Speed Travelling Wave

8 MCQs2 revision cards9-step worked example

Source: NCERT Oscillations and WavesPYQ coverage: NEET 2020, 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

The speed of a travelling wave depends on the medium, not on the source. Two formulas govern this topic at the NEET level, and one common error connects them.

Transverse wave on a string. The speed is v = √(T/μ), where T is the tension in the string and μ is the linear mass density (mass per unit length). This is derived from Newton's second law applied to a small element of the string (NCERT Class 11 Physics Chapter 14, page 4). The key relationship: v scales as √T, not as T. Doubling the tension increases speed by a factor of √2, not 2. This is a high-frequency distractor in NEET — students who treat v as directly proportional to T pick the wrong option.

Speed of sound in a gas. The Newton-Laplace formula gives v = √(γP/ρ), where γ is the adiabatic index, P the gas pressure, and ρ the density (NCERT Class 11 Physics Chapter 14, page 5). Newton's original formula used isothermal bulk modulus (B = P), predicting a value about 15% below the measured speed of sound in air. Laplace corrected it by recognising that sound compressions are adiabatic, replacing P with γP.

The connection that trips students: both formulas have the form v = √(elastic property / inertial property). For the string, the elastic property is tension and the inertial property is linear mass density. For the gas, the elastic property is γP (adiabatic bulk modulus) and the inertial property is density ρ.

Watch out: wave speed on a string is independent of frequency and wavelength — those are set by the source. Changing tension changes v, which then changes the wavelength for a given frequency (v = fλ), but does not change the frequency itself.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

The speed of a transverse wave on a stretched string depends on:

MCQ 2Easy RecallPractice

Laplace corrected Newton's formula for the speed of sound in air by replacing the isothermal process with:

MCQ 3Easy RecallPractice

In the Newton-Laplace formula v = √(γP/ρ), what does γ represent?

MCQ 4Direct ApplicationPractice

The tension in a stretched string is increased to 4 times its original value while the linear mass density remains unchanged. The wave speed becomes:

MCQ 5Direct ApplicationPractice

A transverse wave travels on a string with speed 40 m/s when the tension is 100 N. If the tension is reduced to 25 N, what is the new wave speed?

MCQ 6Direct ApplicationPractice

The speed of sound in oxygen at a given temperature is v. The speed of sound in hydrogen at the same temperature and pressure is (assume both are diatomic):

MCQ 7CalculationPractice

A string of length 2.0 m has a total mass of 20 g. It is stretched with a tension of 80 N. A transverse wave of frequency 200 Hz travels along the string. The wavelength is:

MCQ 8CalculationPractice

The speed of sound in air at 0 °C is 332 m/s. If the temperature is raised to 546 °C, the speed of sound becomes approximately:

Quick recall before you leave

Worked Example

Pattern: Wave speed on a string changes when tension changes (based on PYQ pattern NEET pattern: wave speed tension — observed NEET 2022).

1
Given
- v₁ = 60 m/s - T₁ = 36 N - T₂ = 144 N - μ unchanged
2
Required
- v₂ = ?
3
Concept
Wave speed on a string depends on tension and linear mass density: v = √(T/μ). When μ is constant, speed scales as the square root of tension.
4
Formula
v₂/v₁ = √(T₂/T₁)
5
Substitution
v₂/v₁ = √(144/36) = √4
6
Calculation
v₂/v₁ = 2 v₂ = 2 × 60 = 120 m/s Note on exact values: 36 N, 144 N, and 60 m/s are problem-defined exact values. The integers 2 and 4 are exact. These do not limit significant figures.
7
Final answer
v₂ = 120 m/s
8
Common trap
Treating v as directly proportional to T (linear scaling) would give v₂ = 60 × (144/36) = 60 × 4 = 240 m/s — exactly double the correct answer. The square-root dependence is the key: v ∝ √T.
9
Similar NEET-style question
A wave travels at speed v on a wire under tension T. The wire is replaced by one with the same material but double the diameter (hence 4× the cross-sectional area and 4× the linear mass density). If tension remains the same, what is the new speed? (Answer: v/2, since v = √(T/μ) and μ → 4μ gives v → v/√4 = v/2.) ---

Before solving, remember these

Formula

Speed of wave on string

v = √(T/μ), where T is tension and μ is linear mass density (mass per unit length). Doubling tension increases speed by √2.

