Waves II
Level : Conceptual Physics
Teacher : Kim
Observe video and then answer the following questions
Q1) You are in a spaceship and another ship explodes
i) Can you ‘see’ the explosion? a) Yeah~! b) No way~!
Explain=>
ii) Can you ‘hear’ the explosion? a) Yeah~! b) No way~!
Explain=>
iii) Can the pilots in their ship communicate with each other? a) Yeah~! b) No way~!
Explain=>
What are Waves?
- Wave is a ‘wiggle’ that propagates through space and time through a medium, usually transferring
energy (electromagnetic waves can travel through a vacuum and do not need a medium to travel).
- When energy is transferred by a wave from a vibrating source to a distant receiver, there is no
transfer of matter between the two points
=> It is the disturbance that moves along the medium, not the wave itself
Demo1) ‘Slinky waves’
- When someone speaks to you from across the room, a disturbance travels through the air across the
room and hits your eardrum
=> The air molecules(medium) themselves do not move along, as they would in a wind but the
disturbance that moves
What is a medium?
Mechanical waves require some sort of material to travel.
i) Can sound wave travel through a vacuum of space?
Yeah!
ii) Can sound wave travel through air?
iii) Can sound wave travel through water?
Yeah!
No way!
Yeah!
iv) Can sound wave travel through solid materials?
No way!
No way!
Yeah!
No way!
Any materials a wave use to travel are called a medium.
Example) The alarm in your alarm clock set up vibrations or waves in the surrounding air molecules.
The air would act as a material medium, transferring the disturbance along the air until it reaches your
ears, disturbing the molecules of your eardrum.
If the alarm clock was set off in a vacuum, the medium that was passing the disturbance is
removed. The sound will no longer be able to transfer the disturbance and eventually diminish.
*~ Bye bye Ether~*
Early scientists believed that all waves must travel through a medium.
Light, also being a wave, must travel through a medium.
A medium must exist that surrounds the heavenly bodies in the Universe
This invisible medium in which Earth floated was called ether. Scientist justified the existence of ether
by suggesting it is too small to be detected by current technology but must exist.
However, all experiments designed to detect the ether failed
=> Electromagnetic waves do not require a medium to travel. It is a ‘self’ propagated wave
Mechanical waves require a medium to travel, EM waves(light, radio, gamma rays , . . .) do not
require a medium to travel.
Features of Periodic Waves
i) Draw a wave and define amplitude, wavelength, crest and trough
position
time
ii) Define frequency
=> How often a vibration occurs in 1 second
=> The number of back&forth(cycle) vibrations a oscillating system makes in a given time
=> If 1 cycle occurs in 1 second, than the frequency is (
) cycle per second
=> If 2 cycles occur in 1 second, than the frequency is (
) cycle per second
=> If 6 cycles occur in 2 second, than the frequency is (
) cycle per second
=> If 1 cycle occur in 0.5 second, than the frequency is (
=> The unit of frequency is call (
) cycle per second
) [Hz]
iii) What is a period?
The time it takes to complete one cycle is called the (
=> If the frequency of a wave is 2Hz, than the period is (
)
frequency (f ) = 1 / period(T) or
)
T=1 / f
Q2) The Sears Building in Chicago sways back and forth at a frequency of about 0.1Hz. How many
seconds does it take to make one vibration?
Q3) A nurse counts 76 heartbeats in one minute. What are the period and frequency of the heart’s
oscillations?
Q4) New York’s 300m high Citicorp Tower oscillates in the wind with a period of 6.8s. Calculate its
frequency of vibration.
Wave speed
Wave speed = wavelength × frequency
v = λ׃
The speed that a wave travel through a medium is dictated by the medium, not by how rapidly the
disturbance take place
Example) If you move a rope up and down faster, then you are generating cycles faster by decreasing
the wavelength of the each waveform, and increasing the frequency of the waves. So the frequency
would go up, the wavelength would go down, but the speed would be the same
Wave speed = wavelength × frequency
𝝀
v = λ׃ or v = 𝑻
i) The Speed of Sound in different Mediums
Sound wave travels through gases, liquids, and solids at considerably different speeds.
The speed of sound in air at 20ºC is about 340m/s. In sea water, sound travels at 1522m/s. In solid
material like copper, sound travels at 5010m/s
ii) The Speed of Sound in the same Medium
The following table shows some wavelengths and corresponding frequencies of sound in air at the
same temperature.
Sound Waves
Wavelength(m)
Frequency(Hz)
Wave Speed(m/s)
2.13
160
340
1.29
264
340
0.86
396
340
0.64
528
340
*A high pitch sound and a low pitch sound travel at the same speed
Q5) If the wave of water vibrates up and down two times each second and the distance between wave
crests is 1.5m, what is the frequency of the wave? What is it wavelength? What is its speed?
Q6) What is the wavelength of a 340Hz sound wave when the speed of sound in air 340m/s?
Q7) A certain wave traveling in the water has a wavelength 0.4m apart and has a frequency of 2Hz.
Calculate the speed of the wave.
Q8) The lowest frequency we can hear is about 20Hz. Calculate the wavelength associated with this
frequency for sound that travels at 340m/s.