Introduction
In the previous chapters we studied the different types of motion like uniform motion, accelerated motion, projectile motion and periodic motion. We then learnt to relate the acceleration of the body to the force applied. In this chapter we shall study the motion of collection of particles - wave motion.
Simple Pendulum and Restoring Force
A simple pendulum consists of a heavy mass (spherical in shape) suspended by a long weightless, inextensible and flexible string from a point about which it can oscillate. The mass which is attached to the end of the string is called the bob of the pendulum.
Simple Pendulum - Time Period of Oscillation Varies with Time
A simple pendulum whose time period is two seconds (i.e., on Earth) is called a Seconds Pendulum. A Seconds Pendulum is used in Pendulum clocks.
Forces Acting on the Bob of a Simple Pendulum
weighing with a spring balance or a pan balance is possible due to damping of the oscillations otherwise it would not have been possible to read the scale.
Waves and Wave Motion
Most of us have seen the formation of ripples when a pebble is dropped into a pond. The ripples are produced due to the disturbance created by the pebble. The ripples travel in concentric circles of ever-increasing radius till they strike the boundary of the pond.
Mechanical Waves and Electromagnetic Waves
A mechanical wave is a periodic disturbance, which requires a material medium (solid, liquid or gas) for its propagation. These waves are also known as elastic waves because their propagation depends upon the elastic properties of the medium through which they pass. Examples for mechanical waves are sound waves and water waves. In these waves, the particles of the medium just vibrate to and fro about their mean position.
Longitudinal and Transverse Waves
A wave motion in which the particles of the medium oscillate about their mean positions in the direction of propagation of the wave, is called longitudinal wave.
Graphical Representation of Waves
Whenever a wave passes through a medium, there is a change in some property of the medium. Hence, the waves can be graphically represented by showing the changes in the value of any such property of the medium as the waves travel through it.
Wave Pulse and Periodic Waves
All along when we were discussing about transverse and longitudinal waves the disturbances, which cause them were continuous. But the disturbances always need not be continuous i.e., it might last only for a short duration.
Wavelength, Frequency and Time Period
Wavelength is defined as the distance between the two nearest points on the wave or it can be defined as the distance between two consecutive crests or troughs in the case of a transverse wave or it is the distance between two consecutive compressions or rarefactions.
Sound
Sound is always produced due to the vibration of a body. In some cases the vibrations of the source may be very small or very large that it may not be possible to detect them. This type of vibrations is produced by tuning fork, drum, bell, the string of a guitar etc. Human voice originates from the vibrations of the vocal chords and the sound from the musical instruments is due to the vibrations of the air columns. We have already discussed that the sound travels in the form of longitudinal wave motion. Sound waves require a material medium for its propagation.
Speed of Sound
The flash of lightning due to collision of clouds is seen much before the thunder, although both occur simultaneously. This happens because the velocity of light is greater than the velocity of sound. The speed of sound depends on (1) elasticity and (2) density of the medium through which it propagates.
Practical Applications of Reflection of Sound
Megaphone: Megaphone is a horn-shaped tube. The sound waves are prevented from spreading out by successive reflections and are confined to the air in the tube.
Summary
WAVE is a form of disturbance, which travels through a material medium due to the repeated periodic motion of the particles of the medium about their position.
