Forced Vibration and Forced Vibration and ResonanceResonance

Natural FrequenciesNatural Frequencies

Nearly all Nearly all objects, when objects, when disturbed, will disturbed, will vibrate.vibrate.

Natural FrequenciesNatural Frequencies

Objects tend Objects tend to vibrate at to vibrate at their own their own particularparticular frequencyfrequency

Natural FrequenciesNatural Frequencies

This frequency This frequency is known as is known as the the natural natural frequencyfrequency of of the object.the object.

The glass “harp”The glass “harp” http://http://

www.huffingtonpost.cwww.huffingtonpost.co.uk/2013/02/15/o.uk/2013/02/15/hallelujah-wine-hallelujah-wine-glasses-glasses-video_n_2693037.htmlvideo_n_2693037.html

http://http://www.youtube.com/www.youtube.com/watch?v=S5K-watch?v=S5K-43bjRwU43bjRwU

ResonanceResonance

ResonanceResonance

Definition: when Definition: when the frequency of a the frequency of a vibration on an vibration on an object matches object matches the object’s the object’s natural natural frequency…a frequency…a dramatic increase dramatic increase in height occursin height occurs

ResonanceResonance

An object that is forced at its natural An object that is forced at its natural frequency will frequency will resonateresonate (vibrate) at (vibrate) at that frequency (with increasing that frequency (with increasing amplitude if the forcing continues).amplitude if the forcing continues).

Consider the forced vibration of a child Consider the forced vibration of a child on a swing – pushing at the natural on a swing – pushing at the natural frequency increases the amplitude.frequency increases the amplitude.

ResonanceResonance

An object that is forced at its natural An object that is forced at its natural frequency will frequency will resonateresonate (vibrate) at that (vibrate) at that frequency (with increasing amplitude if frequency (with increasing amplitude if the forcing continues).the forcing continues).

http://www.youtube.com/watch?http://www.youtube.com/watch?v=nHSGd2X1nc8&feature=relatedv=nHSGd2X1nc8&feature=related

ResonanceResonance

An object that is forced at its natural An object that is forced at its natural frequency will frequency will resonateresonate (vibrate) at (vibrate) at that frequency (with increasing that frequency (with increasing amplitude if the forcing continues).amplitude if the forcing continues).

The Tacoma-Narrows Bridge:The Tacoma-Narrows Bridge:

http://www.youtube.com/watch?http://www.youtube.com/watch?v=xox9BVSu7Okv=xox9BVSu7Ok

ResonanceResonance In 1831 soldiers were In 1831 soldiers were

ordered to “break step” ordered to “break step” across bridges to avoid across bridges to avoid producing a frequency producing a frequency equal to the natural equal to the natural frequency of the bridge. frequency of the bridge.

According to accounts According to accounts of the time, the bridge of the time, the bridge broke apart beneath the broke apart beneath the soldiers, throwing soldiers, throwing dozens of men into the dozens of men into the water.water.

Resonance: ExampleResonance: Example

For example, the sound wave For example, the sound wave produced by a vibrating tuning fork produced by a vibrating tuning fork will cause an identical tuning fork to will cause an identical tuning fork to start vibrating.start vibrating.

Examples of resonanceExamples of resonance

1) breaking glass1) breaking glass 2) bridges2) bridges 3) walls (walls of 3) walls (walls of

Jericho)Jericho) 4) radio stations4) radio stations 5) microwave 5) microwave

ovens…water ovens…water molecules in food molecules in food vibrate and resonate vibrate and resonate and heat upand heat up