SOUND AND ITS NATURE & PROPAGATION

HOW SOUND IS PRODUCED?

Sound is a form of energy and it is produced by a vibrating body. Whenever a sound is produced, the vibration of the sounding body can be observed or felt.

Experiment: Place a pan upside down on a table and strike it with a spoon. A particular sound will be heard. We can feel its vibrations by touching it with our fingers. We can also see its vibrations by placing small pieces of paper on the pan, which start jumping.

NATURE AND PROPAGATION O F SOUND

Sound waves are Compressional waves. To verify and study the nature and propagation of sound we consider the motion of a vibrating tuning fork. Strike a tuning fork on a rubber pad. The prongs of such a tuning fork begin to vibrate.

When the prongs move right to the equilibrium position, they compress the layer of air in front of it. The compressed layer transfers its increased pressure to the air layer next to it. This process continues and the compression produced by rightward movement of the prong. Now when the prongs move left to the equilibrium position, the pressure in the adjacent layers of air decreases and a rarefaction is produced here. This first layer transfers its decrease in pressure

To the next layer and after doing so it comes to its normal position. As the tuning fork continues to vibrate to and fro against its mean position compressions and rarefactions are produced and travels outwards one after the other. Such a series of compression and rarefactions are the sound waves.

“SOUND CANNOT TRAVEL IN VACUUM”

The apparatus consists of a jar which is placed on the platform of a vacuum pump. As electric bell is suspended in the jar with the help of two wires passing through a cork fixed in its mouth. The electric bell will begin to ring on connecting the free ends of the two wires to a battery. The sound of the bell can be heard. Now start pumping out air from the jar by means of vacuum pump. The sound of the bell starts becoming feeble. At last the sound of the bell becomes hardly audible in spite of the fact that we see the hammer stinking the bell. We can conclude from the experiment that the presence of air is necessary for the propagation of sound.

WHAT IS AUDIBLE FREQUENCY RANGE?

Answer.         Sound is produced by a vibrating body. A simple pendulum also vibrates but it does not produce any sound. The reason is that its vibrations are very slow. And a human ear can hear a sound only if its frequency lies between 20 and 20000 Hz. Sounds of frequency beyond 20000 Hz are inaudible because the eardrum cannot vibrate so rapidly. The audible frequency range differs a little for different persons. Young children can hear sounds of 20000 Hz frequency, but old people cannot hear sounds even above 15000 Hz frequency.

Read More

1. Important Trigonometric Ratios
2. Waves & Its Types
3. What are Physical Quantities?
4. Scientific Method

AMPLITUDE, FREQUENCY, WAVELENGTH, TIME PERIOD, CREST, TROUGH, REFLECTION, REFRACTION, DIFFRACTION AND INTERFERENCE.

Amplitude: The distance of wave between its mean position and its maximum position is known as amplitude.

Frequency: The number of waves passing through a certain point of a medium in one second is known as the frequency.

Crest: Those parts of the transverse waves where the particles of the medium are above their normal position are called crest.

Trough: Those parts of the transverse waves where the particles of the medium are below their normal position are called crest.

Reflection: When a wave collides with a transparent medium, the waves will always reflect in such a way that the angle at which they approach the medium equals the angle at which they reflect off the barrier.

Refraction: When a ray of light enters from one transparent medium to another transparent medium, it bends from its path. This bending of waves from their incident path is called refraction.

Diffraction: When a wave passes through such a slit whose size is equal or less then the wavelength of wave, then the wave bend around the corners. This bending of waves around the corners is called diffraction.

Interference: When two identical waves passing through a medium, then either it makes a wave of double amplitude or they cancel the effect of each other.

Types: There are two types of interference.

1. Constructive Interference: If two identical waves propagate through a medium such a way that crest of one wave falls on the crest of the other and trough of the one falls on the trough of the other, the resultant displacement of particles get double of the individual wave. Such interference is known as constructive interference.

2. Destructive Interference: If two identical waves propagate through a medium such a way that crest of one wave falls on the trough of the other and trough of the one falls on the crest of the other, they cancel the effect of each other. Such interference is known as destructive interference.

STATIONARY WAVES.

When two identical waves pass through a medium along the same line but in opposite directions, then stationary waves are produced.

Explanation: Fasten one end of a string to a firm support and hold its other free end by your hand. The up and down of the hand will produce waves in the string. If we keep on increasing  the rate of up and down motion of hand, then at a particular frequency, the string will start vibrating making a shape of loops, which is known as stationary waves.

Nodes: At some portions of the string the amplitude is zero are called nodes

Anti nodes: At some portions of the string where amplitude is maximum are called antinodes.

The number of loops of stationary wave depend upon the frequency of that wave. The string makes a single loop at the lowest frequency, known as fundamental frequency or first harmonic. If the string makes two loops the frequency is called second harmonic and so on.

VIBRATORY MOTION.

When a body moves to and fro about a point and repeat its motion again and again about the same point then this type of motion is called vibratory motion.

Example: Pendulum

HOOKE’S LAW.

The external force acting on a body is directly proportional to the increase in length. 

MATHEMATICAL PROBLEMS

Problem 1: If five waves pass through a point of a medium in 10 seconds. What is its frequency and time period? If its wavelength is 5 cm, calculate the wave speed.

Problem 2: The time period of a wave is 5s. If velocity of the wave is 10 m/s, then what will be its wavelength?

Problem 3: A stationary wave is making 5 loops. The distance between two stationary points is 10 cm and its velocity is 20 m/s. what will be its frequency? What will be the fundamental harmonic frequency?

