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NickKloostra
25553141
Section203
LO6- StandingWavesonStrings
Question:Yourfriendstretchesastringacrossa distance of 1m, and attachesboth endsto a wall.He
tellsyouthatwhenhe plucksthe string,itwill vibrate initsfourthharmonic.He betsyouthat you can’t
guessthe distancesthathe can place his fingersdownonthe stringandnot affectthe amplitude of the
wave.Atwhat distancescanthe situationdescribedabove occurat?
Answer:
At lengths
𝐿
4
,
𝐿
2
𝑎𝑛𝑑
3𝐿
4
youcan pressdownonthe stringandnot affectthe amplitude,soatthe distances
of 0.25m, 0.5m and 0.75m.
Explanation:
Whena string of lengthL isstretchedandfixedatbothends,youcan sendwavesback andforth across
the stringby pluckingit. These travellingwavesmovinginopposite directionscreate astandingwave on
the string.A standingwave isa wave ina mediuminwhichthe frequency,amplitude andwavelengthof
the two interferingwavesremainconstantthroughoutthe mediumonwhichitpropagates.
Due to the interference of the initial wave andreflectedwave,the standingwave producesapatternof
nodesandantinodes.Nodesoccurwhenthe amplitude remainsatrestat all times,andantinodesoccur
whenthe amplitude of the wave isata maximum. More specifically, nodesoccurwhensin(
2𝜋
𝜆
x) =0, and
antinodesoccurwhensin(
2𝜋
𝜆
x) =+/- 1. Because these nodesare infixedpositions,anddonotmove
across the medium,the wave doesnotcarry energywithit.
Now, because the string in the question is helddown at both ends, the amplitude is always zero at the
endsof the string.Knowingthatthe locationof the antinodesoccurwhen sin(
2𝜋
𝜆
x) =0, we can define
2𝜋
𝜆
L
= mπ, where mis a positive non-zerointeger.Isolatingforλ at m, we findthat wavelengthisequal to
2𝐿
𝑚
when L= 1, 2, 3, 4, … Therefore, the longest wavelength is equal to 2L, and the lengths of the waves
decrease proportionally to the equation derived above. These standing waves are called the normal
modesof vibrationof the string.The frequenciescorrespondingtothe normal modesof vibration are as
follows:
f=
𝑣
𝜆
=
𝑚
2𝐿
𝑣 =
𝑚
2𝐿
√
𝑇
µ
The frequencieschangewithrespecttowavelengthbecausethe wavespeedof awaveonastringremains
constant, so if wavelength increases, frequency must decrease, and vice versa.
The lowest frequency corresponds to the fundamental frequency or first harmonic. The allowed
frequencies after the first harmonic are called harmonics, or resonant frequencies.
Soas wavelengthdecreases,the numberof nodesonthe standingwaveincrease,the frequency increases
and the harmonic increases. Higher frequencies are associated with higher values of m. In the fourth
harmonic, you can see that the wavelengthis equal to L/2. This means that there are three nodes, and
that they occur at the lengths
𝐿
4
,
𝐿
2
𝑎𝑛𝑑
3𝐿
4
.

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Learning object #2

  • 1. NickKloostra 25553141 Section203 LO6- StandingWavesonStrings Question:Yourfriendstretchesastringacrossa distance of 1m, and attachesboth endsto a wall.He tellsyouthatwhenhe plucksthe string,itwill vibrate initsfourthharmonic.He betsyouthat you can’t guessthe distancesthathe can place his fingersdownonthe stringandnot affectthe amplitude of the wave.Atwhat distancescanthe situationdescribedabove occurat? Answer: At lengths 𝐿 4 , 𝐿 2 𝑎𝑛𝑑 3𝐿 4 youcan pressdownonthe stringandnot affectthe amplitude,soatthe distances of 0.25m, 0.5m and 0.75m. Explanation: Whena string of lengthL isstretchedandfixedatbothends,youcan sendwavesback andforth across the stringby pluckingit. These travellingwavesmovinginopposite directionscreate astandingwave on the string.A standingwave isa wave ina mediuminwhichthe frequency,amplitude andwavelengthof the two interferingwavesremainconstantthroughoutthe mediumonwhichitpropagates. Due to the interference of the initial wave andreflectedwave,the standingwave producesapatternof nodesandantinodes.Nodesoccurwhenthe amplitude remainsatrestat all times,andantinodesoccur whenthe amplitude of the wave isata maximum. More specifically, nodesoccurwhensin( 2𝜋 𝜆 x) =0, and antinodesoccurwhensin( 2𝜋 𝜆 x) =+/- 1. Because these nodesare infixedpositions,anddonotmove across the medium,the wave doesnotcarry energywithit.
  • 2. Now, because the string in the question is helddown at both ends, the amplitude is always zero at the endsof the string.Knowingthatthe locationof the antinodesoccurwhen sin( 2𝜋 𝜆 x) =0, we can define 2𝜋 𝜆 L = mπ, where mis a positive non-zerointeger.Isolatingforλ at m, we findthat wavelengthisequal to 2𝐿 𝑚 when L= 1, 2, 3, 4, … Therefore, the longest wavelength is equal to 2L, and the lengths of the waves decrease proportionally to the equation derived above. These standing waves are called the normal modesof vibrationof the string.The frequenciescorrespondingtothe normal modesof vibration are as follows: f= 𝑣 𝜆 = 𝑚 2𝐿 𝑣 = 𝑚 2𝐿 √ 𝑇 µ The frequencieschangewithrespecttowavelengthbecausethe wavespeedof awaveonastringremains constant, so if wavelength increases, frequency must decrease, and vice versa. The lowest frequency corresponds to the fundamental frequency or first harmonic. The allowed frequencies after the first harmonic are called harmonics, or resonant frequencies. Soas wavelengthdecreases,the numberof nodesonthe standingwaveincrease,the frequency increases and the harmonic increases. Higher frequencies are associated with higher values of m. In the fourth harmonic, you can see that the wavelengthis equal to L/2. This means that there are three nodes, and that they occur at the lengths 𝐿 4 , 𝐿 2 𝑎𝑛𝑑 3𝐿 4 .