NCERT Notes Class 11 Physics Chapter 13 Oscillations

These Class 11 Physics Notes Chapter 13 Oscillations pull together every definition, formula and derivation on simple harmonic motion that the Boards, JEE Main, JEE Advanced, NEET, CUET and NDA papers test in 2026-27. Use them to revise the whole chapter fast, with the SHM equations, energy results and pendulum formulas in one place.

Oscillations is the bridge chapter that turns your motion and energy skills into the study of repeating motion, and it feeds directly into Waves right after it.

• CBSE Weightage: 4 to 5 marks, usually one short answer plus one numerical on the time period or energy of SHM.
• Topics covered: periodic and oscillatory motion, SHM, phase, energy in SHM, spring and pendulum oscillations, damping, and resonance.
• Key formulas: displacement, velocity and acceleration in SHM, plus the time period of a spring and a simple pendulum.

These Class 11 Physics Notes Chapter 13 Oscillations are curated by subject experts, based on the 2026-27 NCERT textbook, and checked against the last five years of CBSE Board, JEE Main and NEET papers.

Topic-by-Topic Summary of Oscillations

The chapter builds up from any to-and-fro motion, narrows it to simple harmonic motion, and then applies that one idea to springs and pendulums. Here is the quick map of what each topic gives you.

• Periodic and oscillatory motion: motion that repeats, and the special back-and-forth kind called oscillation.
• Simple harmonic motion (SHM): the displacement, velocity and acceleration equations that define the whole chapter.
• Phase and circular motion: SHM seen as the shadow of a particle moving in a circle.
• Energy in SHM: how kinetic and potential energy trade places while the total stays fixed.
• Springs, pendulums, damping and resonance: real systems that oscillate, and what slows them or drives them.

Revise the topics in this order, because each one uses the one before it. Once you fix the meaning of ω, every formula in the chapter falls out of it. These Class 11 Physics Notes Chapter 13 Oscillations follow the same sequence as the NCERT textbook.

Periodic and Oscillatory Motion Explained

Periodic motion is any motion that repeats after a fixed time. Oscillatory motion is a special kind where the body moves to and fro about a mean position. Every oscillation is periodic, but not every periodic motion is an oscillation.

• Time period (T): the time for one full oscillation, measured in seconds.
• Frequency (ν): the number of oscillations per second, equal to 1/T, measured in hertz.
• Displacement: the distance of the body from its mean position at any instant.

Any periodic motion can be written as a sum of sine and cosine terms, which is why SHM matters so much. The planet going around the Sun is periodic but not oscillatory; a swinging pendulum is both. Keep this difference ready, because Boards ask it as a one-mark definition question.

Simple Harmonic Motion: Displacement, Velocity and Acceleration

Simple harmonic motion is oscillatory motion where the acceleration is always directed towards the mean position and is proportional to the displacement. This single rule gives the three equations you use in every SHM numerical.

Velocity is largest at the mean position and zero at the extremes; acceleration is the reverse. The defining test of SHM is a = −ω²x — acceleration proportional to displacement and opposite in sign. A useful side result is v = ±ω√(A² − x²), which links speed to position without needing time.

Phase and SHM as a Projection of Uniform Circular Motion

The quantity (ωt + φ) is called the phase, and it fixes the state of the oscillator at any instant. The constant φ is the phase constant, set by where the body starts. Phase is the idea that lets you compare two oscillations.

• Angular frequency (ω): the rate of change of phase, equal to 2π/T = 2πν.
• Phase constant (φ): the phase at time zero, which decides the starting position and direction.
• In phase / out of phase: two SHMs with a phase difference of 0 move together; a difference of π means they are exactly opposite.

SHM is the projection, or shadow, of uniform circular motion on any diameter. Picture a particle going round a circle of radius A; the foot of the perpendicular from it onto a diameter performs SHM. This picture is why ω is called angular frequency, and it makes the sine and cosine forms feel natural rather than memorised.

Energy in Simple Harmonic Motion

An oscillator carries both kinetic and potential energy, and the two keep swapping while the total stays constant. This is one of the most-tested ideas in the chapter, so learn where each energy peaks.

The total energy of SHM stays the same at every point and depends on the square of the amplitude. Both kinetic and potential energy vary with time, but they add up to a fixed value. A frequent exam point is that both energies complete one cycle in half the time period of the displacement.

