Midterm Study Notes

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Published on 20 Jun 2011
School
UTSG
Department
Astronomy & Astrophysics
Course
AST201H1
Professor
* The definition of "Universe"
The Universe is the totality of all space, time, energy, and matter. Therefore, the Universe
has no outside.
* The age and size of the Universe
The Univserse is about 13.7 billion years old. The size of the universe is a tough question to
answer. So, a better question to ask is how much of the universe can we see.
* The observable Universe
But, if the universe is 13.7 billion years old, the light from the galaxy would not have
reached you yet. It will take 86.3 billion years before the light reaches us.
Therefore, there is limit imposed by nature to how far we can see: observable universe vs.
unobservable universe. So, whenever we mention/refer to the term universe we will usually
mean the observable universe.
* Light travel time and the light year
Speed of light = c = 300 million m/s.
A light year is the distance light travels in one year: 1 ly = 9.46 x 1015 m or 9.46 trillion km
By definition of a light year, light takes 1 year to travel 1 light year and 100 billion years to
travel 100 billion light years.
Because light takes time to travel to us, looking far away means literally looking back in
time. For example, the distance to the nearest large galaxy is about 2.5 million light years,
we see it the way it was 2.5 million years ago.
* The contents of the Universe
Planets: spherical bodies which orbit stars directly; can be rocky, gassy, liquid, or icy.
Stars: enormous spheres of very hot gas which produce energy by nuclear fusion. The Sun is
one.
Solar System: one or more planets orbiting a star.
Galaxy: a collection of billions of stars all orbiting a common center.
* Our location in the Universe
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Earth is in the Milky Way Galaxy, Our solar system is somewhere in the middle (between
the center and outer edge of the solar system). Our nearest neighbor galaxy is the
Andromeda Galaxy. Both Andromeda and Milky Way are spiral galaxies.
The Milky Way, Andromeda and a few dozen small galaxies make up the Local Group of
galaxies, all of which orbit one another. All the galaxy clusters in the Universe are moving
apart from one another.
Enormous group of galaxies all orbiting one another are called galaxy superclusters. They
may have 1000s or millions of member galaxies. Our supercluster is called the Virgo
because many of its galaxies appear to us in the direction of the constellation Virgo.
*Science
Science is the process of trying to find natural explanations for natural phenomena by
repeatedly following a cycle of hypothesis, prediction, and observation or experiment.
For a proposition to be scientific, it must be falsifiable. A falsifiable proposition is one which
could, by experiment, be proven false (if indeed it is false).
* Gravity and motion
Speed: rate at which object moves, Example: 10 m/s
Velocity: speed and direction, Example: 10 m/s, due east
Acceleration: Any change in velocity units of speed/time (it may involve a change of
direction, not speed per se”)
All falling objects accelerate at the same rate (not counting friction of air resistance). On
Earth g = 10 m/s2 i.e. speed increases by 10 metres every second of falling.
Galileo showed that g is the same for all falling objects, regardless of their mass. Newton
discovered the laws of motion and gravity. He realized that the same physical laws that
operate on Earth also operate in the heavens (one universe).
* Forces and Newton's Laws
Newtons three laws of motion:
1)An object moves at constant velocity unless a net force acts to change its speed or
direction. Example: spacecraft spend most of their coasting (no engines running).
2)Force = mass x acceleration. Example:
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3)For every force, there is always an equal and opposite reaction force. Example:
Rocket trying to go up, the fuel burning is in the opposite direction.
* Mass vs. weight
Mass the amount of matter in an object
Weight the force that acts upon an object
* Free-fall
You are weightless in free-fall. There is gravity in space, weightlessness is due to a constant
state of free-fall.
* Momentum
Momentum = mass x velocity
A net force changes momentum, which generally means acceleration (change in velocity).
Rotational momentum of a spinning or orbiting object is known as angular momentum. For
a particle moving in a circle with a given radius: Ang. Momentum = mass x velocity x
radius
Conservation of Momentum - The total momentum of interacting objects cannot change
unless an external force is acting on them. Interacting objects exchange momentum
through equal and opposite forces.
Conservation of Angular Momentum the angular momentum of an object cannot change
unless an external twisting force (torque) is acting on it: therefore, angular momentum
stays constant i.e. increase of radius equals decrease in velocity. It also explains why
objects rotate faster as they shrink in radius.
Earth experiences no twisting force as it orbits the Sun, so its rotation and orbit will
continue indefinitely [not exactly true: tidal forces of Sun and Moon exert small changes
over time]
* Conservation of energy and types of energy
Energy makes matter move. Energy is conserved, but it can:
- Transfer from one object to another
- Change in form
Types of energy:
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Document Summary

The universe is the totality of all space, time, energy, and matter. * the age and size of the universe. The univserse is about 13. 7 billion years old. The size of the universe is a tough question to answer. So, a better question to ask is how much of the universe can we see. But, if the universe is 13. 7 billion years old, the light from the galaxy would not have reached you yet. It will take 86. 3 billion years before the light reaches us. Therefore, there is limit imposed by nature to how far we can see: observable universe vs. unobservable universe. So, whenever we mention/refer to the term universe we will usually mean the observable universe. * light travel time and the light year. Speed of light = c = 300 million m/s. A light year is the distance light travels in one year: 1 ly = 9. 46 x 1015 m or 9. 46 trillion km.