Q.A sitar wire is replaced by another wire of same length and material but of three times the earlier radius. If the tension in the wire remains the same, by what factor will the frequency change?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Wave Speed on String
Wave Speed on a String – From Intuition to Formula
Imagine you and a friend hold a long, taut rope between you. If you give your end a quick flick upward, a bump travels along the rope toward your friend. That bump is a wave, and the speed at which it moves is the wave speed.
Now ask yourself: what determines how fast that bump travels? Two things stand out from everyday experience:
- Tension – If you pull the rope tighter, the bump zips along faster. A loose rope makes the wave crawl.
- Mass – If the rope is heavy (like a thick clothesline), the wave moves slower than on a light, thin string under the same tension.
So wave speed increases with tension and decreases with the "heaviness" of the string. That's the core intuition.
The Precise Statement
For a wave traveling along a stretched string, the wave speed v is given by:
v=μT
where:
- T is the tension in the string (in newtons, N)
- μ is the linear mass density – the mass per unit length of the string (in kg/m)
v=μT
This formula is exact for an ideal string (perfectly flexible, no stiffness, no damping). It comes from solving the wave equation for a string, but you can understand it physically.
Why the Square Root? A Quick Physical Argument
Think of a small segment of the string. The tension provides the restoring force that tries to straighten the string when it's bent. A higher tension means a stronger restoring force, so the wave accelerates faster – hence higher speed.
The mass per unit length μ is the inertia of the string. A heavier string resists acceleration more, so the wave slows down.
The square root appears because the relationship between force, mass, and acceleration isn't linear when you derive it properly. But the key takeaway is:
Wave speed on a string depends only on the string's tension and its linear density – not on the frequency or amplitude of the wave.
This is a surprising and important result. Whether you send a slow, gentle ripple or a fast, sharp pulse, both travel at the same speed on the same string.
A Simple Example
A steel guitar string has μ=0.002 kg/m and is under tension T=100 N. What is the wave speed?
v=0.002100=50000≈224 m/s
That's about half the speed of sound in air – fast enough that the wave reaches the other end almost instantly.
Common Mistakes to Avoid
- Do not confuse wave speed with the speed of the string's particles. The string itself moves up and down (transverse motion), but the wave travels horizontally. These are different speeds.
- Wave speed does NOT depend on frequency. Changing how fast you flick your hand changes the frequency, but the wave still travels at v=T/μ.
- Tension is not the same as force applied at the end. If the string is under tension T everywhere (ideal case), that's the value you use – not the force you apply to create the wave.
Where This Formula Comes From (A Glimpse) …
The key idea is that the fundamental frequency of a stretched string depends on its linear mass density, which changes with the cross-sectional area.
Step 1: The fundamental frequency is f=2L1μT, where μ is mass per unit length.
Step 2: For a wire of radius r, μ=ρ⋅πr2 (density × area). If radius becomes 3r, then μ′=ρ⋅π(3r)2=9μ. …
The frequency of a vibrating string depends inversely on its radius when length, material, and tension are fixed. Tripling the radius reduces the frequency to one-third of its original value.
The key to this problem lies in understanding how the fundamental frequency of a stretched string depends on its physical properties. For a sitar wire (or any string fixed at both ends), the fundamental frequency is given by:
f=2L1μT
where L is the length, T is the tension, and μ is the mass per unit length (linear mass density). The wire is replaced by another of the same length and material, with the same tension, but with three times the radius. Since the material is the same, the density ρ is unchanged. The linear mass density μ is mass per unit length, which for a cylindrical wire is μ=ρ⋅(cross-sectional area)=ρπr2.
So μ is proportional to r2. If the radius becomes 3 times larger, μ becomes 9 times larger. Now look at the frequency formula: f∝1/μ. Therefore, if μ increases by a factor of 9, the frequency decreases by a factor of 9=3.
Let’s walk through it step by step.
