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PhysicsMediumClass 11

Damped and Forced Oscillations — Resonance

SHM

8

JEE Qs

8%

Hard

75

min

Focus on understanding the conditions for resonance, the effect of damping on resonance amplitude and sharpness, and the physical significance of the Q-factor.

🧮 Key Formulas

Damped SHM Equation: m(d²x/dt²) + b(dx/dt) + kx = 0
Damped angular frequency: ω_d = sqrt(ω_0² - (b/2m)²) where ω_0 = sqrt(k/m) is natural frequency
Amplitude of damped oscillation: A(t) = A_0 * e^(-bt/2m)
Forced oscillation equation: m(d²x/dt²) + b(dx/dt) + kx = F_0 * cos(ωt)
Steady-state amplitude of forced oscillation: A = F_0 / (m * sqrt((ω_0² - ω²)² + (bω/m)²))
Resonance condition: ω = ω_0 (driving frequency equals natural frequency)
Amplitude at resonance (ω = ω_0): A_res = F_0 / (b * ω_0)
Quality factor (Q-factor): Q = ω_0 * m / b = ω_0 / (2 * γ) where γ = b/2m

✅ Key Points for JEE

  • 1Damping force (F_d = -bv) always opposes motion, causing amplitude to exponentially decay over time for free oscillations.
  • 2Resonance occurs when the external driving frequency (ω) matches the system's natural angular frequency (ω_0 = sqrt(k/m)), leading to maximum amplitude of oscillation.
  • 3The maximum amplitude at resonance is inversely proportional to the damping coefficient (b); less damping results in a larger and sharper resonance peak.
  • 4The quality factor (Q) quantifies how underdamped an oscillator is; a higher Q implies less energy loss per cycle and a sharper, taller resonance curve.
  • 5The phase difference between the driving force and displacement changes significantly near resonance.

⚠️ Common Mistakes

  • Confusing the natural angular frequency (ω_0) with the damped angular frequency (ω_d); resonance occurs at ω_0, not ω_d.
  • Ignoring the crucial role of damping in limiting the amplitude at resonance; without damping, the amplitude would theoretically be infinite.
  • Incorrectly interpreting the effect of changing damping on the sharpness or height of the resonance curve.
  • Not understanding that the damped oscillation angular frequency is always slightly less than the natural angular frequency in underdamped cases.

📝 Practice Questions

See all

Q36.A particle oscillates along the x-axis according to the law, x(t) = x0 sin2 ( 2t ) where x0 = 1 m . The kinetic energy (K) of the particle as a function of x is correctly represented by the graph (1) (2) (3) (4)

2025·Graph basedMedium

Q28.Two bodies A and B of equal mass are suspended from two massless springs of spring constant k1 and k2 , respectively. If the bodies oscillate vertically such that their amplitudes are equal, the ratio of the maximum velocity of A to the maximum velocity of B is (1) k1 (2) k2 √k1k2 (3) (4) k2 k1 √k2k1

2025·MCQEasy

Q41.A light hollow cube of side length 10 cm and mass 10 g , is floating in water. It is pushed down and released to execute simple harmonic oscillations. The time period of oscillations is yπ × 10−2 s, where the value of y is (Acceleration due to gravity, g = 10 m/s2 , density of water = 103 kg/m3 ) 2025 (23 Jan Shift 1) JEE Main Previous Year Paper (1) 6 (2) 2 (3) 4 (4) 1

2025·NumericalMedium

Q36.A particle is executing simple harmonic motion with time period 2 s and amplitude 1 cm . If D and d are the total distance and displacement covered by the particle in 12.5 s , then D is d (1) 16 (2) 10 5 (3) 15 (4) 25 4

2025·MCQMedium

Q35.Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : Knowing initial position x0 and initial momentum p0 is enough to determine the position and momentum at any time t for a simple harmonic motion with a given angular frequency ω. Reason (R): The amplitude and phase can be expressed in terms of x0 and p0 . In the light of the above statements, choose the correct answer from the options given below : (1) (A) is false but (R) is true (2) (A) is true but (R) is false (3) Both (A) and (R) are true but (R) is NOT the (4) Both (A) and (R) are true and (R) is the correct correct explanation of (A) explanation of (A)

2025·Assertion ReasoningMedium

Q24.The displacement of a particle executing SHM is given by x = 10 sin (wt + π3 )m. The time period of motion is 3.14 s. The velocity of the particle at t = 0 is ______ m/s.

2024·NumericalEasy

NCERT Chapters

  • Class 11 Physics Ch 14: Oscillations