Chapter 13

13.1 Special Wiggles – Vibrations and Waves

• When something moving back and forth, side to side, or up and down we say it vibrates.

• When that vibration moves through space and time we call it a wave.

• Waves are rhythmic disturbances that carry energy through matter or space

13.1 Special Wiggles – Vibrations and Waves

• Light and Sound both travel as a wave but they are two very different types of wave.

• Sound requires matter to move through to transport energy from one place to another

• Light can travel through space with out any matter to move through.

• It can even pass through many materials

13.1 Special Wiggles – Vibrations and Waves

Wave Characteristics

13.1 Special Wiggles – Vibrations and Waves

A: Resting point; this is where the wave starts from.

B: Crest; this is the highest point of the wave.

C: Wavelength (λ); the distance from one crest to the next one

13.1 Special Wiggles – Vibrations and Waves

D: Wavelength; wavelength can also be the distance between any two identical parts of the wave

E: Amplitude; the distance from the resting point to the crest.

F: Amplitude; amplitude can also be the distance from the resting point to the trough

13.1 Special Wiggles – Vibrations and Waves

G: Trough; the lowest point of the wave

13.1 Special Wiggles – Vibrations and Waves

• Frequency: How often the wave or vibration passes a point in one second.

• The symbol for frequency is f

• The unit for frequency is hertz (Hz)

• One hertz is one vibration per second

13.1 Special Wiggles – Vibrations and Waves

• Period: The time it take for one complete vibration or one complete wave (crest and trough) to pass a point.

• The symbol for period is T.– Think of is as a special unit of time

• The unit for period is seconds

13.1 Special Wiggles – Vibrations and Waves

• Period can be calculated from frequency and vis-versa

• Suppose a 2 waves passes a pole every second. This would make the frequency 2 Hz.

• This means the period of the wave is ½ seconds or a half a second.

• It takes a half a second for one complete wave to cycle through

periodFrequency

1

frequencyPeriod

1

13.2 Wave Motion – Transporting Energy

• When a wave moves through matter it carries energy

• It does not, however, carry the matter with it.– A wave moving past a bobber moves the bobber up

and down but does not move it in the direction of the wave.

– You don’t feel sound waves from someone speaking because the air that is carrying the sound is not moving to you.

13.2 Wave Motion – Transporting Energy

• Because waves move we often need to know the wave speed.

• Wave speed is related to the frequency and the wavelength of the wave.

• Remember speed is

wavelengthxfrequencyspeedWave

time

dist

13.2 Wave Motion – Transporting Energy

• Wavelength is measured in meters which is the distance between the crests of the waves

• Frequency is equal to

• And Period is measured in seconds which is the time for one wave to complete its cycle or one wavelength to pass

•

• Wave speed is measured just like any other speed in m/s

Period

1

s

m

Periodxmwavelength

(sec)

1)(

13.2 Wave Motion – Transporting Energy

• Try it!

• A wave travels past the bird 3 time every second. It has a wavelength of 1 meter. What is the wave speed?

13.2 Wave Motion – Transporting Energy

Wavelength = 1 m ; f = 3 Hz

Wave speed

Wave speed = wavelength x frequency

Wave speed = 1m x 3 Hz

Wave speed = 3 m/s

13.3 Two types of waves – Transverse and Longitudinal

• Transverse Waves– The medium vibrates in a perpendicular direction of the vibrating

source– Describing Transverse waves

• Crests – the highest point of the wave

• Troughs – the lowest points of the wave

• Wavelength – the distance from crest to crest

• Amplitude – the distance from the crest to the resting position

• Frequency – the number of wave crests that pass a certain point each second.

– Examples• Light

• Water Waves

• Waves in a guitar string

13.3 Two types of waves – Transverse and Longitudinal

• Longitudinal Waves– The medium vibrates in the same direction as the

vibrating source– Describing Longitudinal Waves

• Compression – the dense area of the wave• Rarefaction – the less dense area of the wave• Wavelength – One compression and one rarefaction• Frequency – The number of compressions that pass a point

each second.

– Examples• Sound

13.3 Two types of waves – Transverse and Longitudinal

A: Longitudinal Wave

B: Transverse Wave