Midterm Review 1

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

* The definition of "Universe" • The universe is the totality of all space, time, energy, and matter o Therefore, the universe has no outside *The age and size of the Universe • 13.7 billion years old *The observable Universe • What we can see (there is a limit imposed by nature to how far we can see) *Light travel time and the light year • 1 light year = 1 year • 1 light year = almost ten trillion km • Looking far away means looking back in time *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 • Solar system  one or more planets orbiting a star • Galaxy  a collection of billions of stars all orbiting a common centre * Our location in the Universe • 1 AU = distance from Sun to Earth *Gravity and motion • All falling objects accelerate at the same rate • Speed  the rate at which an object moves (distance/time) o Gravity is the same for all falling objects, regardless of their mass • Velocity  speed AND direction • Acceleration  any change in velocity * Forces and Newton's Laws • Any object moves at constant velocity unless a net force acts to change its speed or direction • Force = mass x acceleration • For every force, there is an equal and opposite reaction force *Mass vs. weight • Mass  the amount of matter in an object • Weight  the force that acts upon an object * Free-fall • Weightlessness is due to a constant state of freefall o Nothing to stop acceleration due to gravity * Momentum • Mass x velocity • Angular momentum = mass x velocity x radius (rotation of spinning object) * Conservation of energy and types of energy • 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 • Kinetic (motion) • Radiative (light) • Potential / stored o Energy can change type, but it cannot be created or destroyed * Mass-energy equivalence 2 • E = mc • Mass itself is a form of potential energy * Kepler's Laws, especially the third law • Orbits are ellipses o Planets move in elliptical orbits, the Sun at one focus • The law of equal areas o As a planet moves around its orbit, it sweeps out equal areas in equal times 2 3 2 • P = A (period in years) = (average distance form the Sun in AU) 3 o More distant planets orbit the Sun at slower average speeds *Newton's laws of gravity • Every mass attracts every other mass • Attraction is directly proportional to the product of their masses • Attraction is inversely proportional to the square of the distance between their centres *The nature of light • 1 watt = 1 joule/s • Light is a form of energy *Colours, wavelengths, and frequencies • White light is made up of many different colours • Wavelength  the distance between two peaks (or troughs) • Frequency  the number of times per second that a wave vibrates up and down • Wave speed = wavelength x frequency * The electromagnetic spectrum • Human eyes cannot see most forms of light • The entire range of wavelengths of light * The interaction of light and matter, especially on an atomic scale • Determines the appearance of everything we see • Emission • Absorption • Transmission • Reflection or scattering o Mirror reflects light in a particular direction o Movie screen scatters light in all directions * Properties of atoms • Particles of light are called photon
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