PhysicsUnit 12

Natural Frequency, n

• 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

struck.

• This “natural frequency” depends on properties of the object that’s vibrating– Size– Shape– material

• Tuning forks of different frequencies have different lengths.

• Which of these do you think will have the highest n and why?– 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

Resonance

• Happens when an object is forced to vibrate at its natural frequency– Standing waves are created as waves interfere with

themselves– Amplitude increases dramatically due to constructive

interference

• 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

stone

More on resonance

• Since objects have only one natural frequency, they will only resonate at one frequency.– 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 n, 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.