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Lecture 19

PHY 2020 Lecture 19: Chapter 19

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PHY 2020

Chapter 19- Vibrations and Waves - A vibration is a wiggle in time - Figure 19.1: If you drop two balls of different mass, they accelerate at g. Let them slide without friction down the incline, and they slide together at the same fraction of g. Tie them to strings of the same length so that they are pendulums, and they swing to and fro in unison. In all cases, the motions are independent of mass. o A heavy person and a light person swing to and from on swings of the same length. Who has the longer period? ▪ They have the same period; mass doesn’t matter. - FORMULA SHEET: o Period= 1/ frequency o Frequency=1/period - Figure 19.2 - Wave speed= wavelength/period - Wave speed= frequency * wavelength o Wont change with air #47. If a pendulum is shortened, does its frequency increase or decrease? What about its period? - Frequency will increase; period will decrease. # 49. You pick up an empty briefcase and let is swing to and fro at its natural frequency. If the case were filled with books, would the natural frequency be lower then, higher than or the same as before. - The frequency is the same because mass is not dependent on frequency. #51. What happens to the period of a wave when the frequency decreases? - Period increases as frequency decrease. #52. What happens to the wavelength of a wave when the frequency decreases? - As the frequencies decrease, the wavelengths get larger o Wave speed= frequency * wavelength. - Transverse wave: a wave in which the medium vibrates perpendicularly to the direction in which the wave speed travels. - Longitudinal wave: a wave in which the medium vibrates parallel to (along) the direction in which the wave travels. - Figure 19.6: Both waves transfer energy from left to right. (a) When the end of a Slinky is pushes and pulled rapidly along its length, a longitudinal wave is produced. (b) When it’s shaken sideways a transverse wave is produced. - Figure 19.7: P and S waves generated by an earthquake. o P waves are longitudinal and travel solid and liquid materials. S wave is transverse and travel only through solids. - Figure 19.10: Constructive and destructive interference in a transverse wave. o First equation- constructive o Second wave- destructive - Figure 19.11: Two sets of overlapping water waves produce an interference pattern. The left image is an idealized drawing of the expanding waves from the two sources. The right image is a photograph of an actual interference pattern. - Standing waves: a stationary interference pattern in a medium when two sets of identical waves pass through the medium in opposite directions. o Figure 19.13: (a) Shake the rope until you set up a standing wave of one segment (b) shake with twice the frequency to produce a wave with two segments. (c) Shake with three times the frequency and produce three segments. - Figure 19.15: Top view of water waves formed by a stationary bug jiggling in still water. The blue circles represent wavefronts in the expanding pattern. o DOPPLER EFFECT - Figure 19.17: The pitch (frequency) of sound is higher when a source is moving toward you, and lower when the source is moving away. o BIG BANG THEORY - Isn’t light a wave too? Can it experience the Doppler effect? o Yes; in space there are different wave/ wavelength. #74. A bat chirps as it flied toward a wall. Is the frequency of the echoed chirps its received higher than, lower
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