel free to do more problems than those that are
  assigned I will count them as extra credit
  towards your homework score (but extra credit
  work must be handed in along with the regular
  problems)

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Important Dates

• January 26 Last day to add/drop
• February 4 Ski Day
• March 13 20 Spring Break
• April 8 Last day to withdraw
• May 9 13 Final Exams

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Finally

• This is not an easy course especially if you
  have not had physics before
• Some of the concepts are difficult to understand
• But by the time we are done, you will be well on
  your way to being able to deal with real-world
  problems in Newtonian mechanics

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Lecture 1

• Measuring Things
• The International System of Units
• Changing Units
• Length
• Time
• Mass

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Measuring Things

• Physics is based on measurement
• All physical quantities are measured in their own
  special units and compared against standards
• Some things we measure are in base units
  others are in derived units
• Certain physical quantities, such as time and
  mass, have specific base quantities that have
  been agreed to by international bodies

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The InternationalSystem of Units

• The International System of Units (a.k.a. the
  metric system) was established in 1971
• Seven fundamental (base) quantities were picked
  we will look more closely at three of them
  starting today
• Length (meter)
• Time (second)
• Mass (kilogram)

19
SI System

• As we go along, we will encounter units for
  physical quantities that are not base units
• These are called derived units
• Example A watt (W) is the SI unit for power and
  is measured in units of kgm2/s3
• It uses the base units of mass, length and time

20
SI System

• Another example
• Speed can be measured in which of the following?
• Miles per hour
• Meters per second
• Kilo-furlongs per fortnight

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Scientific Notation

• In physics we need to deal with very small
  numbers as well as very large numbers
• .0000000000000000000000138
• 89900000000000000
• We will use scientific notation to compactly
  express such large and small quantities
• 1.38 ? 10-23 (Boltzmann constant)
• 8.99 ? 1016 (mass-energy relationship c2)

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Scientific Notation

• To get a good feel for the power of scientific
  notation, lets look atPowers of 10 applet

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Prefixes for SI Units

• To make the use of SI units a little easier,
  prefixes have been established for certain powers
  of 10

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Changing Units

• We will often have to change from one set of
  units to another
• This may be because certain countries (which will
  remain nameless but whose initials are U.S.A.)
  have not fully adopted the metric system yet
• Or it may be because of tradition, or simply that
  it is expedient

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Changing Units

• Review sample problems 1-1 and 1-2
• Now lets figure out the speed of light(2.998 ?
  108 m/s) in kilo-furlongs / fortnight
• Here are the necessary conversion factors
• 1 furlong 10 chains
• 1 chain 4 rods
• 1 rod 25 links
• 1 link 7.92 inches
• 1 inch 2.54 cm
• 1 fortnight 2 weeks

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Changing Units

• First, lets attack the numerator
• So 1 m 4.97096953789867 ? 10-3 furlong

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Changing Units

• Now the denominator
• So 1 s 8.26719576719577 ? 10-7 fortnight

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Changing Units

• Therefore, the speed of light(2.998 x 108 m/s)
  can also be expressed as1.802662849 ? 109
  kilo-furlongs/fortnight

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Changing Units

• Do we really believe that last calculation that
  the speed of light is exactly 1.802662849 ?
  109 kilo-furlongs/fortnight
• If not, then why not?

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Significant Digits Decimal Places

• If the formula for the volume of a sphere is V
  4/3pr3 and the radius is given asr 3.70 cm,
  then the volume of the sphere is?
• 212.1747902 cm3
• 212.17 cm3
• 212.2 cm3
• 212 cm3

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Length

• The meter was originally defined to be one
  ten-millionth of the distance from the north pole
  to the equator
• This was later abandoned in favor of a standard
  based on the distance between two scratches on a
  platinum-iridium bar
• Even that eventually proved to not meet the needs
  of scientists and engineers

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Length

• So in 1960 a new standard was adopted which was
  based on the wavelength of a particular color of
  light
• Specifically, it was defined to be 1,650,763.73
  wavelengths of a particular orange-red light
  emitted by krypton-68

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Length

• However, even this was not good enough so in 1983
  the length of the meter was defined once again
• This time though, the length standard became a
  derived standard a meter is now defined to be
  the distance that light travels in a certain
  amount of time

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Length

• Specifically, a meter is now defined to be the
  distance light travels (in a vacuum) during a
  time interval of 1/299,792,458 second
• Consequently, the speed of light is also now
  defined to be exactly 299,792,458 m/s