- Write the formula for fundamental frequency. For a string fixed at both ends, the fundamental frequency is
f=2L1μT
Here L and T are constant in this problem.
- Express μ in terms of radius. Since the wire is cylindrical and made of the same material (density ρ),
μ=ρ×area=ρπr2
So μ∝r2.
- Relate frequency to radius. Substitute μ∝r2 into the frequency expression:
f∝r21=r1
That is, frequency is inversely proportional to the radius. …
Step 1: Fundamental frequency of a stretched string: f=2L1T/μ.
Step 2: For a cylindrical wire of density ρ and radius r, μ=ρ×(πr2), so μ∝r2. …
Showing the 12 most recent of 23 on this concept.
- TG EAPCET 2026Set eng-2026-05-10-FN1 markMCQQ.A wire of length 1.2 m is subjected to a tension of 240 N. If the frequencies of two successive modes of vibration of the wire are 200 Hz and 250 Hz, then the mass of the wire is (A) 10 g (B) 20 g (C) 30 g (D) 40 g
›Reveal solutionSolution
The difference between two successive mode frequencies gives the fundamental frequency, which together with the tension and length yields the linear mass density and hence the total mass of the wire. The mass is 20 g.
The key idea is that for a stretched string fixed at both ends, the allowed vibration frequencies are integer multiples of the fundamental frequency f1. So if you know two successive mode frequencies, their difference is exactly f1. Once you have f1, the wave speed v follows from v=f1⋅2L (since the fundamental wavelength is 2L). And the wave speed on a string also equals T/μ, where T is tension and μ is mass per unit length. Equating these gives μ, and multiplying by length L gives the total mass.
Let’s work it through.
- Find the fundamental frequency. For a string fixed at both ends, the nth harmonic frequency is fn=nf1. Two successive modes are fn=nf1 and fn+1=(n+1)f1. Their difference is
fn+1−fn=f1.
Here the given successive frequencies are 200 Hz and 250 Hz, so
f1=250−200=50 Hz.
- Relate fundamental frequency to wave speed. For the fundamental mode, the string vibrates with a single loop: wavelength λ1=2L, where L=1.2 m. The wave speed is
v=f1λ1=50×(2×1.2)=50×2.4=120 m/s.
- Relate wave speed to tension and linear density. The speed of a transverse wave on a string under tension T is
v=μT,
where μ is the mass per unit length. Rearranging,
μ=v2T.
Substitute T=240 N and v=120 m/s: …
- TG EAPCET 2026Set eng-2026-05-10-FN1 markMCQQ.In Young's double slit experiment, if the distance between the first dark fringe and the second bright fringe on the same side of the central maximum is 1.5 mm, then the distance between first dark fringe on one side and the second bright fringe on the other side of the central maximum is (A) 2.5 mm (B) 1.5 mm (C) 3.0 mm (D) 2.0 mm
›Reveal solutionSolution
The key idea is to express the positions of dark and bright fringes in terms of the fringe width β, then use the given distance to find β, and finally compute the required distance. The answer is 3.0 mm.
In Young’s double slit experiment, the pattern on the screen is a series of alternating bright and dark fringes. The central maximum is bright, and on either side, the fringes are equally spaced. The distance between two consecutive bright fringes (or two consecutive dark fringes) is called the fringe width, denoted by β.
The position of the nth bright fringe (from the central maximum) is given by yn=nβ, where n=0,1,2,… (with n=0 being the central maximum). The position of the nth dark fringe is given by yn′=(n+21)β, where n=0,1,2,… (the first dark fringe corresponds to n=0).
Now, let’s work through the problem step by step.
-
Identify the given distance.
The distance between the first dark fringe and the second bright fringe on the same side of the central maximum is 1.5 mm.
- First dark fringe on one side: y0′=(0+21)β=2β.
