ASTR 1 - 15:04:00 Last time Kepler Newton Newtons Laws of...

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15/01/2009 15:04:00 Last time: Kepler – Newton Newton’s Laws of Motion Law of universal gravitation Explained data o Predictions of other planets o Comets o Generalized Kepler III Philosophical Shift Copernican Principle Principle of Mediocrity Cosmological Principle Earth does not occupy any special position in the universe. Laws of physics are the same everywhere in the universe What is Light? Light is some kind of energy transfer How is it transferred? What mechanism?
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Two Mechanisms of Energy Transfer Particles carry energy o Matter- carries energy e.g. thrown chalk Waves o Energy transfer with no net motion of matter o Wave in water – matter returns to original position after wave passes Describing Waves (see drawing) Consider a simple wave Lambda = wave length, distance between successive crests/troughs Wave speed, V (m/s) o Speed of crests/troughs o “Local experience” Frequency, f = number of complete up and down motions in one  second, cycle F measures in “cycles per second” 1 cycle per second = 1 hertz Period of a wave, T o The Time for one cycle (measured in seconds) o Relationships T = 0.1 seconds, f = 10 Hz F = 1/T o V = distance/time = lambda/T o Example:  f = 264 Hz (Middle C)
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Lambda = v/f Lambda = 330 m/s/264 = 330 m/s = .3m = 4 ft. V = speed of sound Intuition about waves o Diffraction Amount of sp reading depends on lambda/D o Fuzziness factor o Wave spreads a lot “No sharp shadow” if lambda/D > 1 o If lambda/d < 1, shadow is sharper, less diffraction Collisions o Particles head on collision and rebound o Waves pass through each other, as if the other wasn’t there o What happens at collision Superpose effects add up Momentarily large wave, constructive interference Momentarily cancel, destructive interference Waves in tank (see picture) Particles o Think of two lawn sprinklers alternating wet and dry zones do not happen! Everything twice as wet
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Up and down amplitude when hit two pitches Sound is a Wave:  Beats What about light? o It is a form of energy transfer o How fast?  V = ? o How was it measured? Early recorded attempt to measure speed of light o Galileo- Shuttered lanterns on widely separated hills o Signaling o Failure:  Light is too fast, all he measured was reaction time Olaus Romer – 1675 (Danish) o Accurate measurement of the speed of light o Astronomers were making accurate tables of the rising and setting of  Jupiter’s moons (Galilean satellites)  Accurate calibration of clocks on ships o
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