5.1.4 interference, standing waves, and resonance
DESCRIPTION
Introduction and Terms INTERFERENCE – the result of two waves meeting in a medium. CONSTRUCTIVE – results in greater amplitude. DESTRUCTIVE – results in lower amplitude. “In phase” interferes CONSTRUCTIVELY “180° out of phase” interferes DESTRUCTIVELYTRANSCRIPT
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5.1.4 Interference, Standing Waves, and Resonance
Guitar Strings and Crumbling Bridges
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Introduction and Terms• INTERFERENCE – the result of two waves meeting in a
medium.– CONSTRUCTIVE – results in greater amplitude.– DESTRUCTIVE – results in lower amplitude.
• “In phase” interferes CONSTRUCTIVELY
• “180° out of phase” interferes DESTRUCTIVELY
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ExampleWhat is the direction of motion in the medium?
D.Both waves reach maximum
amplitude at that point!
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Interference of Two Point Sources of WavesWhen viewed from above, a wave source
makes circular patterns
crest (wave front)
trough
TWO TROUGHS CONSTRUCTIVE
TWO CRESTSCONSTRUCTIVE
CRESTS AND TROUGH DESTRUCTIVE
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Phenomenon #3 – Standing Waves
• When a wave encounters a fixed boundary it REFLECTS.
• The reflected wave comes back through the original wave and they INTERFERE.
• The result is a STANDING WAVE. • NODES : always 180° out of phase (destructive
interference) - NO MOTION.• ANTINODES : alternate between in-phase and 180° out of
phase –MAX MOTION.
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Practice
FOUR
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Phenomenon #4 – Resonance• All mediums have a NATURAL FREQUENCY that
corresponds to their atomic structure.
• Exciting this frequency causes large AMPLITUDE vibrations in the medium
• If the frequency is excited with enough ENERGY the medium may become damaged or even shatter.
• If two materials have the same (or close) natural frequencies then vibrations may be passed from one material into the other.
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Practice• The grid below represents a 10 meter long string.
– Sketch the standing wave that this string would produce if it were to have SIX nodes.
– Draw a circle around each ANTINODE on the string.– Determine the wavelength of this standing wave.
_________________m– Assuming that this wave moves at 2.0 meters per second,
calculate its frequency and period.
4
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End of 5.1.4