CBSE Class 9 Science HOTs

SOUND AND WAVES

SOUND PROPAGATES AS A WAVE:

• Sound propagates from one place to another in the form of waves, i.e. because of the disturbance of particles of the medium.
• Wave is a phenomenon or disturbance in which energy is transferred from one point to another without any direct contact between the points. So, sound is considered as a wave.
• Particles of medium only vibrate. They do not move from one point to another.

TYPES OF WAVES :

• On the basis of direction of propagation, waves can be divided into two types –- Longitudinal waves n transverse
i) Longitudinal waves are waves in which particles of the medium do to and fro motion in the same direction in which the wave moves. E.g Sound waves
ii) Transverse waves are waves in which particles of the medium do to and fro motion in the direction perpendicular to the direction of movement of wave. E.g light waves.

Q. How to identify types of waves
In a transverse wave, the particles are displaced perpendicular to the direction the wave travels.

Example of transverse waves include vibration on a string and ripples on the surface of water. We can make a horizontal transverse wave by moving the slinky vertically up and down.

• In longitudinal wave the particles are displaced parallel to the direction the wave travels. An example of longitudinal waves is compression moving along a slinky. We can make a horizontal longitudinal wave by pushing and pulling the slinky

• sound waves are longitudinal Sound travels as a longitudinal wave through a material medium.

The particles of sound wave do not move from one place to another. They just oscillate back n forth about their position of rest. The disturbance is carried forward. Hence, sound waves are longitudinal waves.

Longitudinal waves can be shown through a sling key.

Hold one end of the sling key n fix the other end to the wall nail. Now, stretch sling key by pulling the hand, after a while push the stretched sling key towards the wall. You’ll observe that the coils come closer at some regions whereas it moves apart at other regions forming regions of compressions n rarefactions.

COMPRESSION AND RAREFACTION

• Sound travels as successive compressions and rarefactions in the medium. A vibrating object producing a series of compressions (C) and rarefaction (R)
• When a vibrating object moves forward, it pushes and compresses the air in front of it forming a region of high pressure called compression (C). Compressions are the regions of high pressure and density where the particles are crowded and are represented by the upper portion or peak of the curve called crest.
• When the vibrating object moves backward, it forms a region of low pressure called rarefaction (R). Rarefactions are the regions of low pressure and density where the particles are spread out and are represented by the lower portion of the curve called trough or valleys.

Questions based on the above topics:-

Question. What type of wave is sound?
Answer. Sound is a longitudinal wave.

Question. Name the types of a wave.
Answer. Longitudinal wave and Transverse waves.

Question. The peak of upper portion of a wave is _____.
Answer. Crest.

Question. In which type of waves the compression and rarefaction exist ?
Answer. Longitudinal waves.

Question. Define through in a wave.
Answer. The peak(maximum point) of lower curve of a wave is known as through of a wave.

Question. Write any three example of a longitudinal wave?
Answer. sound waves. ultrasound waves. seismic P-waves.

Question. What are the two main parts of a longitudinal wave?
Answer. A compression is where the particles of the medium are closest together, and a rarefaction is where the particles are farthest apart.

Question. How to identify transverse waves?
Answer. In a transverse wave, the particles are displaced perpendicular to the direction the wave travels.

Question. What two factors determine the speed of a transverse wave?
Answer. The speed of a wave is dependant on four factors: wavelength, frequency, medium, and temperature.

Question. What is one complete oscillation?
Answer. The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation.
The red part shows a complete oscillation of a wave in unit time period.

Please click the below link to access CBSE Class 9 Science HOTs