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Astronomy Chapter 4 notes

by: Lisa Render

Astronomy Chapter 4 notes PHYS 1350

Marketplace > University of Nebraska at Omaha > Physics > PHYS 1350 > Astronomy Chapter 4 notes
Lisa Render
GPA 3.578

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4.1 Describing Motion Examples from Daily Life 4.2 Newton's Laws of Motion 4.3 Conservation Laws in Astronomy 4.4 The Force of Gravity
Principles of Astronomy
Charles St Lucas
Class Notes
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This 5 page Class Notes was uploaded by Lisa Render on Wednesday October 5, 2016. The Class Notes belongs to PHYS 1350 at University of Nebraska at Omaha taught by Charles St Lucas in Fall 2016. Since its upload, it has received 3 views. For similar materials see Principles of Astronomy in Physics at University of Nebraska at Omaha.


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Date Created: 10/05/16
4.1 Describing Motion: Examples from Daily Life Monday, September 12, 2016 1:01 PM Chapter 4 Making Sense of the Universe • Understanding Motion,Energy, and Gravity • Review ○ Pattern in the sky ○ Geocentricsolar system ○ Epicycles ○ Heliocentric solar system ○ Elliptical orbits ○ Mountains on the moon How do we describe motion? • Precise definition ○ Speed  Rate at which object moves  ○ Velocity  Speed and direction  Ex. 10 m/s, due east ○ Acceleration  Any chance in Velocity  I.e. change in speed and/or direction 2  Units of speed/time(m/s ) • Acceleration of Gravity ○ All falling objects accelerate at the same rate (not counting friction of air resistance) ○ On earth, g = 10 m/s 2  Speed increases 10 m/s with each second of falling ○ Galileo showed that g is the same for all falling objects regardless of their mass  On earth, air resistance makes a feather fall slower than a hammer • Momentum and force ○ Momentum = mass x velocity  Ex. A bus can have a large momentum even if it is moving very slowly, because it has a large mass ○ A net force changes momentum  Which means change in velocity (acceleration)  Ex. Two football players pushing against each other ○ Zero net force implies constant velocity,& vice versa ○ The "rotational momentum"of a spinning or orbiting body is known as angular momentum ○ Force is a push or pull 4.2 Newton's Laws of Motion Monday, September 12, 2016 1:19 PM How did Newton change our view of the universe? • Aristotle ○ Universe has 2 different parts ○ Theory of motion  Heavenly spheres of earth, water, fire, air, and the heavenly bodies • Sir Isaac Newton (1642-1727) • He proved that the same physical laws that operate on Earth also operate in the heavens -> one universe ○ According to Aristotle, earth and heavens obeyed different laws • He discovers 3 laws of motion and 1 law of gravity • Experimentedwith light, created first reflecting telescope,calculus… What are Newton's three laws of motion? 1. An object moves at a constant velocity unless a net force acts to change its speed or direction ○ Space is like a frictionless bowling lane ○ Gravity is a force 2. Force = mass x acceleration ○ ○ So big masses are harder to slow down 3. For every force, there is always an equal and opposite reaction force ○ When any two objects interact, each exerts a force on the other ○ These two forces are called "action" and "reaction" How is mass different from weight? • Mass ○ The amount of matter in an object ○ Quantity of matter ○ Mass is a built-in property of the body • Weight ○ The force that acts on an object to support it against gravity ○ Force supporting a mass ○ Weight can be changed easily • In free-fall, you are weightless • In space, we are weightless but we still have mass ○ The is gravity in space ○ A state of free fall implies weightlessness(or zero support) 4.3 Conservation Laws in Astronomy Wednesday, September 14, 2016 1:12 PM What keeps a planet rotating and orbiting the Sun? 1. Earth needs no push 2. Angular momentum,conserved ○ Whenever you have a torque or rotationon a curve ○ Angular movement= mass x velocity x radius  If radius is big, momentumis slower  If radius is bigger, momentumis faster  Ice skater physics ○ The angular momentumof an object cannot change unless an external twisting force (torque) is acting on it ○ Earth experiences no twisting force