Oscillations of a Spring and the Force Law for SHM

A block on a spring is the cleanest example of SHM. The spring provides a restoring force that always points back to the mean position, and this force is what makes the motion simple harmonic.

• Force law: the restoring force is F = −kx, where k is the spring constant.
• Angular frequency: ω = √(k/m), so a stiffer spring oscillates faster.
• Time period: T = 2π√(m/k), which grows with mass and shrinks with stiffness.

The minus sign in F = −kx is the whole point, because it means the force always fights the displacement. Comparing F = −kx with ma = −mω²x gives ω² = k/m in one line. Notice the time period of a spring does not depend on gravity, which is a favourite trick in objective papers.

The Simple Pendulum and Its Time Period

A simple pendulum is a small bob on a light, inextensible string. For small swings, its motion is simple harmonic, and its time period is one of the most useful results in the chapter.

The time period of a simple pendulum does not depend on the mass of the bob or on the amplitude, only on length and gravity. This holds only for small angles, where sinθ ≈ θ. A pendulum that keeps correct time on Earth would run slow on the Moon, because g there is smaller.

Damped Oscillations, Forced Oscillations and Resonance

Real oscillators lose energy and often need a push to keep going. The last part of the chapter covers what friction does to an oscillation and what happens when you drive it with an outside force.

• Damped oscillation: a resistive force slowly reduces the amplitude, so the swing dies out over time.
• Forced oscillation: an external periodic force keeps the body oscillating at the driving frequency.
• Resonance: the amplitude becomes very large when the driving frequency matches the natural frequency of the system.

In damped SHM the amplitude falls off as A e^−bt/2m, where b is the damping constant. Resonance is why soldiers break step on a bridge and why a radio picks out one station. These real-world links are common short-answer prompts, so keep one example of each ready for the exam.

All Formulas for Oscillations

Every formula you need for the chapter sits in one table below, with its meaning and its SI unit. Learn the SHM equations and the two time-period formulas first, since those carry the most marks in both Boards and entrance papers.

Carry the SI unit on every line of your working. Losing the unit is a silent way to drop the final mark even when the number is right. Keep this table open while you solve the back-exercise numericals.

Key Definitions and Derivations for Oscillations

Boards short-answer questions often ask for a clean definition in one or two lines. Learn these word-for-word, because a vague definition loses easy marks. Each one also sets up a derivation you can be asked to show.

A common derivation asks you to show that a loaded spring performs SHM. Start from F = −kx, write ma = −kx, and read off a = −(k/m)x to get ω = √(k/m) and then T = 2π√(m/k). The same method, starting from the restoring torque, gives the pendulum result.

Common Mistakes Students Make in Oscillations

These slips happen while writing or calculating, not because the concept is unclear. Each one costs 1 to 3 marks in the paper, so watch for them at the exact step.

Oscillations Weightage in CBSE Boards, JEE and NEET

This chapter is small but reliable. It usually carries one short answer plus one numerical, and it links closely to the Waves chapter that follows. Here is how the marks split across the main exams for 2026-27.

The time period of a spring and a pendulum is the single most tested idea from this chapter across all four exams. Master those two formulas first, then the energy results, then phase and resonance, in that order of return on effort.

How to Revise Oscillations Quickly

Use these Class 11 Physics Notes Chapter 13 Oscillations for a fast, ordered recap the night before a test. The checklist below takes about 30 minutes and hits every marks-heavy idea.

• First 10 minutes: write the displacement, velocity and acceleration equations of SHM and the meaning of ω, A and φ from memory.
• Next 10 minutes: redo one spring numerical and one pendulum numerical using the two time-period formulas.
• Last 10 minutes: sketch the energy graph and mark where kinetic and potential energy peak.

Close the loop by writing one line each on damping and resonance. If you can do all four blocks without notes, the chapter is exam-ready. Keep the All Formulas table beside you for the first pass only, then try it closed-book.

Student Feedback on the Oscillations Notes

Other Oscillations Class 11 Physics Resources

Pair these notes with the solved answers, the handwritten notes, the formula sheet, and the textbook PDF for the same chapter.

NCERT Notes for Class 11 Physics: All Chapters

Jump to the revision notes for any other Class 11 Physics chapter below.

FAQs on Oscillations Class 11 Physics Notes