-- NCERT, p. 4

Formula

Speed of sound in gas

v = √(γ P/ρ) (Newton-Laplace formula). For air at STP: v ≈ 343 m/s. Increases with temperature: v ∝ √T.

-- NCERT, p. 5

Formulas

10 formulas — click to collapse

Beat frequency

f_{beat} = ∣ f_{1} - f_{2} ∣

When two waves of nearly equal frequencies superpose, amplitude oscillates at the difference frequency.

Symbol	Quantity	SI Unit
f_beat	beat frequency	Hz
f1, f2	superposed frequencies	Hz

Valid when

Linear superposition
f1, f2 close in value

Period of simple pendulum (small angle)

T = 2 π \frac{L}{g}

Period of simple pendulum of length L. Holds for small amplitudes (sin theta ~ theta).

Symbol	Quantity	SI Unit
T	period	s
L	pendulum length	m
g	gravity	m/s^2

Valid when

Small angular amplitude (typically <15°)
Massless string
Point bob

SHM displacement

x (t) = A cos (ω t + ϕ); ω = 2 π f

Displacement in simple harmonic motion. Velocity = -A*omega*sin(omega*t+phi); a = -omega^2 * x.

Symbol	Quantity	SI Unit
A	amplitude	m
omega	angular frequency	rad/s
phi	phase	rad
T	period	s
f	frequency	Hz

Valid when

Restoring force linear (F = -kx)
No damping

Total energy in SHM

E = \frac{1}{2} k A^{2} = \frac{1}{2} m ω^{2} A^{2}

Total mechanical energy is constant. Oscillates between KE (max at x=0) and PE (max at x=±A).

Symbol	Quantity	SI Unit
E	total energy	J
k	spring constant	N/m
A	amplitude	m
m	mass	kg
omega	angular frequency	rad/s

Valid when

Conservative SHM (no damping)
Elastic regime

Period of mass-spring oscillator

T = 2 π \frac{m}{k}

Period of horizontal spring with mass m, spring constant k. Independent of amplitude.

Symbol	Quantity	SI Unit
T	period	s
m	mass	kg
k	spring constant	N/m

Valid when

Hooke's law spring
No damping
Small enough amplitude to stay in elastic regime

Standing wave in closed-end pipe

f_{n} = \frac{( 2 n - 1 ) v}{4 L}

Pipe closed at one end has only odd harmonics: f, 3f, 5f, ...

Symbol	Quantity	SI Unit
f_n	n-th harmonic	Hz
v	sound speed	m/s
L	pipe length	m

Valid when

Closed at one end (open at other)
End correction neglected

Standing wave in open-open pipe

f_{n} = \frac{n v}{2 L}

Pipe open at both ends has all harmonics. Same formula as string.

Symbol	Quantity	SI Unit
f_n	n-th harmonic	Hz
v	sound speed	m/s
L	pipe length	m

Valid when

Open at both ends
End correction neglected

Standing wave frequencies on fixed-fixed string

f_{n} = \frac{n v}{2 L}

Allowed frequencies on string fixed at both ends. n=1 fundamental; harmonics 2f, 3f, ...

Symbol	Quantity	SI Unit
f_n	n-th harmonic	Hz
v	wave speed on string	m/s
L	string length	m
n	harmonic number	-

Valid when

String fixed at both ends
Wave speed v as defined above

Speed of sound in gas (Newton-Laplace)

v = \frac{γ P}{ρ}

Speed of sound in gas. Adiabatic index gamma, pressure P, density rho. Increases with sqrt(T).