as it orbits the sun so it's rotation and orbit with continue indefinitely What is energy? • Energy makes matter move • Energy can change type but cannot be destroyed ○ Cannot be created or destroyed ○ Can be transferred, converted,moved • Basic types of energy 1. Kinetic (motion)  thermal  Thermal energy (micro motion) □ The collective kinetic energy of many particles  Ex. In a rock, air, water □ Related to temperature  (on the Kelvin scale) reflects the average kinetic energy per particle in a substance □ Kelvin  Absolute zero (zero kinetic energy) ◊ 0 degrees K ◊ -273.15 degrees C ◊ -459.67 degrees F ◊ All particles are at rest with zero kinetic energy  No maximum □ Total kinetic particles  Hot air (gas) vs. hot water  Electrical  Sound  Wind  Light 2. Radiative (light) 3. Stored or potential  E = mc ("mass-energy")  Gravitational,chemical, electrical, elastic, nuclear ○ Gravitational (stored)  Depends on □ Objects mass □ Strength of gravity □ The distance an object could potentially fall □ The distance an object could potentially fall  In space, an object or has cloud has more gravitational energy when it is spread out than when it contracts □ A contracting cloud converts gravitational potential energy into thermal energy □ This is how stars form and becomevery hot ○ Mass-energy (potential)  Mass itself is a form of potential energy   C = speed of light (300,000,000m/sec)  A small amount of mass can release a great deal of energy  Concentrated energy can spontaneouslyturn into particles (for example, in particle accelerators) ○ Conservation of energy  Energy can be neither created nor destroyed  It can change form or be exchanged between objects  The total energy content of the universe was determined in the Big Bang and remains the same today 4.4 The Force of Gravity Monday, September 19, 2016 1:15 PM What determines the strength of gravity • The Universal Law of Gravitation 1. Every mass attracts everyother mass 2. Attraction is directly proportionalto the product of their masses 3. Attraction is inversely proportional to the square of the distance between their centers  Inverse squared law =  D = distance between M and M 1 2 • Newton's Law of Gravity ○ Force of gravity decreases with the square of the distance Why doesn’t gravity pull all bodies together? • Orbital energy ○ Sum of kinetic and gravitational potential energy • Orbit ○ Gravitation infall + force-free motion How does Newton's law of gravity extend Kepler's laws • Kepler's first two laws apply to all pairs of orbiting objects, not just planets ○ Third law needs to be revised • Ellipses are not the only orbital paths. Orbits can be: ○ Bound (ellipses) ○ Unbound (parabola, hyperbola) • Newton's version of Kepler's third law ○ ○ P = orbital period (in years) ○ A = average orbital distance (between centers) (in AU) ○ (M1 = M2) = sum of object masses (relative to mass of the Sun) ○ If a small object orbits a larger one and you measure the orbiting object's orbital period (p) and average orbital distance (a) then you can calculate the mass of the larger object How do gravity and energy together allow us to understand orbits • Total orbital energy (gravitational potential = kinetic) stays constant • Angular momentumalso stays constant • Orbits cannot change spontaneously • What can you do to make an object gain or lose orbital energy ○ Friction or atmosphericdrag ○ A gravitational encounter (e.g. of a comet with Jupiter)  Jupiter has magnetized many comets that could've hit the earth • Escape velocity ○ If an object gains enough orbital energy, it may escape (change from a bound to unbound orbit) ○ Escape velocity from earth = 11km/sfrom sea level  Vs. circular orbit near Earth = 8km/s How does gravity cause tides • The moon's gravity pulls harder on near side of earth than on far side • The difference in the moons gravitationalpull stretches earth, both its rocky and watery parts • Moon also gets stretched by gravity of earth ○ For Io, a moon of Jupiter, this big tidal effect melts it's interior, and creates lots of volcanic activity • Tides and phases ○ Size of tides depends on the phase of the Moon  I.e. tides maximal (when sun and moon aligned); tides minimal (when they pull of earth at 90 degrees too each other)


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