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Time

• In a similar vein, the base unit for time has had
  a number of different definitions
• Clearly, the rotation of the earth would seem
  like a likely source of a standard and it was
  for many years
• But as you can see in Figure 1-2 (page 6), the
  length of time it takes for the earth to rotate
  through 360 varies somewhat

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Time
37
Time

• The standard unit of time is of course the second
  and it is now defined to be the length of time it
  takes for 9,192,631,770 oscillations of light (of
  a particular wavelength) emitted by cesium-130
  atoms
• The mechanism for detecting and counting these
  oscillations is called an atomic clock
• The accuracy of a cesium-based atomic clock is
  such that it would take 6,000 years for two such
  clocks to differ by 1 second

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Sample Problem 1-4

• To calculate the height of the clouds above the
  earth you only need a stopwatch and some facts
  about the earth

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Sample Problem 1-4
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Sample Problem 1-4

• We begin by noting that just as the sun
  disappears, your line of sight to it is tangent
  to the surface of the earth
• When the sun no longer illuminates the clouds,
  there is again a line that is tangent to the
  earth
• We will assume that the earth is perfectly round
  which we know is untrue but

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Sample Problem 1-4

• We note that the angle between the 1st and 2nd
  tangent lines is ?
• We know that the earth rotates through 360º in 24
  hours, so from this we can calculate the amount
  of rotation in 38 minutes

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Sample Problem 1-4

• Let t 38 min
• We therefore have

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Sample Problem 1-4

• We can see that a right triangle is formed by
  connecting the radii at the two tangent points
• One leg of the triangle is of length r wherer
  6.37 106 m (the radius of the earth)
• The 2nd leg of the triangle is r H where H is
  the height of the clouds above the earth

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Sample Problem 1-4

• The angle subtended by these two legs is also ?
  (as can be seen from the geometry)
• Going back to our basic trigonometry, we can see
  that

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Sample Problem 1-4

• Rearranging and then filling in the values we get

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Sample Problem 1-4

• Besides assuming that the earth is perfectly
  round, what other approximation did we implicitly
  make as we went through this derivation?

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Sample Problem 1-4

• The assumption is that there are 24 hours in a
  full day
• Beyond the fact that there arent exactly 24
  hours in a day because there arent exactly 365
  days in a year, there also is the issue of a
  solar day vs. a sidereal day

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A digression onthe length of a day

• A solar day is the amount of time when the sun is
  directly overhead until it is again directly
  overhead
• This is nominally taken to be 24 hours (86,400
  seconds)
• A sidereal day is the amount of time it takes the
  earth to rotate 360º w.r.t. a distant star, e.g.,
  a star we are not orbiting

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A digression onthe length of a day

• For more information on a solar vs. sidereal day,
  see also http//csep10.phys.utk.edu/astr161/lect/
  time/timekeeping.html

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Sample Problem 1-4

• OK so whats the point???
• There are many places in physics especially at
  this level where we make assumptions,
  approximations, and simplifications
• We need to at least be aware of them and, when
  appropriate, document them

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Order of Magnitude

• The current standard for time the cesium-based
  clock might gain or loose a second in
  approximately 6,000 years or 3 ? 107 seconds. We
  would express this error as 3 ? 10-7.
• This means that the order of magnitude of the
  error is 7.
• New clocks under development will have much high
  accuracy their accuracy will be on the order of
  1 second in 1 ? 1018 seconds (about 3 ? 1010
  years)!

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Mass

• The standard unit for mass is a physical object
  a platinum-iridium cylinder kept at the
  International Bureau of Weights and Measures near
  Paris
• By international agreement, that cylinder of
  metal has been assigned a mass of exactly 1
  kilogram (1 kg)

53
Mass

• Because we need to measure the mass of very small
  objects (atoms or even smaller) the physical
  cylinder in Paris (or copies of it) may not be
  very convenient
• So a second standard has also been adopted one
  for use at the atomic level
• Again by international agreement, the carbon-12
  atom has been assigned a mass of 12 atomic mass
  units (u)

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Mass

• The relationship between the two mass standards
  is1 u 1.6605402 ? 10-27 kgwith an
  uncertainty of 10 in the last two decimal places

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Next

• Homework Problems 2, 7, 8, 10, 19, 20,
  27, 58
• Read Chapter 2