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Lecture 11

AS101 Lecture 11

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Wilfrid Laurier University
Patrick Mc Graw

Lecture 11 3/25/2013 9:10:00 AM Our Window on the Universe: Near Earth Asteroids:  Rocky objects orbiting the sun at a distance similar to earths orbit.  Objects do not stay in such orbits for very long‖ within a few million years they are likely to be kicked into another orbit by gravitational perturbations.  Most came from the asteroid belt (between mars’s and jupiter’s orbits) fairly recently. Meteoroid:  A small rocky object (generally less than 10m across) in the solar system. Meteor:  The visible phenomenon of a meteoroid or small asteroid entering the earth’s atmosphere, glowing as it heats up. Meteorite:  A rock (formerly a meteoroid or fragment of an asteroid) that has fallen to the ground from outer space. (most objects smaller than 10m vaporize completely in the atmosphere. Tides: Another Consequence of Gravity:  Newton’s Law of Universal Gravitation: o Any two object attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centres.  F = G M1M2/r^2 o That means that, for example, the moon exerts a greater force on a piece of earth close to the moon than on the centre, or the opposite side.  Tides are mostly noticeable in the oceans, but the earth’s crust itself also bulges slightly (about 10-50cm) in response to tidal forces.  Over time, tidal bulges can affect an objects rotation.  The synchronous rotation of the moon (same side always faces the earth) is probably a result of ―tidal locking.‖ Light: Our Messenger From the Universe:  What is light: o The warmth of sunlight reminds us that light is a form of energy o Energy can take many forms and can be converted from one form to another o Some other forms of energy: kinetic energy (energy of motion), heat, sound, chemical energy, etc… o When light from th sun warms the ground or your skin, light energy is being converted into heat. Solar power plants convert light into electrical energy. Green plants convert light into chemical energy through photosynthesis.  Light is a form of energy: o Units of energy: joules o 1 Joule of energy is enough to:  start a 2-kg object moving at 1m/s  heat of 1 gram of water by approx. 0.25 degrees C o Watts are a unit of power: energy per unit time.  1 watt = 1 joule/sec. so:  a 100W light bulb is using 100 joules per second (only part of that energy is actually emitted as light—the rest of it mostly becomes heat) Quantifying Light Intensity:  intensity of light can be measured in W/M^2  what does this mean?  Well, if the intensity of light from a source is measured to be 1/W/m^2, then that means the amount of energy falling every second on every square meter of surface is 1 joule (if that surface is perpendicular to the direction the light is coming from)  The intensity of light is related to its brightness.  To summarize: light involved a flow of energy. intensity tells you how much energy is passing through each unit of perpendicular area each second.  But if a surface Is at an angle, then the same energy is spread over a larger area. Inverse Square Law (for light intensity):  As you get farther from the source, the same energy gets spread over a larger area.  That means that the intensity decreases at you get farther away from the source.  Stars that are farther away look dimmer than they would up close.  Mathematically speaking, light acts like gravity: if you double the distance, the intensity becomes ¼ as much—triple the distance, intensity becomes 1/9 as much, etc.  This is because if you double the distance, the same amount of light energy gets spread over four times as much area.  That’s the same type of mathematical behaviour as in Newton’s law of gravity (gravity gets weaker as distance increases but never really goes away.) Light: wave or Particle?  1670, Hooke, Huygens: light behaves like a wave.  1700, Newton: light consists of particle (corpuscles)  1800, Young, Fresnel: showed wavelike properties of light (interference)  1873, Maxwell: identified light as electromagnetic waves  1905, Einstein: light consists particles of energy called ―photons‖  so is light made of particles or waves?  1920’s-30’s: quantum Mechanics: light behaves as both, particles and wave. Which properties are apparent depends on how you observe it.  Is light a wave or a
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