© John Parkinson 1 VIBRATIONS VIBRATIONS & & RESONANCE RESONANCE
Dec 25, 2015
©John Parkinson
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VIBRATIONSVIBRATIONS
&&
RESONANCERESONANCE
©John Parkinson
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Natural Frequency / Free Vibrationsthe frequency at which an elastic system naturally tends to vibrate, if it is displaced and then released
The natural frequency of a body depends on its elasticity and its shape.
At this frequency, a minimum energy is required to produce a forced vibration.
Free vibration is the vibration of an object that has been set in motion and then left.
©John Parkinson
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Forced vibrations
are the result of a vibration caused by the continuous application of a repetitive
forceUnless the forcing frequency is equal to the
natural frequency, the amplitude of oscillation will be small.
e.g. a swing pushed at “the wrong frequency”
©John Parkinson
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the result of forced vibrations in a body when the applied frequency matches
the natural frequency of the body
The resulting vibration has a high amplitude --
and can destroy the body that is vibrating.Resonance allows energy to be transferred
efficiently
©John Parkinson
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ON NOVEMBER, 7 1940 THE TACOMA NARROWS BRIDGE IN WASHINGTON STATE WAS BUFFETED BY 40 MPH WINDS
AT APPROXIMATELY 11:00 AM, IT COLLAPSED DUE TO WIND-INDUCED VIBRATIONS
http://www.civeng.carleton.ca/Exhibits/Tacoma_Narrows/
http://www.glendale-h.schools.nsw.edu.au/faculty_pages/ind_arts_web/bridgeweb/commentary.htm
WATCH A VIDEO AT
OR AT
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Other Resonance Examples
RUMBLE STRIPS Wheels hit the strips at regular time intervals as the car travels at a steady speed and this makes the suspension resonate so the car vibrates with a larger and larger amplitude and makes the driver slow down.
Bus windows
At low engine revs the windows natural frequency can be the same as that of the engine.
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Tuning Circuit
The circuit contain the coil and the capacitor resonates to a certain frequency of AC that is picked up in the aerial.
The variable capacitor enables different frequencies to be received
A wine glass can be broken by a singer finding its resonant frequency
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DRIVER FREQUENCY IN PURPLE
DRIVEN FREQUENCY IN ORANGE
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applied frequency
amp
litu
de
Resonant frequency f0
RELATIONSHIP BETWEEN AMPLITUDE AND DRIVER
FREQUENCY
LIGHT DAMPING
HEAVY DAMPING
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Phase lag in degrees
f0
0
180
90
Applied frequency
Phase lag of the driven system behind the driver frequency
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Damping
Damping is the term used to describe the loss of energy of an oscillating system(due to friction/air resistance/ elastic hysteresis etc.)
slight damping
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time
DAMPING
DISPLACEMENT
INITIAL AMPLITUDE
THE AMPLITUDE DECAYS EXPONENTIALLY WITH TIME
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With Critically Damped motion the body will return to the equilibrium in the shortest time - about T/4.
Heavy damping or overdamping
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Longitudinal Waves
• Each point or particle is moving parallel or antiparallel to the direction of propagation of the wave.
• Common examples:- Sound, slinky springs sesmic p waves
• Longitudinal waves cannot be polarised
Direction of travel
VIBRATION
©John Parkinson
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A longitudinal sound wave in air produced by a tuning fork
Observe the compressions and rarefactions
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transverse wave
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Transverse
• Each point or particle is moving perpendicular to the direction of propagation of the wave.
• Common examples:- Water, electromagnetic, ropes, seismic s waves
• You can prove that you have a transverse wave if you can polarise the wave (especially important with light (electromagnetic) as you cannot “see” the wave!!)
Direction of travel
vibration
©John Parkinson
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Formation of a STANDING WAVE
Two counter-propagating travelling waves of same frequency and amplitude superpose to form a
standing wave, characterised by nodes (positions of zero disturbance) and antinodes (positions of maximum
disturbance
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NODESANTINODES
Node to Node = ½ λBETWEEN ANY PAIR OF ADJACENT NODES, ALL PARTICLES
ARE MOVING IN PHASE
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STANDING WAVES ON A STRING
Fundamental length = λ/2
length
length
length
First overtone length = λ
Second overtone length = 3λ/2
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LONGITUDINAL STANDING WAVES
OPEN ENDED PIPE
FUNDAMENTAL l = λ/21st harmonic
actual air vibration
1st overtone l = λ2nd harmonic
2nd overtone l = 3λ/23rd harmonic
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CLOSED PIPE
FUNDAMENTAL l = λ/41st harmonic
1st overtone l = 3λ/43rd harmonic
2nd overtone l = 5λ/45th harmonic