Waves

Physical ScienceGoal 3.04

Warm-up

• When you go to a sporting event, what happens when you do a wave?– How do you know what to do?

– How do you know when to do it?

– Do you end up back in your seat when you are done

A wave is a repeating disturbance or movement that transfers energy through matter or space

A

A rock is dropped in the water at point A.

After 10 waves, where will each float be?

Waves transfer ENERGY, not the material through which they move.

A

The floats will be in the same places after the waves pass. They will bob up and down as the waves go by.

Mechanical Waves

• Mechanical waves are waves that require a material, or a medium, to travel

• Mechanical waves can be: – compressional (longitudinal)

– transverse

Transverse Waves

• Transverse waves are the waves that happen in the ocean.

• They go up and down.

Parts of a Transverse Wave

Crest

Origin

Trough

Wavelength

Amplitude

Compressional Waves

• Compressional waves is what occurs when you play with a slinky!

• The waves go back and forth.

• Examples of compressional waves: – Sound waves (we’ll talk more about this later! )

Parts of a Compressional Wave

(parts where it is “uncompressed”)

Properties of a Wave• Wavelength (): measured in meters– Transverse: distance from crest to crest or trough to

trough – Compressional: distance from compression to

compression

• Frequency: number of wavelengths that pass a fixed point each second– In transverse waves – counting the number of crests or troughs that pass by a

point each second– In compressional waves – the number of compressions that pass a point every

second– measured in Hz (1/s)– F = 1/T

• Period: time (s) it takes for 1 wavelength to pass a point – As the frequency of a wave increases, the period decreases. – Measured in seconds

Properties of a Wave

Properties of a Wave• Amplitude: related to the energy carried by a wave (greater the amplitude, the

more energy carried by the wave)– Compressional: how tightly the medium is compressed

• Denser the compression, the larger the amplitude

– Transverse: distance from crest or trough of the wave to the rest position of the medium

– measure in meters

Wave Speed Equation:Speed (m/s) = frequency (Hz) x wavelength (m)

v = f

v = /T

Practice Problems

1. The frequency of a sound wave is 440 Hz. It has a wavelength of 0.775 m. Find its speed.

2. The speed of a sound wave is 350 m/s. It has a frequency of 220 Hz. What is its wavelength?

Warm-Up

• Draw and diagram two wave:(Crest, Trough, Amplitude, wavelength)

– A compression wave

– A transverse wave

Sound is a WAVE!!!!

• Sound waves are compressional waves!!

How does sound work? • Compressions and

rarefractions move away from a speaker as molecules in the air collide with their neighbors. As the speaker continues to vibrate, more molecules in the air are pushed together and spread apart. A series of compressions and rarefractions form and travel from the speaker to your ear! The sound wave is what you hear!!!!!!!!!!!!

Speed of Sound• Most sounds travel through air to reach your ears, but

have you ever been underwater and heard sounds???

• Sound waves can travel through many different materials – solids, liquids or gases.

• What about where there is no particles?? Do you think sound can travel on the moon???

• It can’t!!!!! Sound can ONLY travel through particles! Astronauts must use special electronic equipment to communicate

Speed of Sound

• Sound doesn’t travel at the same speed through each material.

• Sound travels the fastest though solids because the particles are very close together and the collide quickly!

Medium Speed of Sound (m/s)

Air 347Cork 500Water 1,498Brick 3,650Aluminum 4,877

Speed of Sound

• The speed of sound DOES NOT depend on the loudness of the sound!!

• Loud sounds travel through a medium at the same speed as soft sounds.

Using Sound

• Acoustics is the study of sound

• Concert halls are built to reflect sound in some directions and absorb them in others

Warm Up

• What is the wave equation?

• What is the relationship between period and frequency?

1. A wave traveling with a frequency of 23 Hz has a wavelength of .34m. What is the speed of the wave?

Harder wave equation problems

2. What is the velocity of wave with a period of .34 s if the wavelength is 10 m?

3. What is the wavelength of a wave with a velocity of 300 m/s and a frequency of 1.5KHz?

4. What is the wavelength of a sound with a pitch of 440 Hz?

• Most waves needed something to move through. The material a wave moved through was called the medium

• Electromagnetic waves need no such medium

Propagation

How to make them

• EM waves are also called rays

• They are made by changing an electric or magnetic field. This then transfers energy!

EM spectrum problems

• EM spectrum problemsuse the same wave equation as always

The velocity of anything on the electromagnetic spectrum is constant = 300,000,000 m/s… this is the speed of light

If you have a wavelength you can look at an EM spectrum to determine what type of wave you are dealing with?