Waves

What is a Wave? A wave is an energy vibration that usually takes place in matter Key word: vibration. Waves repeat A single “wave” is called a pulse

Wave Types Transverse Waves vibrate at a right angle to their direction of motion Examples: Light, ocean waves, waves on a rope

Longitudinal Waves vibrate in the direction of motion Examples: Sound, P-Waves, “Slinky” Waves

Wave Properties We use four different characteristics to talk about waves: frequency, wavelength, amplitude, period, and speed Wavelength is the distance between identical parts of a wave (crestcrest, troughtrough) Amplitude is half the wave’s total height, or the distance between the crest and the rest position

Wave Time A wave’s period is the time it takes for one vibration to occur If you divide wavelength by time, you get a wave’s speed (speed = distance/time)

Frequency A wave’s frequency tells us how often waves occur (2 waves a second, 4 waves a second, etc) The units of frequency are Hertz, or Hz 1 Hz = 1 wave/second Frequency = 1/period

Source or Medium Dependence? Amplitude? Source Energy Period Source Speed Medium Wavelength Medium/Source

Some Wavelengths Microwaves: .03 m Radio Waves: 10 m Ocean Waves: 200 m Tsunami: 11,000 m 200,000 m

Two Views Position/Time View Snapshot View We focus on how one point on the wave evolves in time Snapshot View We focus on how the wave evolves in multiple dimension

Time View

Which wave properties can we deduce from this graph?

Snapshot

What can we deduce from this graph?

Interference with Standing Waves

Resonance Questions of Interest: Can you break a wine glass with sound? Why did the Tacoma Narrows bridge collapse?

Forced Vibrations Vibrating an object at a specific frequency Think back to the lab from last week (part C) Forced vibrations can create either organized or unorganized patterns On what does the pattern’s organization depend?

Natural Frequency Most objects (media) have so called natural frequencies If we vibrate an object at one or more of its natural frequencies, something interesting occurs

Resonance When a source vibrates an object at its natural frequency, we call it resonance You can create large amplitudes with fairly small input

Other Examples? Swing sets Singing in the shower Tuning fork Standing waves on spring

Raising the stakes If you continue vibrating something at its resonance frequency, the amplitude of the vibration increases

Shattering a Wine Glass?