Admin. 9/13/16
1.
Class website http://www.astro.ufl.edu/~jt/teaching/ast1002/
2.
Optional Discussion sections: Tue. ~11.30am (period 5), Bryant 3; Thur. ~12.35pm
(end of period 5 and period 6), start in Pugh 170, then Bryant 3
3.
Office hr: Tuesday 12.30-1pm; Wed. 12.30-1.00pm, Bryant 302 (but email me if
coming on Wed.).
4.
5.
Homework 3: is due Mon. Sept 19th 11.59pm via Canvas e-learning under “Quizzes”
Reading this week: Chapters 0, 1, 2.1-2.4, 4.1, 5
6.
Midterm 1: Tue. Sept. 27th, in class BRING YOUR UF ID TO THE EXAM. For
review go to Discussion Sections or Office Hrs. Best preparation is to review class
notes and homework & quizzes, then also the textbook. You will not be able to use a
calculator or refer to your class notes in the exam. You will receive a list of formulae
(see next slide). Exam is multiple choice, similar to HWs. Bring a pencil.
7.
Observing project deadline: Thursday Oct. 27th 2016, however, you are strongly
advised to complete observing by Fri. Oct. 7th.
8.
Email me Astro-news, jokes, tunes, images: [email protected]
9.
Printed class notes? Name tags?
Formulae we have met so far
(will be displayed for you in Midterm 1):
Speed = distance / time
Angular size: θ = size / distance
Kepler’s 3rd Law: P2 = a3
[ Newton’s version of Kepler’s 3rd: P2 ∝ a3/(m1+m2) ]
Newton’s 2nd Law: F = m a
Newton’s Law of Gravity: F ∝ m1 m2 / r2
Density = mass / volume
Volume of a sphere = (4/3)πr3
Surface area of sphere = 4πr2
Frequency: f = 1/Period
Speed of wave (light) = frequency x wavelength: c = f λ
Key Concepts: Lecture 10
Tides as a consequence of Newton’s Laws
Structure of the Earth
Intro. to Elements and Atomic Structure, Radioactivity
Evolution of Earth’s Surface
Weight
• Recall Newton’s Law of Universal
Gravitation:
Mass1 x Mass2
Force ∝ Separation 2
• Weight is the force of gravity pulling you to
the ground: F = constant myou mEarth
r2
How much do you weigh on the Moon? (see table 5.1 C&M)
What does it mean to be
“weightless” in orbit?
Are you massless?
Is there a gravitational force from the Earth acting on you?
Are you accelerating?
Tides
• Tides - cyclic change in water depth
– 2 high tides per day
• Primarily due to gravitational pull of the Moon.
Sun’s effect is about half as strong.
• Inverse square law of gravity implies:
• Moon pulls hardest on the water nearest the Moon
• Moon pulls less hard at the center of the Earth
• Moon pulls least on the water on the far side
Spring & Neap Tides
• Strong “Spring” tides when Sun and
Moon line up. (nothing to do with
Spring season)
• Weak “Neap” tides when Sun and
Moon are at 90 degrees from each
other as viewed from Earth: tidal
forces partially cancel each other
out.
• The local ocean floor also has a
large effect on the size of tides
leading to variations in the height of
tides from place to place.
Moon is Tidally-Locked to Earth
• Moon rotates about once per month so that it always
presents the same face to Earth.
• Initially it rotated faster, but was slowed down by tidal
forces from the Earth
• The tides on the Earth, raised by the Moon, are also
slowing down the Earth’s rotation: the day is getting
longer!
Moon always presents the
same face to the Earth
The Earth’s Interior
• The Earth is very dense: average density is 5500 kg/
m3
– Water 1000 kg/m3
– Normal Rocks 2000-4000 kg/m3
– Pure Iron 7800 kg/m3
• The Earth’s interior is differentiated
– Distinct layers - dense material sinks, light
material floats
Overview of the Earth
• Dense Rocky Composition
• Evolving Surface (most is
younger than <600 million years)
• Atmosphere of Nitrogen and
Oxygen
• Oceans of Water
• Magnetic Field
Seeing Inside the Earth
• Interior structure is probed by
studying how waves travel through the
Earth.
• Earthquakes generate seismic waves,
similar to sound waves.
• Reflect at boundaries
• Travel at different speeds in different
materials, e.g. faster in higher density
material.
• 2 main types of wave (P, S). S-(Shear/
Secondary) waves cannot travel
through liquid. P-(Pressure/Primary)
waves can. From this we can tell Earth
has a liquid outer core.
Radioactivity is an important heat source in the Earth: to understand
radioactivity we need to know a little about atomic structure…
The Elements and Atomic Structure
• -All matter in the Earth, other planets & stars is
composed of various elements, e.g. Hydrogen (H),
Helium (He), Carbon (C), Oxygen (O), Iron (Fe), etc.
• -Atoms of these elements have nuclei of protons (p)
and neutrons (n) surrounded by electrons (e).
Nucleus (protons) is positively charged, has most of
the mass and is very small. Electrons are negatively
charged and have little mass, but occupy most of the
volume.
• -Different chemical elements are due to different
numbers of protons in the nucleus: e.g. 1p=H, 2p=He,
6p=C, 8p=O, 26p=Fe
• -Nuclei with larger numbers of protons are more
unstable because the positive electric charges repel
each other. This leads to radioactive decay of some
nuclei, e.g. nuclei of Uranium (92 protons)
• Accretion
– Material comes together
– 4.6 Billion years ago= age of Sun
– Cratered by impacts
Nuclear decay, e.g. fission
Isotopes of an element have different numbers of neutrons in their nuclei.
Importance of Radioactive Decay
• Primary source of heat in the
interior
– Number of protons = type of element
– Some nuclei spontaneously change, e.g.,
undergoing fission into two smaller
nuclei, and this also releases energy
(heat)
• Used to determine ages
– Half life - time it takes for half
the material to change
– As time passes one element
changes into another
Evolution of the Earth
• Differentiation
– Interior is molten
– Materials of different densities separate
– 4.5 Billion years ago
– Crust and core forms
• Crustal Formation
– Cooling and thickening of crust
– 3.7 Billion years ago
– Tectonic movement, volcanoes & mountains and ocean
basins
Shaping the Crust
• Plate Tectonics/Continental Drift
• Volcanic activity
• Impacts
• Erosion
Geological Evidence for an Old Earth
Cycles of erosion, sedimentation, rock formation, uplift
Plate Tectonics &
Continental Drift
• Continents are light and float
• Heat from the Mantle and Core
– Escape of heat drives convection
– New crust rises from mantle along spreading center
– Old crust is pulled down in subduction zones
• Process takes billions of years
• Evidence
Layers on layers... early geologists argued this process must take
millions or even billions of years.
Continental Drift
– Lines of mountains and volcanoes
– Magnetic striping along ocean spreading zones
– Motion of a few cm/year is now measured
Pangea
Alfred Wegener (1912)
Volcanic Activity
• Many volcanoes occur
in regions where the
crust is weakened by
continental drift.
• Some volcanoes are
due to hotspots caused
by rising Mantle
plumes: e.g. Hawaiian
island chain.
• It is not necessary to
have continental drift
to have volcanoes.
Impacts
• Objects strike the earth
• They create craters
• The craters are eroded
and eventually
subducted
• Since the surface of
the Earth is active it
has only a few young
craters left
Erosion
• Gravity naturally
flattens the surface into
a perfect sphere
• Wind, water and ice
slowly flatten mountains
and craters
• Takes hundreds of
millions of years
Question
• If the Earth was a geologically “dead” planet
with no plate tectonics or atmosphere what
would its surface look like today?