Physics 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 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 wn 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 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.