Sound (part -2) class 8

 SOUND

Q.5. Give reasons:

i) Sound can travel through a medium but it cannot travel through a vacuum. 

Ans. The substance through which sound travels is called a medium. Sound needs a material medium like solid, liquid or gas to travel and be heard but it can not travel through vacuum. 

The sound waves involve the vibrations of the particles of the medium through which they travel. The layers of particles only vibrate back and forth, and transfer the sound energy from one layer to the next till it reaches our ear. Hence, the molecules of medium carry the sound waves from one place to another through their vibrations.

Sound cannot travel through vacuum because vacuum has no molecules which can vibrate and carry sound waves. 

ii) If we turn on a gas tap for few seconds, a person standing a few meters away will hear the sound of escaping gas first and the smell of gas reaches him afterwards.

Ans. The sound of gas travels through the vibrations of air layers (not by actual movement of air from gas tap to the person). So it reaches first, but the smell of gas reaches the person through the actual movement of the air layers, which takes more time.

iii) Sound travels fastest in solids, faster in liquids and slowest in gases.

Ans. The particles in solids are tightly packed to each other than liquids followed by gases. So, the vibrations get transferred easily and quickly in solids as compared to other mediums.

iv) If you want to hear a train approaching from far away. It is more convenient to put the ear to the track.

Ans. This is because sound travels much more faster through the solids (railway line made of steel) than through air.

v) Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later.

Ans. This is because the speed of light (lightning) is more than the speed of sound (thunder). 

vi) Sound cannot be heard directly on the surface on moon.

Ans. This is because the moon has no air or atmosphere at all to carry the sound waves (or sound vibrations). It is all vacuum on the surface of moon.

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