Properties of Waves

Unit 9: Waves and Sound

Search: PhET Waves Intro

• Choose Water
• Check “Side View” (bottom left)
• Choose the multi wave option
• Set the Frequency and Amplitude in the middle of the scale
• Click on the green dot on the faucet and start the water flowing

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PhET Waves Intro

• Increase the frequency. What happens with the number of waves?
• What happens to the distance between waves (wavelength)?
• Increase the amplitude. What happens with waves?
• Decrease the frequency. What happens with the number of waves?
• What happens to the distance between waves (wavelength)?
• Decrease the Amplitude. What happens with the waves?

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Waves

• A wave is a disturbance that carries energy through matter or space
• During the “wave” at a baseball game, the people do not move, but the energy moves through the people

Waves

• Water waves, sound waves, and waves that travel down a rope or spring are types of mechanical waves
• Mechanical waves must travel through a medium (a solid, liquid, or gas)

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Waves

• Electromagnetic waves do not require a medium and can travel through a vacuum (like space)
• Light waves are electromagnetic waves

Transverse Waves

Transverse Waves

• Crest: highest point
• Trough: lowest point
• Wavelength: distance between two crests or two troughs
• Amplitude: the maximum distance the wave moves from equilibrium
• Equilibrium: the location of a wave at rest

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Transverse Waves

• A transverse wave is one that vibrates perpendicular to the direction of the wave’s motion

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Longitudinal Waves

Longitudinal Waves

• Wavelength: the distance between two compressions
• Compression: a region of high (maximum) pressure where the particles are squeezed together
• Rarefaction: a region of low (minimum) pressure where the particles are spread out

Longitudinal Waves

• A longitudinal wave is one that vibrates parallel to the direction of the wave’s motion

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Properties of ALL Waves

Pulse vs Period

• A pulse is a single bump or disturbance that travels through a medium
• If the wave moves up and down at the same rate, a periodic wave is generated

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Amplitude and Energy

• Waves with a high amplitude have high energy 
• Waves with a low amplitude have low energy 

Frequency

• Defined as the number of cycles per second 
• The higher the frequency, the shorter the period and the shorter the wavelength 
• Measured in Hertz (Hz) with the unit f

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Period

• Defined as the time it takes for one complete cycle
• A cycle is one complete wave
• The longer the cycle, the longer the wavelength and the lower the frequency
• Measured in seconds with symbol T

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Period and Frequency are inverses of each other

Wave Speed

• The speed of a wave depends on the density of the medium  
• Within a medium:
• speed is the same regardless of frequency and wavelength 
• its wavelength and frequency do not change
• The wavelength and speed of a wave CAN change as the wave moves from one medium to the next
• The frequency of a wave DOES NOT CHANGE

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The Wave Equation

OR v = d

t

Example 1

You want to describe the motion of a swing. You find out that it takes 2 seconds for the swing to complete one cycle. The swing passes through 48 degrees as it goes from high point to high point.

1. What is the period of the swing?

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• What is the frequency of the swing?

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T (Period) = time/cycles

T = 2/1

T = 2 s

f (frequency) = cycles/time

f = 1/2

f = 0.5 Hz

Example 2

A wave oscillates 6 times in 3 seconds and has a wavelength of 4 meters. What is the frequency, period, and speed of the wave? 

Givens:

Cycles = 6

t = 3 s

λ = 4 m

f = cycles/time

f = 6/3

f = 2 Hz

T = 1/f

T = 1/2

T = 0.5 s

v = fλ

v = (2)(4)

v = 8 m/s

9.1 Lesson Check