## Standing Waves and Normal Modes – Class 11 | Chapter – 15 | Physics Short Notes Series PDF for NEET & JEE

Standing Waves and Normal Modes: Standing waves, also known as stationary waves, are a type of wave that appears to remain in a fixed position while oscillating. Standing waves are formed by the interference of two waves that are traveling in opposite directions and have the same frequency. The wave crests of one wave line up with the troughs of the other wave, resulting in a pattern of stationary points along the medium.

## Standing Waves and Normal Modes

There are two types of standing waves: standing transverse waves and standing longitudinal waves. Standing transverse waves occur in a medium that can be displaced perpendicular to the direction of wave propagation, such as a stretched string or a guitar string. Standing longitudinal waves occur in a medium that can be displaced parallel to the direction of wave propagation, such as a pipe filled with a gas or a column of air.

The points along the medium where the wave amplitude is zero are called nodes, while the points where the wave amplitude is maximum are called antinodes. The distance between two consecutive nodes or antinodes is known as a wavelength, and the number of wavelengths that fit into a given distance is known as the wave number.

In conclusion, standing waves, also known as stationary waves, are a type of wave that appears to remain in a fixed position while oscillating. Standing waves are formed by the interference of two waves that are traveling in opposite directions and have the same frequency, and can occur in both transverse and longitudinal waves. The wave pattern of standing waves consists of nodes and antinodes, and the distance between these points is related to the wavelength and wave number of the wave.

### Standing Wave Equation

We can consider that, at any point in time, you and time t, there are generally two waves, one which moves to the left-hand side and the other which moves to the right-hand side. The wave when keeps traveling in the positive direction of the x-axis is given as,

y1(u, t) = a sin (Ku – ωt),

and that when traveling in the negative direction of the x-axis is given as,

y2(u, t) = a sin (Ku + ωt),

By the principle of superposition, the combined wave is stated as,

y (u, t) = y1(u, t) + y2(u, t)

= a (sin) of (ku – ωt) + a (sin) of (ku + ωt)

= (2a sin ku) cos ωt

This is the standing wave equation.

### How Are Standing Waves Formed?

Standing waves are formed by the interference of two waves traveling in opposite directions and having the same frequency. The wave crests of one wave line up with the troughs of the other wave, resulting in a pattern of stationary points along the medium.

Here’s an example of how standing waves can be formed in a string:

1. An initial disturbance is applied to one end of the string, causing it to vibrate and generate a wave that travels along the length of the string.
2. When the wave reaches the other end of the string, it is reflected and travels back in the opposite direction.
3. When the two waves meet, they interfere with each other, resulting in a pattern of stationary points along the string. The wave crests of one wave line up with the troughs of the other wave, resulting in a pattern of nodes and antinodes.
4. The pattern of stationary points is maintained as long as the two waves continue to interfere with each other. This creates the appearance of a wave that remains in a fixed position while oscillating.

In conclusion, standing waves are formed by the interference of two waves traveling in opposite directions and having the same frequency. The wave crests of one wave line up with the troughs of the other wave, resulting in a pattern of stationary points along the medium. This pattern of stationary points is maintained as long as the two waves continue to interfere with each other, creating the appearance of a wave that remains in a fixed position while oscillating.

### Normal Modes

Normal modes are a type of standing wave pattern that describe the motion of a physical system when it oscillates. Normal modes are characterized by their frequency, wavelength, and pattern of motion. In a system with multiple normal modes, each mode is independent of the others and has its own frequency, wavelength, and pattern of motion.

Normal modes are used to describe the vibration of objects such as strings, membranes, and solid bodies. For example, the normal modes of a guitar string can be used to predict the pattern of vibration when the string is plucked or strummed. Similarly, the normal modes of a drumhead can be used to predict the pattern of vibration when the drum is struck.

In conclusion, normal modes are a type of standing wave pattern that describe the motion of a physical system when it oscillates. Normal modes are characterized by their frequency, wavelength, and pattern of motion, and are used to describe the vibration of objects such as strings, membranes, and solid bodies.

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