A Wave That Continues to Vibrates Back and Forth

Selina Solutions Class 8 Physics Solutions for Exercise Short/Long Answer Questions in Chapter 7 - Sound

Question 3 Short/Long Answer Questions

Q3) Explain the mechanism of formation of a longitudinal wave when source vibrates in air.

Answer:

Solution :

When a source vibrates in the medium, it creates disturbances which travel in the medium in the form of longitudinal waves.

Example -

• A vibrating string can create longitudinal waves as depicted in the animation below.
• As the vibrating string moves in the forward direction, it begins to push upon surrounding air molecules, moving them to the right towards their nearest neighbor.
• This causes the air molecules to the right of the string to be compressed into a small region of space.
• As the vibrating string moves in the reverse direction (leftward), it lowers the pressure of the air immediately to its right, thus causing air molecules to move back leftward.

The lower pressure to the right of the string causes air molecules in that region immediately to the right of the string to expand into a large region of space.

• The back and forth vibration of the string causes individual air molecules (or a layer of air molecules) in the region immediately to the right of the string to continually vibrate back and forth horizontally.
• The molecules move rightward as the string moves rightward and then leftward as the string moves leftward. These back and forth vibrations are imparted to adjacent neighbors by particle-to-particle interaction.
• Other surrounding particles begin to move rightward and leftward, thus sending a wave to the right.
• Since air molecules (the particles of the medium) are moving in a direction that is parallel to the direction that the wave moves, the sound wave is referred to as a longitudinal wave. The result of such longitudinal vibrations is the creation of compressions and rarefactions within the air.

Video transcript

hello students welcome to leader learning's question and answer video my name is pallabi and i'm the math and science tutor at leader let's have a look at this question now this is a long question and a lot of concept is there inside the question so we will develop along the concept let's see how it goes explain the mechanism of formation of a longitudinal wave when source vibrates in air so to understand this i have two pictures over here actually i have three but let's focus on the two pictures first so this is a vibrating body or a source which is producing sound let's say it's producing sound in this case sound is a longitudinal wave we will come to that also that y sound is longitudinal wave so this is how the source is vibrating the particles of the medium are these blue dots which you can see and this is the receiver's ear this is the medium and the vibrating body produces vibrations throughout the medium right when a source vibrates in the medium it creates disturbances which travel in the medium in the form of longitudinal waves we will come back to that but before that let's try understanding this through a vibrating string experiment so a vibrating string can create longitudinal waves as depicted in the diagram here as you can see this is a vibrating string so look at these and observe this as i speak so as the vibrating string moves in the forward direction it begins to push upon the surrounding air molecules so the vibrating string is moving along this direction it is pushing the air molecules moving them to the right towards their nearest neighbor this causes the air molex to the right of the string to be compressed so this is how the air molecules end up getting compressed let's have a clearer picture of this this is what is happening into a small region of space so as the vibrating string moves in the reverse direction now the string is moving like this and this right so when it moves in the reverse direction that is leftward it lowers the pressure of the air immediately to its right and thus the air molecules they move back leftward so the air molecules are again moving back leftward right now the lower pressure to the right of the string causes air molecules in that region immediately to the right of the string to expand into a large area so this is what happens when the pressure has become low so that's why they expand into a large area and what happens after that now this the back and forth vibration of the string causes the individual air molecules or the layer of the air molecules in the region to vibrate back and forth horizontally the molecules they move rightward as the string moves rightward then leftward as the string moves leftward so as you can see what i'm explaining from this that the molecules are also moving rightward and leftward that is along the direction of propagation of the wave right okay the back and forth vibrations are imparted to adjacent neighbors by particle to particle interaction so this vibration is imparted to the neighbors through particles they are vibrating and the vibration continues or moves ahead other surrounding particles now begin to move rightward and leftward thus sending a wave to the right so the particles the surrounding particles they transfer the energy to the surrounding particles and like this the whole wave starts propagating since air molecules that is the particles of the medium are moving in a direction that is parallel to the direction of the wave moves that is this is the direction of the longitudinal wave and we can see the particles of the air are also moving in this direction that is left or right so since that is happening so some wave is therefore called a longitudinal wave result of such longitudinal vibrations is the creation of compression and rarely functions in the air so this is how a longitudinal wave is formed and how it travels in the air when air is the medium so i hope this point was clear the explanation was clear if you have any further questions please post your comments below thank you

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