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Astronomy & Astrophysics
Michael Reid

AST 201 Midterm Part 1 * The definition of "Universe" The universe is the totality of all space, time, energy, and matter. * The age and size of the Universe 13.7 billion year old; the universe has no outside * The observable Universe * Light travel time and the light year 1 light year almost 10 trillion km A light year is the distance light travels in one year. * The contents of the Universe Planet, star, solar system, galaxy, super cluster, * Our location in the Universe Earth, Solar System, Milky Way Galaxy, Local Group, Virgo Super cluster Summary from slides •The Universe is the totality of everything •The Universe is 13.7 billion years old, but we haven’t talked about why or how we know this •We measure astronomical distances in light years •A light year is almost ten trillion km •Looking far away means looking back in time On Slides: Science is falsifiable. Part2 Chapter 4 Understanding Motion, Energy, and Gravity * Gravity and motion Motion: 1 speed: rate at which object moves  speed = distance/time 2 velocity: speed and direction 3 acceleration: any change in velocity units of speed/time Gravity: all falling objects accelerate at same rate of g = 10m/s(square)  speed increases 10m/s with each second of falling regardless of their mass * Forces and Newton's Laws 1 law: an object moves at constant velocity unless a net force acts to change its speed or direction nd 2 law: force = mass x acceleration 3 law: for every force, there is always an equal and opposite reaction force On Slides: Newton changed our view of the universe by discovered laws of motion and gravitation; realized these same laws of physics were identical in the universe and on Earth. * Mass vs. weight Mass: the amount of matter in an object  never changes!! Weight: the force that acts upon an object  objects are weightless in free-fall(nothing to stop acceleration due to gravity) * Free-fall Falling without any resistance to slow the object down  free-fall makes one weightless Question: why are astronauts weightless in space?  there is gravity in space  weightlessness is due to a constant state of free-fall * Momentum Momentum = velocity * mass Only way to change an object’s momentum is to apply a force on it. Generally acceleration could change in velocity and change momentum. Angular momentum = velocity * mass * radius  for a particle moving in a circle Question ask for no force  find the option with no acceleration  no change in direction or velocity On Slides: Conservation of momentum 动能守恒 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 Angular momentum = mass x velocity x radius The angular momentum of an object cannot change unless an external twisting force is acting on it: therefore angular momentum stays constant, i.e. increase of radius decrease of velocity 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 can be neither created nor destroyed, but 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. Types: kinetic (motion) radiative (light) potential (stored) Thermal energy: the collective kinetic energy of many particles; is related to temperature and density  temperature is the average kinetic energy of the many particles in a substance Gravitational potential energy: on earth, depends on m/g/distance object could potentially fall in space, an object or gas could has more gravitational energy when it is spread out than when it contracts  a contracting cloud converts gravitational potential energy to thermal energy * Mass-energy equivalence E = mc 2 E-energy released/absorbed M-mass lost/gained C-speed of light Summery from slides: • Why do objects move at constant velocity if no force acts on them? – Conservation of momentum • What keeps a planet rotating and orbiting the Sun? – Conservation of angular momentum • Where do objects get their energy? – Conservation of energy: energy cannot be created or destroyed but only transformed from one type to another. – Energy comes in three basic types: kinetic, potential, radiative. C 4.4 * Kepler's Laws, especially the third law 1 orbits are ellipses nd 2 the area swept out in 30-day periods are all equal  near perihelion bigger v; near aphelion smaller v (Result of/ a statement of conservation of angular momentum) 3 P 2 =A3 P- period (year); A- average distance from Sun(AU) From slides: Kepler's Laws For planets in the Solar System: 1. Planets move in elliptical orbits, the Sun at one focus. 2. As a planet moves around its orbit, it sweeps out equal areas in equal times. 3. More distant planets orbit the Sun at slower average speeds, precisely obeying: (Period in years)2 = (average distance from Sun in AU)3 On slides: Universal Law of Gravitation: 1 every mass attract every other mass 2 attraction is directly proportional to the product of their masses 3 attraction is inversely proportional to the square of the distance between their centers. * Newton's law of gravity his laws of gravity and motion showed that the relationship between the orbital period and average orbital distance of a system tells us the total mass of the system.  Newton’s version of Kepler’s Third Law for 2 bodies of comparable mas A = G (M +1 )P2/(42 2) Summery from slides: • What determines the strength of gravity? – Directly proportional to the product of the masses (M x m) – Inversely proportional to the square of the separation • How does Newton’s law of gravity allow us to extend Kepler’s laws? – Applies to other objects, not just planets. – Includes unbound orbit shapes: parabola, hyperbola (e.g. Orbits of comets) – Can be used to measure mass of orbiting systems. Chapter 5 Light and Matters * The nature of light Electromagnetic waves On slide: Particles of light are called photons. The energy of a photon depends on its frequency. * Colours, wavelengths, and frequenc
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