Symbol	Quantity	SI Unit
v	speed of sound	m/s
gamma	adiabatic index	-
P	pressure	Pa
rho	density	kg/m^3

Valid when

Ideal gas
Adiabatic compression/expansion of sound waves

Wave speed on string

v = \frac{T}{μ}

Speed of transverse wave on string under tension T, linear mass density mu.

Symbol	Quantity	SI Unit
v	wave speed	m/s
T	tension	N
mu	linear mass density	kg/m

Valid when

Stretched uniform string
Small amplitude

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

6 items — click to collapse

Trap

Pendulum: mass-dependence claim

Category: Overthinking

Student writes T as depending on bob mass. Simple pendulum T = 2π√(L/g); independent of m.

When it triggers

Question changes pendulum bob mass and asks for new period.

How to avoid

Mass cancels in derivation (gravitational mass = inertial mass). Mass changes the bob's KE and PE proportionally; period unaffected.

Trap

SHM period: amplitude dependence claim

Category: Overthinking

Student claims SHM period depends on amplitude. For ideal SHM (Hooke's law spring or simple pendulum at small angle), period is INDEPENDENT of amplitude.

When it triggers

Question gives changes in amplitude and asks for new period.

How to avoid

T = 2π√(m/k) (spring) or 2π√(L/g) (pendulum, small angle) — neither depends on A. Only at large pendulum angles does T pick up a small amplitude correction.

Trap

Pipe harmonics: open vs closed end

Category: Similar Terms

Student includes even harmonics in a closed-end pipe. Closed pipe has only ODD harmonics (f, 3f, 5f, ...).

When it triggers

Question describes pipe closed at one end (e.g. resonance tube).

How to avoid

Open both ends: all harmonics, f_n = nv/(2L). Closed one end: odd only, f_n = (2n-1)v/(4L). Fundamental of closed pipe is HALF that of open pipe of same L.

Mistake

Includes bob mass in simple pendulum period formula.

Root cause: concept gap

Correction

Simple pendulum T = 2π√(L/g) — independent of mass. Equivalence of inertial and gravitational mass cancels m.

Mistake

Claims SHM period depends on amplitude.

Root cause: concept gap

Correction

Ideal SHM: T = 2π√(m/k) (spring) or 2π√(L/g) (pendulum, small angle) — no amplitude dependence. Doubling amplitude does not change period.

Mistake

Includes even harmonics in a closed-end pipe.

Root cause: concept gap

Correction

Closed-end pipe has only ODD harmonics (f, 3f, 5f, ...). Open-both-ends pipe has all (f, 2f, 3f, ...). Reason: closed end has displacement node and pressure antinode.

Past Year Questions

11 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys. — click to collapse

NEET 2025

A pipe open at both ends has a fundamental frequency f in air. The pipe is now dipped vertically in a water drum to half of its length. The fundamental frequency of the air column is now equal to: f

12f

22 3f

NTA Answer: Option 3(final)

NEET 2025

Two identical point masses P and Q, suspended from two separate massless springs of spring constants k 1 and k 2, respectively, oscillate vertically. If their maximum speeds are the same, the ratio (AQ/AP) of the amplitude AQ of mass Q to the amplitude AP of mass P is k k

22 k k 2 1

3k 1

4k 2 k k 2 1

NTA Answer: Option 1(final)

NEET 2024Revised key

 π If x =5sinπt + m represents the motion of a particle executing simple harmonic motion, the amplitude  3 and time period of motion, respectively, are

15 cm, 2 s

25 m, 2 s

35 cm, 1 s

45 m, 1 s

NTA Answer: Option 2(revised_final)

NEET 2024Revised key

If the mass of the bob in a simple pendulum is increased to thrice its original mass and its length is made half x its original length, then the new time period of oscillation is times its original time period. Then the value of 2 x is:

32 3

NTA Answer: Option 2(revised_final)

NEET 2023

The ratio of frequencies of fundamental harmonic produced by an open pipe to that of closed pipe having the same length is

