Class 9 Science, Chapter 10 (NCERT). 8 questions by concept. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
Why does a ringing bell produce sound?
(b) Its vibrating surface makes nearby air particles oscillate.
The source vibration creates pressure variations in the surrounding medium.
An alarm rings inside a jar. All the air is pumped out, leaving a perfect vacuum. What does the sound need in order to reach a listener outside?
(d) Particles that can pass on the disturbance
Sound is a mechanical wave. It travels only when particles of a medium lie close enough to pass the disturbance from one particle to the next. Each particle merely oscillates about its own position, so no particle travels from the alarm to the listener. A perfect vacuum holds no particles, so no sound reaches the listener.
A sound wave has frequency 200 Hz and wavelength 1.5 m. What is its speed?
(d) 300 m/s
Speed is frequency times wavelength: m/s.
Two sounds have equal frequency but one has greater amplitude. How do they differ?
(d) They have the same pitch, but the greater-amplitude sound is louder.
Pitch follows frequency, whereas loudness is associated with amplitude and intensity.
In which of these does sound travel fastest?
(b) Steel
Particles in a solid are packed closely and are tightly linked, so steel passes the disturbance on fastest. Sound is slower in water and slower still in air. Sound needs a material medium, so it does not travel through vacuum at all.
A 40 kHz sound reaches a human ear. Why is it not normally heard?
(c) Its frequency is above the upper human audible limit.
Forty kilohertz exceeds about 20 kilohertz and is ultrasonic to humans.
An echo returns 0.4 s after a shout. If the speed of sound is 340 m/s, how far away is the wall?
(c) 68 m
The sound goes to the wall and comes back, so it covers twice the wall distance. Total path = 340 × 0.4 = 136 m. So d = 136 / 2 = 68 m.
Which change reduces excessive reverberation in an auditorium?
(b) Adding curtains and other sound-absorbing materials
Soft porous surfaces absorb sound and reduce repeated reflections.
Wrong
The speaker sends its air particles across the room.
Why it feels right. Sound arrives at my ear, so I imagine the particles arriving too.
Right
A sound wave passes along a disturbance through local particle motion.
Worked example
