Physics CH # 12




Definitions

1. Vibration:
One complete round trip of a simple harmonic motion is called vibration.
or

If a body in periodic motion moves to and fro over the same path, this motion is called Oscillation.

2. Time Period (T):
The time required to complete vibration is known as time period.

3. Frequency:

It is the number of vibrations executed by an oscillating body in one second.

4. Displacement:
It is the distance of a vibrating body at any instant from the equilibrium position.

5. Amplitude:
The maximum distance of the body on either side of its equilibrium position is known as amplitude.

6. Wave Length:
The distance between two consecutive crests and troughs is called wavelength.

7. Natural Frequency:
The frequency at which an object will vibrate freely (without any external periodic force or resistance) is known as natural frequency of that object.

8. Audible Sound:
Our ear can hear only those sounds whose frequency is between 20Hz and 20000Hz. This range is known as audible sound.

9. Ultrasonic Sound:
Sound with frequency greater than 20000 Hz is known as ultrasonic sound.

Simple Harmonic Motion (S.H.M):

Definition:

“To and fro motion of a body in which acceleration is directly proportional to displacement and always directed towards mean position is known as Simple Harmonic Motion.”

Condition For S.H.M:

The conditions for simple Harmonic Motion are given below:

* Some resisting force must act upon the body.
* Acceleration must be directly proportional to the displacement.
* Acceleration should be directed towards mean position.
* System should be elastic.

Examples:

Following are the examples of S.H.M:

* Body attached to a spring horizontally on an ideal smooth surface.
* Motion of a simple and compound pendulum.
* Motion of a swing.
* Motion of the projection of a body in a circle with uniform circular motion.

Resonance

Definition:

“The large amplitude vibration of an object when given impulses at its natural frequency is known as Resonance.”

Experiment:

Consider a long string stretched tightly between two pegs. Four pendulums A, B, C and D of different lengths are fastened to the string. Another pendulum E of same length as A is also fastened.

When pendulum E is set to vibrate, it will be observed that all the pendulums start to swing but pendulum A begins to vibrate with larger amplitude, as pendulum E is set into vibration. It imparts its motion to the string. This string in turn imparts the same periodic motion to the pendulums. The natural frequency of all other pendulums except A is different. Due to the same natural frequency only A vibrates as the same vibration of E. This phenomenon under which pendulum A begin to vibrate is called resonance.

Example:

March of Soldiers while Crossing the Bridge
Each bridge has its own natural frequency and marching of soldiers is another vibrating system. So there may occur a force on vibration in bridge. This may damage the bridge. So, for safely precautions, it is written that soldiers must march out of stop while crossing the bridge.

Wave:

Definition:


” A method of energy transfer involving some form of vibration is known as a wave.”

Wave Motion:

Wave motion is a form of disturbance, which travels through a medium due to periodic motion of particles of the medium about their mean position.
Experiment:

We see that if we dip a pencil into a tap of water and take it out a pronounced circular ripple is set up on the water surface and travels towards the edges of the tub. However if we dip the pencil and take it out many times, a number of ripples will be formed one after the other.
Waves can also be produced on very long ropes. If one end of the rope is fixed and the other end is given sudden up and down jerk, a pulse-shaped wave is formed which travels along the rope.

Transverse Wave:

Definition:

“The wave in which amplitude is perpendicular to the direction of wave motion is known as Transverse Wave.”

Examples:
* Radio Waves             * Light Waves  * Micro Waves             * Waves in Water         * Waves in String

Longitudinal Wave:

Definition:

“The wave in which amplitude is parallel to wave motion is called longitudinal wave.”

Example:

* Sound Waves
* *Seismic Waves

Sound:

Definition:
“A vibration transmitted by air or other medium in the form of alternate compressions and rarefactions of the medium is known as Sound.”


Production Of Sound:

Sound is produced by a vibrating body like a drum, bell, etc, when a body vibrates. due to the to and fro motion of the drum, compressions and rarefactions are produced and transmitted or propagated in air.

Propagation Of Sound Waves:
When a body vibrates in air, it produces longitudinal waves by compressions and rarefactions. These compressions and rarefactions are traveled by the particles of the medium and transferred into the next particles. Due to this transference, sound propagates in a medium.

Velocity Of Sound:
It is a matter of common experience that the flash of lightning is seen earlier than hearing the thunder of cloud. Similarly when a gun is fired its sound is heard a little after seeing its flash. The reason is that light is faster than sound. Due to its slow velocity sound lags behind.

Experiment:
Select two stations at a distance of 8 km (or any more distance) such that there is no obstacle between them. Fire a gun at station A and note the time of sound taken for such distance. Repeat the process and note the time taken by the sound to travel from B to A. If we substitute the mean of the two times recorded and distance S (8km) in the formula V = S/t, we will get the velocity of sound.

Factors Effecting Velocity Of Sound:
The factors are given below:

* Velocity of air or any other medium.              * Density of the medium.
* Temperature of the medium.                                    * Nature of the medium


Characteristics Of Sound:

The characteristic properties of sound by which we can distinguish between noise and music, shrill and grave sounds or sound of men and women are known as characteristics of sound. The properties of sound are given below:

1.Loudness:

Definition:
“Loudness is the magnitude of auditory sensation produce by sound.”
Intensity can be defined as the energy carried by the sound waves through a unit area placed perpendicular to the direction of waver per second.

Factors Effecting Loudness Of Sound:
Loudness depend on following factors:

Area of Vibration of Body: Greater will be the surface area more will be the loudness.

Amplitude of Motion of Vibrating Object: Greater will be the amplitude, more will be the loudness.

Density of Medium: Loudness is directly proportional to the density of medium.
Motion and Direction: If source of sound is moving towards the listener loudness will be greater or if wind supports the velocity of sound the loudness will be greater.

2. Pitch:

Definition:
“The sensation that a sound produces in a listener as a result of its frequency is known as Pitch.”
This is the property of sound by virtue of which we can distinguish between a shrill and grave sound.

Factors Effecting Pitch Of Sound:
Pitch depends on following factors:

Frequency of Vibrating Body: The greater the fundamental frequency, more shrill will be the sound.

Relative Motion of Sound: If source and listener both are coming closer pitch will increase.

3. Quality Or Timbre Or Tone:

Definition:
“The characteristic of a musical note that is determined by the frequency present is known as Quality or Timbre or Tone of that sound.”
This is the property of sound by virtue of which it is possible to identify a sound of the same loudness and pitch but originating from different instrument.

Factors Effecting Quality:
Quality depends upon the following factors:

* Phase of the Sound Wave.
* Shape of Waves

Harmful Effects Of Sound (Noise):
Nowadays noise is considered as a great pollution, which is very dangerous for us. Some of them are as follows:

* Continuous noise damages hearing and can result in complete deafness.
* Noise has become a great cause for depression and blood pressure.
* Mental system shows less efficiency due to noise.
* Consequently it is harmful in all respects for living body.

Musical Sound:
The sound producing pleasing effect on our ears are called musical sounds.

Difference Between Longitudinal And Transverse Waves:

Longitudinal Waves:

1. In longitudinal waves, particles of the medium vibrate in the direction of the waves.

2. The portion of wave in which particles of medium are very close to each other is called compression.

3. Examples of longitudinal waves are sound wave and seismic waves.

4. Distance between the centre of two compressions and rarefactions is called wavelength.



Transverse Waves:


1. In transverse waves, particles of the medium vibrate in the direction perpendicular to the direction of wave.

2. The portion in which particles of medium are higher than their normal position is called crest.

3. Examples of transverse wave are microwaves and radio waves.

4. Distance between two crests and troughs is called wavelength.

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