NEET Physics Notes Waves-Wave Motion
When a large number of particles vibrates simultaneously in a medium, then disturbance propagates in the medium. The motion of disturbance is called wave motion. There are two types of wave motion as given below.
When particles of the medium vibrate parallel to the direction of propagation of wave, then wave is called longitudinal wave. These waves propagate in the form of compressions and rarefactions.
They involve changes in pressure and volume. The medium of propagation fnust possess elasticity of volume. They are set up in solids, liquids and gases.
These waves travel in the form of crest and trough set up alternatively. The medium must possess the elasticity of shape. There is no change in density of medium. These waves can be set up in solids, on surface of liquids but never in gases. Transverse waves undergo polarisation as against longitudinal waves, which do not get polarised. Some of the important terms of the wave motion are described below:
It is the number of waves travelled in per unit length. It is measured in (metre)-1.
It is the velocity of the particle executing in simple harmonic motion.
where, y denotes displacement at any instant.
The velocity of transverse wave motion is given by
Particle velocity changes with time but the wave velocity is constant. Acceleration of wave is zero but acceleration of particle is not zero.
Differential Equation of Wave Motion
Speed of Waves
Speed of waves are divided two types of waves. These are given below
Speed of Transverse Wave
The expression for speed of transverse waves in a solid and in case of a stretched string can be obtained theoretically given as
where η is the modulus of rigidity and d is the density of the medium
In a stretched string
where, T = the tension in the string,
m = the mass per unit length of the string,
M = mass suspended from the string,
r = radius of the string
and d = density of the material of the string.
Speed of Longitudinal Wave (or Sound Wave)
According to Newton formula, speed of sound in a gas is
Where B denotes the bulk modulus of the elasticity
D denotes the density of the medium
For gases, E = coefficient of adiabatic elasticity
Effect of Temperature on Velocity
With rise in temperature, then velocity of sound increases as
Effect of Pressure for Gases Medium
remains constant. Pressure has no effect on the velocity of sound, provide temperature remains constant.
Effect of Humidity
When humidity in air increases, its density decreases and so velocity of sound increases
where, Y = Young’s modulus of elasticity
K = bulk modulus of elasticity.