- Second bright fringe on the same side: y2=2β (since n=2 for the second bright fringe; the first bright fringe is n=1). The distance between them is 2β−2β=23β. So, 23β=1.5 mm, which gives β=1.0 mm.
-
Now find the required distance.
We need the distance between the first dark fringe on one side and the second bright fringe on the other side of the central maximum.
- First dark fringe on one side: +2β (say to the right). …
-
- TG EAPCET 2026Set eng-2026-05-11-FN1 markMCQQ.A particle executes simple harmonic motion with an amplitude of 40 cm. If the time period of the particle is 6 s, then the minimum time taken by the particle to move from mean position to a point at a distance of 20 cm from the extreme position is (A) 0.5 s (B) 1.5 s (C) 3 s (D) 2.5 s
›Reveal solutionSolution
A point 20cm from the extreme is 20cm from the mean, i.e. x=A/2; from x=Asinωt this first occurs at t=0.5s — option (A).
Locate the point. Amplitude A=40cm, so the extreme is 40cm from the mean. A point 20cm from the extreme is
40−20=20cm from the mean ⇒ x=2A.
Angular frequency.
ω=T2π=62π=3π rad/s.
Time from the mean. Starting from the mean, x=Asinωt: …
- TG EAPCET 2026Set eng-2026-05-11-FN1 markMCQQ.A wire of length 100 cm is clamped between two rigid supports and is made to vibrate in its fundamental mode. If the amplitude at the midpoint of the wire is A, then the distance between two points having amplitude of 2A is (A) 50 cm (B) 60 cm (C) 40 cm (D) 25 cm
›Reveal solutionSolution
For a wire vibrating in its fundamental mode, the displacement profile is a sine wave with a node at each end and an antinode at the centre. The amplitude at a distance x from one end is Asin(πx/L). Setting this equal to A/2 gives x=L/4 and 3L/4, so the distance between these two points is L/2=50 cm.
The key idea is that the standing wave on a wire fixed at both ends has a well-defined shape. In the fundamental mode, the wire vibrates as a single loop — the midpoint is the antinode (maximum amplitude A), and the ends are nodes (zero amplitude). The amplitude at any other point is simply A times the sine of the distance from a node, scaled to the half-wavelength.
Let’s work through it.
- Set up the standing wave equation. For a wire of length L=100 cm fixed at both ends, the fundamental mode has wavelength λ=2L=200 cm. The displacement at a point a distance x from one end is
y(x,t)=Asin(Lπx)cos(ωt)
The amplitude of oscillation at that point is the coefficient of the time-dependent part:
Amplitude(x)=Asin(Lπx)
- Find where the amplitude equals A/2. Set
Asin(Lπx)=2A
Cancel A (non-zero):
sin(Lπx)=21
The principal solutions in [0,L] are
Lπx=4πandLπx=43π
because sin(π/4)=sin(3π/4)=1/2.
- Solve for x. …
- TG EAPCET 2026Set eng-2026-05-11-AN1 markMCQQ.The speed of a progressive transverse wave on a string is 18ms−1. If the phase difference between two points on the string separated by a distance of 3cm is 90∘, then the frequency of the transverse waves on the string is (A) 300 Hz (B) 225 Hz (C) 75 Hz (D) 150 Hz
›Reveal solutionSolution
The key idea is that phase difference relates to path difference via the wavelength, and wave speed equals frequency times wavelength. Using the given phase difference of 90∘ for a 3cm separation, we find the wavelength and then the frequency: f=150Hz, so option (D) is correct.
The problem gives us the wave speed and the phase difference between two points a known distance apart. The natural link is that phase difference corresponds to a fraction of a full cycle, which in turn corresponds to a fraction of the wavelength. Once we have the wavelength, the frequency follows directly from the wave equation.
Why this works:
A wave's phase changes by 360∘ (or 2π radians) over one full wavelength. So if two points are separated by a distance Δx, the phase difference Δϕ is simply the fraction of a wavelength they are apart, multiplied by 360∘. This gives us a direct proportion:
360∘Δϕ=λΔx.
