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Mention factors that the time-period of a simple pendulum depends and does not depend on?
The time period of a simple pendulum depends on:
(i) The length of the pendulum, and
(ii) Acceleration due to gravity.

The time period of a simple pendulum does not depend on
(i) The mass of the bob,
(ii) The nature of material of the bob, and
(iii) Amplitude of oscillations


What happens to the period of a simple pendulum when its length is made one-fourth?

When the length of a simple pendulum is made one-fourth, the period of a simple pendulum becomes one-half.


Define compression and rarefaction.

Compression is that part of a longitudinal wave in which the particles of the medium are closer to one another than they normally are, and there is a momentary reduction in volume of the medium.

A rarefaction on the other hand, is that part of a longitudinal wave in which the particles of the medium are farther apart than normal, and there is a momentary increase in the volume of the medium.


Show how the speed of sound depends on the humidity of air?

The speed of sound is less in dry air but more in humid air. In other words, sound travels slower in dry air but faster in humid air. In fact, as the humidity of air increases, the speed of sound through it also increases.


Define amplitude ?

Amplitude is the objective measurement of the degree of change (positive or negative) in atmospheric pressure (the compression and rarefaction of air molecules) caused by sound waves.


What keeps the simple pendulum oscillating?

When the pendulum bob is pulled to one side and released, it is the restoring force exerted by gravity and tension in the thread, which tends to bring the bob back to its mean position and keeps it oscillating.


What is the frequency in hertz, if 25 waves are produced per second?

The frequency in hertz is equal to the number of waves produced per second. In this case, since 25 waves are being produced per second, the frequency of the waves is 25 hertz (which is also written as 25 Hz).


Sound waves travel with a speed of about 330m/s. What is the wavelength of sound whose frequency is 550 hertz?

Here, speed of waves, v = 300m/s

Frequency of waves, f = 550Hz

And, Wavelength, = ? (To be calculated)

Now, = f x

So, 330 =550 x

= ; = 0.6m

Thus, the wavelength of sound wave is 0.6 meter.


Sound waves require a material medium. Illustrate.

The substance through which sound travels is called a medium. The medium can be a solid substance, a liquid or a gas. Solids, liquids and gases are called material media. Sound needs a material medium like solid, liquid or gas to travel and be heard. In other words, sound can travel through solids, liquids and gases but it cannot travel through a vacuum or empty space.


Write a note on echoes.

The repetition of sound caused by the reflection of sound waves is called an echo. When a person shouts in a big empty hall, we first hear his original sound. After a little while, we hear the reflected sound of shout. This ‘reflected sound’ is an ‘echo’. So, when we hear an echo, we are actually hearing a reflected sound, a short while after the original sound. Thus an echo is simply a reflected sound. If we shout at a wall from 300 meters away, the sound takes one second to reach the wall. The sound reflects from the wall, and takes 1 second to return. So, we hear the echo 2 seconds after we have shouted.


What makes the simple pendulum oscillate?

When the bob is in oscillatory motion it moves back, displaced from the extreme position towards equilibrium position and falls under gravity and therefore gains momentum. As a result the bob moves and overshoots the equilibrium position. The gravity acting on the bob and the tension in the string together provides the restoring force, which tends to bring the bob to its mean or equilibrium position.


Why does a wave require a material medium for its propagation?

A wave can travel sufficiently long distances with minimum loss of energy through a material medium.


How do we classify wave on the basis of material medium?

The waves, which can produce or propagate only in a material medium are called mechanical waves. Water waves, waves on stretched strings, coiled springs, sound waves etc are some examples.
The waves, which do not require any material medium for its propagation are called non-mechanical
waves. Light waves and electromagnetic waves are some common examples.


Give examples of waves, which have both longitudinal and transverse components.

The waves, which have both longitudinal and transverse components, are:
(i) Ocean waves
(ii) Seismic waves, which travel along the surfaces of the earth after an earthquake or an explosion.
(iii) The wave pulses produced by a beetle’s motion along the sand’s surface.


Explain how transverse and longitudinal waves are called simple harmonic waves.

In transverse waves, the simple harmonic oscillations of the particles of the medium about the mean position are perpendicular to the direction of wave propagation. On the other hand, in longitudinal waves, the constituents of the medium oscillate along the direction of the wave propagation. Thus, both transverse and longitudinal waves are simple harmonic waves.


Derive the relation between wave velocity, frequency and wavelength for a periodic wave.

When a periodic wave propagates through a medium, particles of the medium oscillate with simple harmonic motion.
The frequency n of the wave is the reciprocal of its time period T, i.e.,

ν = 1/T
Wave velocity is the distance covered by a wave in one time period. Therefore
V     =
or  V =
Substituting equation (1), we get

V = ν λ
or Wave velocity = Frequency × Wavelength
Thus, wave velocity is a product of frequency and wavelength.


Longitudinal waves can be propagated through all media, solid, liquid and gas, but transverse waves can propagate only through solids. Explain why.

A longitudinal wave motion travels in a medium in the form of compressions and rarefactions, which involves changes in volume and density of the medium. Thus longitudinal waves can be transmitted through a medium, which can offer an opposition to changes in volume. Solids, liquids and gases possess this property. Hence longitudinal waves can be propagated through all media.

A transverse wave motion is transmitted through a medium in the form of crests and troughs. This involves changes in the shape of the medium. The transverse waves can be transmitted only through the medium, which offers an opposition or resistance to changes in its shape. Solids possess this property and so the transverse waves can be propagated through them.


Explain how an echo is heard?

If we want to hear a distinct echo, the reflected sound or echo must reach the ear 0.1 s after the direct original sound. If we take the sound speed to be 344 m/s, the sound must go to the obstacle and reach the ear of the listener on reflection after 0.1s. This implies that the total distance covered should be at least 344 (m/s) × 0.1 (s) = 34.4 m. Thus for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance i.e., 17.2 m from the source.


How can bats move about freely even in a total darkness?

A bat can hear ultrasound at frequencies upto 120 kHz. During their flight, bats emit a series of high frequency ultrasound pulses. It is able to detect the presence and location of any nearby object from the reflected sounds for which the bat has a sophisticated built-in receiver. Therefore the bat is able to sense the time it takes for the ultrasound pulses to return after being reflected by an object. Thus, bats can move freely even in a total darkness.


What is ultrasonography? How is it used?

A versatile technique of three-dimensional photography with the help of ultrasonic waves is called ultrasonography. It is used by physicians to locate the exact position of an eye tumour. The removal of the eye tumour helps in restoring the normal vision to the patient.

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