WavesWaves

What is a Wave?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 TypesWave 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 PropertiesWave 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 (crestcrest, troughtrough)

• Amplitude is half the wave’s total height, or the distance between the crest and the rest position

Wave TimeWave 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)

FrequencyFrequency

• 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?Source or Medium Dependence?

• Amplitude?– Source Energy

• Period– Source

• Speed– Medium

• Wavelength– Medium/Source

Some WavelengthsSome Wavelengths

• Microwaves: .03 m

• Radio Waves: 10 m

• Ocean Waves: 200 m

• Tsunami: 11,000 m

• 200,000 m

Two ViewsTwo Views

• Position/Time 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 ViewTime View

• Which wave properties can we deduce from this graph?

SnapshotSnapshot

• What can we deduce from this graph?

Interference with Standing WavesInterference with Standing Waves

ResonanceResonance

• Questions of Interest:

• Can you break a wine glass with sound?

• Why did the Tacoma Narrows bridge collapse?

Forced VibrationsForced 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 FrequencyNatural 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

ResonanceResonance

• 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?Other Examples?

• Swing sets

• Singing in the shower

• Tuning fork

• Standing waves on spring

Raising the stakesRaising the stakes

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

Shattering a Wine Glass?Shattering a Wine Glass?