Then we use v=fλ to find f.
Step-by-step solution:
- Convert the phase difference to a fraction of a cycle. A phase difference of 90∘ is one-quarter of a full cycle (360∘). So
360∘Δϕ=360∘90∘=41.
- Relate this fraction to the distance between the points. The distance Δx=3cm=0.03m corresponds to that same fraction of the wavelength λ:
λΔx=41.
Hence,
λ=4×Δx=4×0.03m=0.12m. …
- TG EAPCET 2026Set ap-2026-05-04-AN1 markMCQQ.Two wires A and B made of same material have equal lengths. If the volumes of the wires A and B are in the ratio 1:8 and the tensions applied to the wires A and B are in the ratio 1:2, then the ratio of the speeds of the transverse waves in the wires A and B is (A) 1:1 (B) 2:1 (C) 4:1 (D) 8:1
›Reveal solutionSolution
The speed of a transverse wave in a wire depends on tension and linear mass density. Using volume and length to relate cross‑sectional area and mass, the ratio of speeds comes out to 1:1.
The key idea is that the wave speed in a stretched string is v=T/μ, where T is the tension and μ is the mass per unit length. Since both wires are made of the same material and have equal lengths, their volumes tell us about their cross‑sectional areas, and hence about their masses. The ratio of tensions is given directly. So we just need to combine these pieces carefully.
- Relate volume to cross‑sectional area. For a wire of length L and cross‑sectional area A, volume V=AL. Since L is the same for both wires, the ratio of areas is the same as the ratio of volumes:
ABAA=VBVA=81.
- Find the ratio of masses. Both wires are made of the same material, so density ρ is identical. Mass m=ρV, hence
mBmA=VBVA=81.
- Find the ratio of linear mass densities. Linear mass density μ=m/L. Since L is equal,
μBμA=mBmA=81.
- Apply the wave speed formula. Speed v=T/μ. So
vBvA=TBTA⋅μAμB.
Given TA:TB=1:2, we have TA/TB=1/2. And μB/μA=8 (from step 3). Therefore
- TG EAPCET 2026Set ap-2026-05-05-FN1 markMCQQ.The equation of transverse displacement of a wire of mass 20 g and length 100 cm clamped at its ends is y(x,t)=0.05sin(52πx)cos(80πt), where ‘x’ is in metre and ‘t’ is in second. The tension in the wire is (A) 200 N (B) 600 N (C) 400 N (D) 800 N
›Reveal solutionSolution
We determine the wave speed from the given standing wave equation and then use the formula relating wave speed, tension, and linear mass density to find the tension in the wire. The tension in the wire is 800 N.
When a wire clamped at its ends vibrates, it forms a standing wave. The equation describing this transverse displacement contains information about the wave's characteristics, such as its wave number and angular frequency. These characteristics, in turn, allow us to calculate the speed at which transverse waves propagate along the wire. The speed of a transverse wave on a string is fundamentally determined by the tension in the string and its linear mass density. By extracting the wave speed from the given equation and calculating the linear mass density from the provided mass and length, we can then solve for the tension.
Here's how we can find the tension:
-
Identify the standard standing wave equation and extract wave parameters:
The general equation for a standing wave on a string, formed by two waves travelling in opposite directions, can be written as y(x,t)=Asin(kx)cos(ωt).
Comparing this with the given equation for the transverse displacement:
y(x,t)=0.05sin(52πx)cos(80πt)
We can directly identify the wave number (k) and the angular frequency (ω):
k=52π rad/m
ω=80π rad/s
-
Calculate the wave speed (v):
The speed of a wave (v) is related to its angular frequency (ω) and wave number (k) by the formula:
v=kω
Substituting the values we found:
v=52π rad/m80π rad/s=2π80π×5 m/s=40×5 m/s=200 m/s
So, the speed of the transverse wave on the wire is 200 m/s.
