Unit 12
Natural Frequency, wn
• All objects have a frequency at which they will
sustain vibrations with the minimum energy input
– A 256 Hz tuning fork always vibrates at 256 Hz when
• This “natural frequency” depends on properties
of the object that’s vibrating
– Size
– Shape
– material
• Tuning forks of
have different
• Which of these
do you think will
have the
highest wn and
– Hint: think
about inertia
Forced Vibration
• Occurs when the vibration in one object
causes a nearby object to vibrate at the
same frequency
• Examples
– Car radio makes the
windows rattle
– “feeling” loud sounds
through walls or the floor
• Happens when an object is forced to vibrate
at its natural frequency
– Standing waves are created as waves interfere with
– Amplitude increases dramatically due to constructive
• Objects in resonance can literally tear
themselves apart!
– Opera singer shatters a glass with her voice
– Doctor uses ultrasound to break up a patient’s kidney
More on resonance
• Since objects have only one natural
frequency, they will only resonate at one
– If singing an “A” (440 Hz) shatters a wine
glass, singing a “C” (256 Hz) will not
– Since an object’s shape, size, and material
affect wn, these factors also affect the point at
which they will resonate.
Tacoma Narrows Bridge Disaster
• Keep these wave concepts in mind as you
watch the video of the bridge collapse in
Washington state.