Topic 1 - Sound

What is sound?

Sound is just a vibration.  Vibration is a to and fro or back and forth motion of an object. So, the sound is a vibration in our ear caused by the vibration of the air in our ear. When objects vibrate, they produce sound. An object vibrating back and forth rapidly pushes air forward to make way for itself.  When the vibration stops, the sound stops. 


In some cases, we can see the vibration and in some cases we can’t as the amplitude is very small.

Sound is mechanical wave, a pressure wave and longitudinal wave. In pressure waves, the high-pressure zone is called compression and low-pressure zone is called rarefaction.


The distance between two consecutive compressions or two consecutive rarefaction is called wavelength. It is expressed in meters or m.


How do sounds propagate?

Sounds propagate through a medium. It can go through solid, liquid and gas. No there is no sound in a vacuum or space. Also, another example, decreasing the amount of air will lead to a reduction of the sound level.


Vocal cord

The human voice is produced by the voice box or Larynx. It is at the upper end of the windpipe. Two vocal cords leave a narrow slit between them for the passage of air. When the lung forces air out through the voice box, the vocal cords vibrate producing sound (or voice). Depending on the vocal cord, the sound will vary. The vocal cords in men are about 20 mm long and in women about 15mm long. The vocal cord of children is very small.



Sound Wave

The to and fro motion is called vibration or oscillation. The number of oscillations is called frequency. It is measures in number of times per second (or Hertz). Hertz is written as Hz. 1 Hz means 1 oscillation per second. 50 Hz means 50 oscillations per second. 

The loudness as perceived by our ear is the amplitude of the vibration. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small, the sound produced is very faint. The loudness of sound is proportional to the square of the amplitude of the oscillatory motion.

So, if the amplitude is increased by 3 times, loudness will get increased by squared value of the amplitude i.e. 9 times.

Frequency and amplitude are two properties of sound.

Pitch is nothing but frequency of the sound produced. Babies produce sounds with very high pitch. This means the frequency is very high. Women produce sounds of higher pitch than men.

Sound having same amplitude and frequency can differentiate by sound quality called timbre. Guitar sound varies from Piano sound due to timbre.

Loudness is a relative term. It is perceived with respect to the receiver.


How does our ear function? 

The outer part of the ear is like a funnel. When sound enters it, it travels down to the end of the funnel where there is a thin stretched membrane called the eardrum. Sound vibrations make the eardrum vibrate. The eardrum sends vibrations to the inner ear. Inner ear sends it further to the brain.



Audible Range for humans

20 Hz to 20000 Hz i.e. we can’t hear noises whose frequency is less than 20 Hz or more than 20000 Hz. Different animals have different range. Ultrasound medical devices used frequency more than 20000 Hz or 20 Kilo Hertz or 20KHz

Speed of sound in air is 331 meters per second at 0 ºC and 344 meters per second at 22 ºC. The speed of sound decreases when we go from solid to gaseous state. In any medium as we increase the temperature, the speed of sound increases.

When the speed of any object exceeds the speed of sound it is said to be travelling at supersonic speed. We hear sonic boom there.

Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound, Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound.

For hearing a distinct sound, the time interval between the original sound and the reflected one must be at least 0.1 s.

The minimum distance of the obstacle from the source of sound must be half of this distance, that is, 17.2 m.  Echoes may be heard more than once due to successive or multiple reflections.

The rolling of thunder is due to the successive reflections of the sound from a number of reflecting surfaces, such as the clouds and the land.

The sound gets reflected at the surface of a solid or liquid. Change in density leads to reflection of sound.

A sound created will persist by repeated reflection from the reflecting surfaces until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation. The roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster or draperies to reduce reverbnation.



Noise Pollution

An unpleasant sound is called noise. It causes a lot of health problems in humans. The planting of trees reduces the propagation of noise in the air as they absorb the sounds.



  • The conical shape of megaphones, trumpets or shehanais are designed to send sound in a particular direction without spreading it in all directions.

  • Stethoscope - the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflections of sound.

  • Bats use sound reflection to measure distance and navigate.

  • Ultrasound used for cleaning purposes, detect cracks as the change in medium affects reflection.

  • ‘Echocardiography’ - Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart.  

  • An ultrasound scanner is an instrument that uses ultrasonic waves for getting images of internal organs of the human body

  • SONAR stands for Sound Navigation and Ranging. Sonar is a device that uses ultrasonic waves to measure the depth, direction and speed of underwater objects.

Topic - Gravitation

What is gravity?

Gravity or gravitation is a natural phenomenon by which all things with mass or energy are attracted to (or gravitate toward) one another. Ex- Earth, and Moon or an object dropped from a height falls towards the earth. 


If the earth attracts an object to fall, so it must be attracting to fall all the time.  The moon falls towards the earth, so it follows an orbit rather than going off in a straight line.



Centripetal Force is the force that causes acceleration and keeps the body moving along the circular path is acting towards the center. The motion of the moon around the earth in a circular orbit is due to the centripetal force provided by the force of attraction of the earth.


The force of attraction between objects is called the gravitational force.

Other objects do attract the earth towards them, but the mass of the earth is so high the movement is negligible.


The force of attraction between two objects ‘m’ & ‘n’, with the distance between them ‘r’ ; F = Gmn / r2

G is the universal gravitation constant.

The accepted value of G is 6.673 × 10–11 N m2 kg–2


Kepler’s laws

There are 3 laws. The important is the orbit of a planet is an ellipse with the Sun at one of the foci.


Gravitation in nature

(i) the force that binds everything to the surface of the earth;

(ii) the motion (revolution) of the moon around the earth

(iii) the motion (revolution) of planets around the Sun

(iv) the tides happen due to the gravitation pull of the moon and the Sun


Free Fall

Whenever objects fall towards the earth under gravitation force alone, it is said the objects are in free fall. The acceleration during free fall is ‘g’.

‘g’ is the acceleration due to gravity.  Its value is 9.81 m sec-2


Mass vs Weight

Mass (m) is the amount of matter in an object. SI unit of mass is kilograms

Weight (w) is the force exerted by an object. SI unit of weight is Newtons.

Weight (Force) = Mass x Acceleration


On the moon, the acceleration due to gravity is 1/6th that on earth. It is g/6

So, weight is mg/6.



Thrust vs Pressure

Thrust is the net force acting on an object perpendicular to the surface. (Unit N)

Pressure is the force per unit area acting on the object. (Unit Nm-2)

Why do needles hurt human skin? It is because of the pressure. Due to the small area, the pressure is too high.



Liquids and Gases are known as fluids.

When buoyant force is more than gravitational force, the object floats in fluids.

It is dependent on density. Density is mass per unit volume (Kg/m3). High-density items sink but low-density items float.


Relative density = Density of Matter/ Density of water

Since the relative density has no unit (it is a ratio of similar quantities)


Archimedes’ Principle

When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

Uses - designing ships and submarines; Lactometers (determine the purity of a sample of milk); hydrometers (determining density of liquids)