Chapter 25: Vibrations and Waves
What is a wiggle in time called? A vibration
What is a wiggle in space and time called? A wave
Vibration of a Pendulum The time for one complete forward and back swing is called the period.
Simple Harmonic Motion Back and forth vibratory motion of a pendulum Pendulum bob filled with sand (Oppenheimer, page 373)
Calculating the Period frequency
Wave Description Sine curve
Parts of a Wave
Frequency of a Wave How frequent or how often a vibration occurs Measured in cycles/second or Hertz (Hz) 1-cycle/second = 1-Hz frequency = 1 period
What is the frequency in Hz of a wave vibrating at 75-cycles/second?
AM and FM Radio Frequencies AM = kilohertz kilo = 1,000 FM = megahertz mega = 1 million
Wave Speed wave speed = wavelength x frequency What is the speed of a wave that has a wavelength of 2 meters and a frequency of 4 Hz? 8 m/s
Wave Speed (cont’d) Light waves travel much faster than sound waves 300,000,000 m/s Sound waves = 340 m/s (average)
Wave Motion When energy is transferred by a wave from a vibrating source to a distant receiver, there is no movement of matter between the two points. To put it simply, only the disturbance/energy is transferred, not the medium (stuff).
Transverse Waves Medium moves at right angles to the direction in which the wave travels Ocean, Light (electromagnetic waves)
Longitudinal Waves Medium moves parallel (same direction) to the direction in which the wave travels. Sound waves
Interference of Waves Occurs when two waves are at the same place at the same time Two types of interference: Constructive Interference Destructive Interference
Constructive Interference Occurs when waves are in phase Crest + Crest or Trough + Trough Amplitude increases
Constructive Interference
Destructive Interference
Standing Waves Standing waves are caused by interference They appear to be “standing still” Reflected wave interferes with the incoming wave and they’re in phase
A Standing Wave
Standing Waves The high points are called antinodes. They’re caused by constructive interference. The low points are called nodes and they’re caused by destructive interference.
The Doppler Effect Movement is needed, either the source of the sound or the observer. Frequency/pitch increases as the source gets closer to you and it decreases when the source moves away.
Bow Waves and Shock Waves
Bow Waves Two-dimensional Produced by a boat (bug, etc.) when it’s moving faster than the waves it’s producing.
Shock Waves Three-dimensional cone Results in a sonic boom due to high pressure