Oscillations | Class 11 Physics Notes

13.2 Periodic and oscillatory motions

Periodic motion is defined as any motion that repeats itself identically after a fixed interval of time called the period (T). Examples include an insect climbing up and down a ramp, a child climbing steps repeatedly, or a ball bouncing between the palm and the ground. The height of the object above the ground plotted against time shows repetitive patterns characteristic of periodic motion.

Oscillatory motion is a special kind of periodic motion where the body moves to and fro about an equilibrium position. At this equilibrium position, no net external force acts on the body, so if left undisturbed, it remains at rest there. When displaced slightly, a restoring force acts to bring it back, causing oscillations. For example, a ball placed in a bowl oscillates about the bottom point when displaced.

Not all periodic motions are oscillatory. Circular motion is periodic but not oscillatory because the motion is along a closed path without a to-and-fro nature. The terms oscillation and vibration are often used interchangeably; oscillations usually refer to lower frequency motions (like a swinging branch), while vibrations refer to higher frequency motions (like a vibrating string).

Simple harmonic motion (SHM) is the simplest form of oscillatory motion, where the restoring force is directly proportional to the displacement and directed towards the mean position. In real systems, oscillations eventually dampen due to friction and other dissipative forces, but they can be sustained by external periodic forces.

Periodic motions of coupled oscillators in materials give rise to waves, such as water waves, seismic waves, and electromagnetic waves, which will be studied in the next chapter.

📊 Diagram: Fig. 13.1 Examples of periodic motion. The period T is shown in each case.

🔗 Connection: This section introduces periodic and oscillatory motions, leading to the mathematical description of periodic functions and displacement variables in the next subsection.