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Lecture

# November 6.doc

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School
Department
Physics
Course
PHYS 101
Professor
Kenneth Ragan
Semester
Fall

Description
Phys 101 Alanna Houston November 6, 2007 - Intensity = power/area = - Example: A particular guitar string is supposed to vibrate at 200 Hz but vibrates at 205 Hz. By what percent should the tension be changed to correct the frequency? o f ~ sqrt(F ) T o f newfold= sqrt(F TnewFTold = 200/205 = 0.976 o F Tnew/FTold (0.976) = 0.952 o –4.8% - Example: A tsunami of wavelength 250 km and speed 750 km/h (typical values) approaches a coastline, and the water level drops along the coast. How long do you have to get to safety? o Trough o ½ λ, ½ T o v = λf o 750 km/h = 250 km f o f = 3 h -1 o FINISH - Reflection, refraction, diffraction and interference are associated with ALL waves…not just light waves - Waves can be reflected by an object at the end of the medium: - o A sound wave echoes (bounces) off a rock wall o A water wave will reflect off a cliff-side o A wave on a rope will bounce off the end of a rope - Reflection occurs when the medium density changes - If a section of light rope turns to a section of heavy rope, there will be reflection and transmission at the interface - - Phys 101 Alanna Houston - When a wave on a string hits a boundary with a section of the string having a different mass, the wave frequency will be the same between the two sections of the string - In 2 or 3 dimensional waves, the angle of incidence = the angle of reflection - To measure these angles we use rays (line that cross wave fronts at right angles: - - When the velocities of the waves in the 2 media are different, we also have refraction - - Refraction towards the normal: high wave velocity to low wave velocity Superposition: - Two crests or two troughs add to give a new crest or trough that is the sum of the old ones (constructive interference) - A crest and a trough add to produce destructive interference
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