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Chapter 19
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What are Vibrations, Waves, and Sound?
• Vibration is another word used for a back and forth motion.
• Waves carry energy from place to place without the transfer of matter.
• Sound is energy that objects produce when they vibrate.
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Repetitive motion makes rhythm.
• Back and forth motion like:
• rocking chair
• pendulum of grandfather clock
• ????
• Circular motion like:
• earth’s orbit
• ferris wheel
• cooling fan
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What is Harmonic Motion?
• Harmonic motion includes motion that goes around and around or repeats.
• Ex. bicycle tire, swing• Objects or systems that make
harmonic motions are called oscillators.
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What is oscillation?• The word oscillation means a motion that
repeats regularly. • A system with harmonic motion is called an
oscillator. • Ex.- a pendulum is an oscillator • Ex.- your heart and its surrounding
muscles. • Ex.-our solar system is a large oscillator
with each planet in harmonic motion around the sun.
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What is Vibration?
• Vibration is another word used for back and forth motion.
• People tend to use “vibration” for motion that repeats fast and “oscillation” for motion that repeats more slowly.
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Music is sound vibrating and oscillating in harmonic motion.
• Sound is a traveling vibration of air molecules. Musical instruments and stereo speakers are oscillators.
• The speaker on a stereo has cyclic back-and-forth motion, pushes and pulls on air, creating tiny oscillations in pressure.
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Understand how music affects your ear.
• The pressure oscillations travel to your eardrum and cause it to vibrate.
• Vibrations of the eardrum move tiny bones in the ear setting up more vibrations that are transmitted by nerves to the brain.
• There is harmonic motion at every step of the way, from the musical instrument’s performance to the perception of sound by your brain.
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Your cell phone and radio relies on harmonic motion.
• A cell phone uses an electronic oscillator that makes more than 100 million cycles each second.
• When you tune into a station at 101 on the FM (frequency modulation) dial, you are actually setting the oscillator in your radio to 101,000,000 cycles per second.
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Describe harmonic motion• Period is the time to complete one cycle.• Frequency is the number of cycles per second. • “FM” stands for frequency modulation on the
radio. It uses frequencies between 95 million and 107 million cycles per second.
• Your heartbeat has a frequency between one-half and two cycles per second.
• The musical note “A” has a frequency of 440 cycles per second.
• The human voice contains frequencies mainly between 100 and 2,000 cycles per second.
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Unit of Measure for a Cycle• Hertz is the unit of one cycle per second.
• You hear music when the frequency of the oscillator in your radio exactly matches the frequency of the oscillator in the transmission tower connected to the radio station. A radio station dial set to 101 FM receives music broadcast at a frequency of 101,000,000 hertz or 101 megahertz.
• Your ear can hear frequencies of sound in the range from 20 Hz to between 20,000 Hz.
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Define Amplitude• You know the period is the time to complete a
cycle. • The amplitude describes the “size” of a cycle.• With mechanical systems (such as a pendulum),
the amplitude is often a distance or angle.
• How do you measure amplitude?• The amplitude is the maximum distance the
oscillator moves away from its equilibrium position. For a pendulum, the equilibrium position is hanging straight down in the center.
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What properties of a system determine whether its motion will be linear motion or harmonic motion?
• Linear Motion- goes in a straight line
• Ex. If you set a wagon on a hill and let it go, the wagon rolls down and does not come back. So this is not harmonic motion.
• Harmonic motion goes back and forth.
• Ex. push a child on a swing, the child goes away from you at first, but then comes back. The child on the swing shows harmonic motion.
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Systems maintain harmonic motion
By always moving back and forth around a central or equilibrium position ( zero net force).
• Equilibrium is maintained by restoring forces such as gravity pulling the swing back toward equilibrium.
• Inertia- object in motion will stay in motion
• Mass
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Define Resonance• Resonance occurs when:
• there is a system in harmonic motion, like a swing;
• there is a periodic force, like a push;
• the frequency of the periodic force matches the natural frequency of the system.
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