-
Calculate the linear mass density (μ) of the wire: …
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- TG EAPCET 2025Set eng-2025-05-02-FN1 markMCQQ.The fundamental frequency of transverse wave of a stretched string subjected to a tension T1 is 300 Hz. If the length of the string is doubled and subjected to a tension of T2, the fundamental frequency of the transverse wave in the string becomes 100 Hz, then T2:T1= (Linear density of the string is constant) (A) 1:2 (B) 3:4 (C) 2:3 (D) 4:9
›Reveal solutionSolution
The fundamental frequency of a stretched string is given by f=2L1μT. Using the given data for two different tensions and lengths, we set up a ratio and solve for T2:T1, obtaining 4:9. The correct option is (D).
The key concept here is the standing wave on a stretched string. For a string fixed at both ends, the fundamental (lowest) frequency is determined by the length L, the tension T, and the linear mass density μ (mass per unit length). The formula is:
f=2L1μT
This comes from the wave speed v=T/μ and the fact that the fundamental wavelength is 2L. So frequency is inversely proportional to length and proportional to the square root of tension. Since the linear density is constant, we can compare two situations directly.
- Write the frequency for the first case. Given: f1=300 Hz, tension T1, length L1=L (say).
300=2L1μT1
- Write the frequency for the second case. Given: f2=100 Hz, tension T2, length L2=2L (doubled).
100=2(2L)1μT2=4L1μT2
- Set up the ratio of the two equations. Divide the first equation by the second:
100300=4L1T2/μ2L1T1/μ
Simplify the left: 300/100=3.
On the right, the 1/L and μ cancel, and we have:
3=41T221T1=1/41/2⋅T2T1=2⋅T2T1 …
- TG EAPCET 2025Set eng-2025-05-02-AN1 markMCQQ.The equation of a transverse wave propagating on a stretched string is given by y=3sin(4x+200t), where x and y are in metre and the time ‘t’ is in second. If the tension applied to the string is 500 N, the linear density of the string is (A) 0.25 kgm−1 (B) 0.4 kgm−1 (C) 0.2 kgm−1 (D) 0.1 kgm−1
›Reveal solutionSolution
The wave speed is found from the wave equation’s angular frequency and wave number, then related to tension and linear density via v=T/μ. The linear density comes out to 0.2 kgm−1, so option (C) is correct.
The key idea is that for a transverse wave on a stretched string, the wave speed is determined by the tension and the linear density: v=T/μ. But we can also read the wave speed directly from the wave equation y=Asin(kx±ωt): the speed is v=ω/k. Equating these two expressions lets us solve for the unknown linear density μ.
-
Identify the wave parameters from the given equation
The wave is y=3sin(4x+200t). Compare with the standard form y=Asin(kx+ωt) (the plus sign means the wave travels in the negative x-direction, but speed magnitude is what matters).
- Wave number k=4 radm−1
- Angular frequency ω=200 rads−1
- Amplitude A=3 m (not needed for speed).
-
Find the wave speed from k and ω
The wave speed is v=kω.
v=4200=50 ms−1.
- Relate wave speed to tension and linear density For a string under tension T, the transverse wave speed is v=μT, …
-
- TG EAPCET 2025Set ap-2025-04-29-FN1 markMCQQ.In a medium, a source produces 60 crests and 60 troughs in a time of 0.2 second. If the distance between a crest and its adjacent trough is 100 cm, then the speed of sound in the medium is (A) 600 ms−1 (B) 1200 ms−1 (C) 300 ms−1 (D) 200 ms−1
›Reveal solutionSolution
The key is to find the frequency from the number of waves per second and the wavelength from the crest-to-trough distance. The speed comes out to 600 ms−1, option (A).
