Class 9 Science Chapter 11 Assertion and Reason Questions - Sound

Direction: In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as:

Q1: Assertion: Sound would travel faster on a hot summer day than on a cold winter day.
Reason: Velocity of sound is directly proportional to the square of its absolute temperature.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans : (c)
The velocity of sound in a gas is directly proportional to the square root of its absolute temperature. Since, temperature of a hot day is more than cold winter day, therefore sound would travel faster on a hot summer day than on a cold winter day.

Q2: Assertion: The velocity of sound changes as we go up in the atmosphere.
Reason: Pressure decreases as we go up in the atmosphere.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (b)  
As we move up the pressure of air, density of air, both decrease. But, velocity of sound will not change as long as temperature of air remains constant. Since temperature falls as we ascend up, hence the velocity of sound also decreases.  

Q3: Assertion: Two persons on the surface of moon cannot talk to each other.
Reason: There is no atmosphere on moon.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (a)
Sound waves required material medium to travel. As there is no atmosphere, i.e., vacuum is on the surface of moon, therefore the sound waves cannot reach from one person to another. Because of this reason, for communication on moon astronomers use phones, which convert sound wave into radio waves (electromagnetic waves need no medium for propagation). 

Q4: Assertion: Waves produced in a cylinder containing a liquid by moving its piston back and forth are longitudinal waves.
Reason: In longitudinal waves, the particles of the medium oscillate parallel to the direction of propagation of the wave.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (b)  
When piston is moved back and forth, the molecules of the liquid also move along the direction of motion of the piston (direction of disturbance) also the volume and density of liquid is changing with motion of piston.

Q5: Assertion: Waves produced by a motor boat sailing in water are both longitudinal and transverse in nature.
Reason: The longitudinal and transverse waves cannot be produced simultaneously.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (c)  
The propeller of a motor boat cuts the water surface laterally and also pushes it is backward direction. Hence, it will result in both longitudinal and transverse waves. 

Q6: Assertion: The flash of lightening is seen before the sound of thunder is heard.
Reason: Speed of sound is greater than speed of light.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (c) Assertion is true but reason is false.
The speed of sound is 340 m/s and speed of light is 3 ×10⁸ m/s in air. Thus flash of lightening is seen before the sound of thunder is heard. 

Q7: Assertion: The longitudinal waves are called pressure waves.
Reason: Propagation of longitudinal waves through a medium involves changes in pressure and volume of air, when compression and rarefaction are formed.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (a)  
In longitudinal wave, pressure variations travel along the tube with the density variations, when the density reaches its maximum at a particular location, the pressure also reaches its maximum at that location. 

Q8: Assertion : Transverse waves can be produced in liquids.
Reason: Light waves are transverse waves.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (b)

Q9: Assertion: Echo is produced when sound is incident on hard and polished surface.
Reason: Sound energy can be totally reflected by objects with soft and loose texture.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (c)
Sound energy gets partially absorbed by objects with soft and loose texture.

Q10: Assertion: The velocity of sound increases with increase in humidity.
Reason : Velocity of sound does not depend upon the medium.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (c)  
Velocity of sound increases with increase in humidity so it depends on the medium.

Q11: Assertion: Transverse waves travel through air in an organ pipe.
Reason: Air possesses only volume elasticity.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (d)
Since mechanical transverse wave can propagate through medium which possesses elasticity of shape. Air possesses only volume elasticity therefore transverse wave cannot propagate through air.

Q12: Assertion: Sound would travel faster on a hot summer day than on a cold winter day.
Reason: Velocity of sound is directly proportional to the square of its absolute temperature.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (c)  
The velocity of sound in a gas is directly proportional to the square root of its absolute temperature  Since temperature of a hot day is more than cold winter day, therefore sound would travel faster on a hot summer day than on a cold winter day.

Q13: Assertion: A vibrating tuning fork sounds louder when its stem is pressed against a desktop.
Reason: When a wave reaches another denser medium, part of the wave is reflected.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (c)  
If the tuning fork is held in hand and hit with hammer, a sound is produced as the prongs of the tuning fork set surrounding air particles into vibrational motion. The sound produced by the tuning fork in air is barely audible. However, if the tuning fork is set on a desktop. The desktop begins vibrating at the same natural frequency of the tuning fork. The tuning fork forces surrounding desktop particles into vibrational motion. The vibrating desktop in turn forces surrounding air particles into vibrational motion. As the surface area of desktop is greater than the surface area of tuning fork, more surrounding air particles will be forced into vibration. This causes an increase in the amplitude and thus loudness of the sound.

Q14: Assertion: The speed of sound in solids is maximum though their density is large.
Reason: The coefficient of elasticity of solid is large.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (a)  
The velocity of sound in solid is given by, Thought ρ is large for solids, but their coefficient of elasticity E is much larger (compared to that of liquids and gases). That is why v is maximum in case of solid.

Q15: Assertion: Two persons on the surface of moon cannot talk to each other.
Reason: There is no atmosphere on the moon.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (a)
Sound waves require material medium to travel. As there is no atmosphere (vacuum) on the surface of moon therefore the sound waves cannot travel from one person to another. Because of this reason, for communication on moon astronomer use phones, which convert sound wave into radio waves (electromagnetic waves which need no medium for propagation).

Q16: Assertion: Compression and rarefaction involve changes in density and pressure.
Reason: When particles are compressed, density of medium increases and when they are rarefied, density of medium decreases. 
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (a)
A compression is a region of medium in which particles are compressed i.e., particles come closer i.e., distance between the particle becomes less than the normal distance between them. Thus there is temporary decrease in volume and a consequent increase in density of medium. Similarly in rarefaction, particle get farther apart and a consequent decrease in density.

Q17: Assertion: The velocity of sound in hydrogen gas is less than the velocity of sound in oxygen gas. 
Reason: The density of oxygen is more than the density of hydrogen.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.
Ans: (d)  
The velocity of sound in gaseous medium is given by Clearly, the velocity of sound in a gas is inversely proportional to the square root of density of the gas. The density of oxygen is 16 times the density of hydrogen, therefore, the velocity of sound in hydrogen is four times the velocity of sound in oxygen.