Tiêu đề WAVES and SOUNDS.pdf ✅

CLASS VIII PHYSICS Introduction of wave : There are two ways of transfer of energy from one point to other. 1. By the transport of matter and 2. Without the transport of matter (i.e., through wave motion) wave : A wave is produced by the vibrations of the particles of the medium through which it passes. When a wave passes through a medium, the medium itself does not move along the direction of the wave, only the particles of the medium vibrate about their fixed positions. For example, when a water wave passes over the surface of water in a pond, it does not drive water to one side of the pond, only the water molecules vibrate up and down about their fixed position. Similarly, when sound waves produced by ringing bell, come to us through air, there is no actual movement of the air from the bell to our ears. Only the sound energy travels through the vibrations of the air molecules. The light from the sun also comes to us in the form of light waves, there being no direct contact between the sun and the earth. Thus A wave is a disturbance by which energy is transferred from one point to the other through vibrations of the me- dium particles without their actual movements. (or) A periodic disturbance produced in a material medium due to the vibrating motion of the particles of the me- dium is called a wave. wave motion : The movement of a disturbance produced in one part of a medium to another involving the transfer of energy but not the transfer of matter is called wave motion. Example : The following are examples of wave motion 1. Formation of ripples on the water surface. 2. Propagation of sound wave through air or any other material medium. ➢ The speed of a travelling wave 1. Phase : The points on a wave which are in the same state of vibration are said to be in the same phase. In the above figure A, B are in the same phase and C, D are in the same phase. Example: All points that lie on the crests are in the same phase similarly all points that lie on the trough are in the same phase. 2. Time period (T) or Periodic time : The time in which a vibration body completes one vibration is called time period (or) The time required to produce one complete wave (or cycle) is called time period of the wave. WAVES AND SOUND SYNOPSIS - 1

CLASS VIII PHYSICS Here ' A ' is the amplitude of the wave. nit : The S.I unit of amplitude is metre (m). Note: The amplitude of a wave is a measure of its energy. Thus the greater the amplitude of wave, the greater the energy carried by the wave. Relation between time Period (T) and frequency (f) : The frequency of a wave is the reciprocal of its time - period. i.e., frequency = 1 time period ⇒ f = 1 T ⇒ T = 1 f Where f = frequency of the wave and T = time period of the wave. Relationship between, time period, wavelength and velocity: We know that, velocity of the wave = distance travelled the wave time taken Suppose a wave travels a distance λ in time T, then v = λ T (i.e. the relation between wave velocity, wavelength and time period) Hence T is the time taken by one wave. We know that 1 T becomes the number of waves per second and this is known as frequency (f) of the waves. So the above equation can also be written as, v = f × λ Where, v = velocity of the wave f = frequency λ = wavelength In other words velocity of a wave = frequency x wavelength Thus, the velocity (or speed) of a wave in a medium is equal to the product of its frequency and wavelength. Thus formula v = f × λ is called wave equation. 1. The time period of a wave is equal to reciprocal of the frequency of a wave. i.e., T = 1 f ⇒ f = 1 T 2. Wave velocity = distance travelled by the wave timetaken ⇒ v = λ T 3. Velocity of wave = frequency × wavelength v = f × λ ⇒ f = v λ ⇒ λ = v f The Principle of superposition of waves: The displacement at any time due to a number of waves meeting simultaneously at a point in a medium is the vector sum of the individual displacements due to each one of the waves at that point at the same time. If y ̅1, y⃗ 2 , y⃗ 3 ... ... ... are the displacements at a particular time at a particular position, due to individual waves, then the resultant displacement. y = y⃗ 1 + y⃗ 2 + y⃗ 3 + ⋯ ⋯ ⋯