Physics Unit 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.