The problem gives you two separate pieces of information: one about how many waves pass per unit time, and one about the spatial size of a wave. The speed of a wave is always the product of its frequency and wavelength, v=fλ. So the task is to extract f and λ from the given data.
Let’s break it down.
- Finding the frequency. The source produces 60 crests and 60 troughs in 0.2 seconds. One complete wave consists of one crest and one trough. So 60 crests and 60 troughs together mean 60 complete waves. The number of waves per second — the frequency — is therefore
f=0.260=300 Hz.
- Finding the wavelength. The distance between a crest and its adjacent trough is given as 100 cm. That distance is exactly half a wavelength (crest to trough is λ/2). So
2λ=100 cm=1 m,
which gives
- TG EAPCET 2024Set eng-2024-05-09-AN1 markMCQQ.If the length of a string is P when the tension in it is 6 N and its length is Q when the tension in it is 8 N, then the original length of the string is (A) 3P+4Q (B) 3P−4Q (C) 4P+3Q (D) 4P−3Q
›Reveal solutionSolution
The problem uses Hooke’s law for a string under tension: extension is proportional to tension. The original length is found by eliminating the spring constant from two equations, giving the result 4P−3Q.
The key idea is that for an elastic string obeying Hooke’s law, the extension (change in length) is directly proportional to the applied tension, provided the string is not stretched beyond its elastic limit. So if the original (unstretched) length is L, then when tension is T, the length is L+kT, where k is a constant (the reciprocal of the spring constant times the original length, but we don’t need that detail). We have two data points, and we can solve for L without ever finding k.
- Let the original length of the string be L (in metres, say). When the tension is 6 N, the length is P. So:
P=L+k⋅6
where k is the constant of proportionality (extension per unit tension).
- Similarly, when the tension is 8 N, the length is Q:
Q=L+k⋅8
- We have two linear equations in L and k. To eliminate k, we can solve for k from one equation and substitute into the other. From the first equation:
k=6P−L
- Substitute into the second equation:
Q=L+8⋅6P−L
- Multiply both sides by 6 to clear the denominator:
6Q=6L+8(P−L)
- Expand and simplify:
6Q=8P−2L…6Q=6L+8P−8L
- TG EAPCET 2024Set eng-2024-05-09-AN1 markMCQQ.A message signal of peak voltage 12 V is used to amplitude modulate a carrier signal of frequency 1.2 MHz. The amplitude of the side bands is (A) 12 V (B) 3 V (C) 6 V (D) 8 V
›Reveal solutionSolution
In standard AM, each sideband amplitude is half the modulating signal’s peak voltage. With a 12 V peak message, each sideband is 6 V, so the correct option is (C).
The key idea here is that amplitude modulation (AM) creates two sidebands — one above and one below the carrier frequency — and each sideband carries half the amplitude of the modulating signal. This is not a guess; it follows directly from the mathematics of AM.
Why this works:
When you multiply a carrier wave Accos(ωct) by a message signal m(t)=Amcos(ωmt), the product expands using the cosine product identity into two new frequencies: ωc+ωm and ωc−ωm. Each of these terms has amplitude 2Am. That’s the sideband amplitude — no more, no less.
Let’s walk through it step by step.
- Write the standard AM wave The general expression for an amplitude modulated wave is
s(t)=Ac[1+μcos(ωmt)]cos(ωct)
where μ=AcAm is the modulation index. Here, the peak message voltage is given as 12 V, so Am=12 V. The carrier amplitude Ac is not explicitly given, but that’s fine — we only need the sideband amplitude.
- Expand the product Using the identity cosAcosB=21[cos(A+B)+cos(A−B)], we get:
s(t)=Accos(ωct)+2Acμcos[(ωc+ωm)t]+2Acμcos[(ωc−ωm)t]
The first term is the carrier. The second and third terms are the upper and lower sidebands.
- Identify the sideband amplitude Each sideband has amplitude 2Acμ=2Ac⋅(Am/Ac)=2Am …
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