12 : 1

21 : 3

33 : 1

41 : 2

NTA Answer: Option 1(final)

NEET 2023

The x-t graph of a particle performing simple harmonic motion is shown in the figure. The acceleration of the particle at t = 2 s is π2 π2

1– ms–2

2ms–2 8 16 π2 π2

3– ms–2

4ms–2 16 8

NTA Answer: Option 3(final)

NEET 2022

If the initial tension on a stretched string is doubled, then the ratio of the initial and final speeds of a transverse wave along the string is

11 : 2

21 : 1

32 : 1

41 : 2

NTA Answer: Option 4(final)

NEET 2022

Two pendulums of length 121 cm and 100 cm start vibrating in phase. At some instant, the two are at their mean position in the same phase. The minimum number of vibrations of the shorter pendulum after which the two are again in phase at the mean position is:

211

410

NTA Answer: Option 2(final)

NEET 2021

A spring is stretched by 5 cm by a force 10 N. The time period of the oscillations when a mass of 2 kg is suspended by it is

10.628 s

20.0628 s

36.28 s

43.14 s

NTA Answer: Option 1(final)

NEET 2020

U U U U A U B Sfl U U U 6 Hz fl 25. In a guitar, two strings A and B made of same flS U U – B fl U U material are slightly out of tune and produce beats , flS fl U of frequency 6 Hz. When tension in B is slightly decreased, the beat frequency increases to 7 Hz. 7 Hz – A fl 530 Hz , B If the frequency of A is 530 Hz, the original fl — frequency of B will be :

1537 Hz

2523 Hz

3524 Hz

4536 Hz

NTA Answer: Option 3(final)

NEET 2020

U fl U flS U fl U 29. The phase difference between displacement and U — acceleration of a particle in a simple harmonic motion is :

1zero

2rad 3

3rad 2

4rad 2

NTA Answer: Option 2(final)

How NEET usually asks this

6 recurring patterns from past papers — click to collapse

Two strings/forks slightly out of tune produce beats; find one frequency given the other and beat frequency.

Direct ApplicationEasy

Common distractors

treats beat as sum

Adds frequencies instead of subtracting

Given pendulum length and g, find T. Or given mass change, observe T unchanged.

Direct ApplicationEasy

Common distractors

expects mass dependence

Assumes T depends on bob mass

Open-open pipe (all harmonics) vs closed-end pipe (odd only). Compare frequency ratios.

InterpretationMedium

Common distractors

treats closed pipe like open

Includes even harmonics in closed pipe

Phase between displacement, velocity, acceleration in SHM. v leads x by π/2; a leads x by π.

InterpretationEasy

Common distractors

uses pi 2 instead of pi

Confuses v-x phase with a-x phase

Given spring extension/compression with given force, find k, then find period when given mass m. T = 2pisqrt(m/k).

Multi StepEasy

Common distractors

forgets 2pi factor

Drops 2*pi

uses amplitude in period

Believes T depends on amplitude

Given tension change, find new wave speed on string. v ∝ sqrt(T).

Direct ApplicationEasy

Common distractors

uses linear tension scaling

Treats v ∝ T not sqrt(T)

Sources

NCERT refs: Class 11 Physics Chapter 14, p.4 | Class 11 Physics Chapter 14, p.5

Test yourself on this topic with real past-paper questions:

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Simple Pendulum Spring Oscillations

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Speed Travelling Wave

Lesson

Practice MCQs

Quick recall before you leave

Wave speed on a string

Speed of sound in a gas (Newton-Laplace)

Worked Example

Before solving, remember these

Formulas

Valid when

Valid when

Valid when

Valid when

Valid when

Valid when

Valid when

Valid when

Valid when

Valid when

Exam Traps & Common Mistakes

When it triggers

How to avoid

When it triggers

How to avoid

When it triggers

How to avoid

Correction

Correction

Correction

Past Year Questions

How NEET usually asks this

Common distractors

Common distractors

Common distractors

Common distractors

Common distractors

Common distractors

Sources

Free NEET study resources