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Lecture
4: Perspectives
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| Astronomy
101/103 |
Terry
Herter, Cornell University
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Lecture
Goals
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Review
the basic physical vocabulary to describe:
- How
big, how tall, how fast, how massive, etc. ...
Understand
terminology and learn the language.
Only
then can you appreciate and understand astronomical concepts.
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Basic Units
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All
physical quantities have units. The four basic ones we will
use are:
- TIME
: seconds, hours, years, ...
- DISTANCE
: cm, m, light years, ...
- MASS
: gm, kg, ...
- TEMPERATURE
: Centigrade, Kelvin
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Examples
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Here
are a few examples using units for time and distance.
| Time |
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1
yr =
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3.15x107
s |
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Lifetime
~
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2.5x109
s |
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| Distance/Size |
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Thumb
=
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2
cm = 0.02 m |
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Grain
of sand =
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1
mm = 10-3 m |
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Atom
=
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10-10
m (1 A = 1 Angstrom) |
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Nucleus
=
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10-15
m (1 Fermi) |
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Diameter
of Earth =
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12,800
km |
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Astronomical
Distances
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Astronomers
use a number of units to express astronomical distances.
The most common ones are listed below.
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Astronomical
Unit
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(AU)
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=
1.5x1011 m |
=
Earth-Sun distance |
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Light
year
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(LY)
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=
9.5x1015 m |
=
63,240 AU |
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Parsec
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(pc)
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=
3.1x1016 m |
=
206,265 AU |
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1
PC
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=
3.26 LY |
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Mass
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Mass
is a measure of the amount of matter in an object -- Not
weight!
The
mass of an object is the same on the earth or in space,
but its weight depends upon gravity.
Here
is a sample table that shows the difference between mass
and weight.
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Place
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Mass
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Weight
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Comments
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Earth
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70
kg
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154
lb.
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size
of a person
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Moon
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70
kg
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26
lb.
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1/6th
gravity of earth
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Jupiter
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70
kg
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391
lb.
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no
solid surface
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Sun
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70
kg
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2.1
tons
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no
solid surface
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White
Dwarf
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70
kg
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25,300
tons
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So What?
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Mass
allows us to characterize an object independent of gravity.
- Mass
characterizes an objects "resistance" (inertia) to acceleration.
- How
hard it is to move an object or change its direction.
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Temperature
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- Characterizes
the internal motions of an object
- Example:
Air in a box
- Particles
(molecules) move faster when the temperature is higher.
- There
is a limit to how low temperatures can go -- absolute
zero.
| Absolute
Zero is the limit to how low temperatures can
go. |
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Temperature
Scales
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There
are a number of temperature scales. Astronomers use the
one you are probably least familiar, the Kelvin or "Absolute
Temperature" scale.
- Celsius
(Centigrade)
- 0
C = Freezing point of water
- 100
C = Boiling point of water
- Kelvin
- 0
K = Absolute Zero (-273.2 C)
- K
= C + 273
- Fahrenheit:
C = (5/9)*(F - 32)
- The
one most commonly used in the United States, but NOT
used in the sciences.
- Water
freezes at 32 F and boils at 212 F.
| It
is somewhat amusing to note that the only place
the Centigrade and Fahrenheit scale agree is
at -40, that is, -40 F = -40 C. |
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Example
Temperatures
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Like
all units, any time you list a temperature indicate the
scale you are using.
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Object
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F
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C
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K
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Ithaca
(July)
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95
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35
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308
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Ithaca
(Feb)
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-10
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-23
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250
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Sun
(surface)
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9981
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5527
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5800
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Pluto
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-382
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-230
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43
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Derived
Units
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- Originate
from combinations of the basic units.
- Used
for convenience to let us speak more "simply" about things.
- Some
are:
- velocity,
acceleration, angles, and density
- force,
pressure, energy, and power
- luminosity
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Velocity
and
Acceleration
Examples
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- Velocity
- Walking:
3-5 mph = 1.3-2.2 m/sec
- Sprinter:
25 mph = 11 m/sec
- Velocity
of earth around the Sun: 30 km/sec
- Acceleration
- Ex:
0 to 60 mph in 10 seconds => 2.7 m/sec2
- Ex:
Earth gravity accelerates objects at 9.8 m/sec2
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Angular
Size
and
Density
Examples
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- Angular
size
- Moon
& Sun: 30 arcminutes
- Jupiter:
30 to 49 arcseconds
- Pluto:
~ 0.1 arcsecond
- Resolution
of the eye: ~1 arcminute
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Your
thumb at arms length subtends an angle of
about 3 degrees.
[Your thumb is about 2.5 cm (1 inch) wide.
Extending your arm your thumb will be about
50 cm from your eye.]
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Density
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Density
represents the mass (or number of particles) per unit volume
of a substance, material, or object.
Densities
can vary greatly between materials (e.g. lead vs. gold vs.
aluminum) and states of matter (e.g. liquid vs. gas).
Some
Example Densities
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Material
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Mass
Density
(g/cm3)
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Particle
Density
(particles/cm3)
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Water
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1
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3.7x1022
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Lead
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11.3
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3.3x1022
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Gold
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19.3
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5.9x1022
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Air
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1.3x10-3
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3x1019
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Space
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2x10-24
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1
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Density
and
Mass
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If
a body is of uniform density then mass = density x volume.
Examples
- 1
gallon (3.8 liters) of water:
- 3
x 4 x 8 inch (96 in3) bar of gold is 30 kg
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Force
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Force:
F = m*a or a = F/m
- A
force is that which can change the velocity of an
object (either speed or direction).
- 1
dyne = g-cm/s2
- 1
Newton = kg-m/s2 = 105 dynes
= 0.2248 lb
- Forces
cause acceleration.
- Example
Forces
- Friction
(Electromagnetic Force)
- Gravity
- Earth's
gravity: 980 g-cm/sec
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A Poem
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- by
Jack Prelutsky about Energy and Power
"The Turkey Shot out of the Oven"
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