IMPORTANT POINTS TO REMEMBER

10. The wavelength of the stationary wave is twice the distance between two successive nodes or antinodes.

Read More

1. Important Trigonometric Ratios

2. Contribution of Muslim Scientists in the field of Science

3. Physics & Its Branches

4. Classification of computers

SIMPLE HARMONIC MOTION

Simple harmonic motion is a simple vibratory motion in which any body continues to move to and fro about its mean position. For example: motion of pendulum, motion of perpendicularly hanged body with spring.

Explanation: Consider the motion of a mass attached with a spring. Its one end is attached to a firm support and a mass “m” is attached to its other end. Currently the spring is in equilibrium position. If an external force “F_ext”is applied on the mass “m”, length of the spring increases by an amount “x”. The external force “F_ext” acting on the body is directly proportional to the increase in length “x”.

This means that the acceleration of the body is directly proportional to its displacement and is always directed towards its mean position. As the mass “m” moves towards the equilibrium position its displacement goes on decreasing. Resultantly the acceleration of the body also decreases. On reaching the equilibrium position, displacement becomes zero also acceleration becomes zero. But at this point velocity is maximum, so due to inertia the mass does not stop at this position but continues its motion towards left and the spring is being compressed. Hence the body of mass “m” keeps on vibrating.

RESTORING FORCE.

When an external force is applied on a spring, the length of the spring increases. After removing the force the spring moves towards its equilibrium position. The force due to which it moves towards its equilibrium position is known as restoring force.

SIMPLE PENDULUM & ITS MOVEMENT

Definition: A simple pendulum consists of a single isolated bob suspended from a frictionless support by means of a light inextensible string.

Explanation: The motion of simple pendulum is also simple harmonic motion. In equilibrium, the pendulum is held stationary in a vertical position. If the bob is disturbed from its equilibrium position, it will start moving towards its mean position under the action of one component of gravitational force. At equilibrium position the velocity of the bob is maximum and due to inertia it passes by its mean position towards the other end. But now the velocity of the bob decreases, and becomes zero as it reaches at its maximum point on the other end. And then bob starts moving between one end to the other end. The acceleration of the bob always remains toward the mean position.

CHARACTERISTICS OF SIMPLE HARMONIC MOTION

(i)         A body executing simple harmonic motion always vibrates about its equilibrium position.

(ii)        Its acceleration is always directed towards its mean position.

(iii)       Its acceleration is directly proportional to its displacement from the mean position. i.e 0 at mean position and maximum at extreme position.

(iv)       Its velocity is maximum at the mean position and zero on the extreme position.

Mathematical Problems

Problem 1: The time period of a simple pendulum is 2s. What will be its length on earth and on moon?

Problem 2: Length of a spring is 8 cm. when a mass of 4 Kg is hung to it, its length becomes 16 cm. what is its spring constant?

Read More

1. Important Trigonometric Ratios

2. Contribution of Muslim Scientists in the field of Science

3. Physics & Its Branches

4. Classification of computers

WAVES & ITS TYPES

WAVES

Wave motion in a medium is due to the generated disturbance which causes the constituent particles to repeat its to and fro motion about its mean position in equal interval of time, and this disturbance is passed over from one end of the medium to the other. Wave is form of energy.

Example: Dip one end of a pencil at the edge of a tub containing water. Rapidly move the pencil up and down vertically. The ripples spread outwards on the surface of the water. Place some small pieces of cardboard equally spaced in the direction of the waves, and observe the movement of pieces as the wave pass. Every piece of cardboard moves up and down about its mean position. They are not displaced forward from their original positions along with the water waves. The motion of these pieces about their mean position is known as vibratory motion.

TYPES OF WAVES

There are two types of waves.

1. MECHANICAL WAVES: The waves which require a medium for their propagation is known as mechanical waves.  It is further divided into two types.

(i) Transverse waves: Transverse waves are such waves in which the particles of the medium vibrate perpendicular to the direction of propagation of the wave.

Example: Water waves, waves produced by the string

(ii) Compressional or longitudinal waves. Waves in which the direction of the vibratory motion of the particles of the medium is parallel to the direction of propagation of the wave.

Example: Motion of a spring system, sound waves.

2. ELECTROMAGNETIC WAVES: The waves which do not require a medium for their propagation is known as electromagnetic waves.

Example: Radio waves, Television waves, X-rays, etc.

WAVES ARE CARRIERS OF ENERGY.

Energy can be transferred through matter and waves.

Experiment: Dip one end of a pencil at the edge of a tub containing water. Rapidly move the pencil up and down vertically. The ripples spread outwards on the surface of the water. Place a cork on the surface of water near the other end of the tub opposite to the pencil. When waves pass through this cork, the cork vibrates up and down perpendicular to the water surface at its place. We impart energy in moving the pencil up and down. This energy reaches the cork through water waves due to which cork vibrates.

RIPPLE TANK

Ripple tank is a simple apparatus which used to study the properties of waves like reflection, refraction, etc.

Construction: This tank consists of a rectangular tray with glass bottom and is placed nearly half meter above the surface of table by means of four supporting legs. Waves are produced on the surface of water present in the tray by means of a vibrator. This vibrator is an oscillating electric motor which is fixed on a wooden plate. This plate is suspended by means of rubber bands and its lower end touches the water surface. On setting vibrator ON, this plate starts vibrating and straight waves are generated on the water surface.

Read More

1. Important Trigonometric Ratios

2. Contribution of Muslim Scientists in the field of Science

3. Physics & Its Branches

4. Classification of computers