Physics [2 Mark Question Answer]

(a) A vibrating tuning fork is an example of natural vibrations.
(b) When a body vibrates with some other frequency, rather than its own natural frequency under influence of some external force, the body is said to produce forced vibrations.
Example : When a large table produces a loud sound when the stem of vibrating tuning fork is placed on it, the table is producing forced vibrations.

(a) Pitch is the subjective property related to the frequency of sound.
(b) In addition to fundamental frequency, all musical instruments produce forced vibrations, the pitch which depends upon the shape and size of instrument. It is these forced vibrations which impart quality to the note of each instrument, and hence sound produced by them is different.

(a) When the vibrations produced by the feet of the person are impressed on the rails, they vibrate with forced vibrations, thereby producing a ringing sound.
(b) (i) With the increase in amplitude, the loudness of musical note increases.
(ii) With the increase in frequency the pitch of musical note increases, i.e., the musical note becomes shrill.

(a) Decibel ( dB ) is the unit used for measuring sound level.
(b) The phenomenon is resonance. It is because, at some particular length of air column, the natural frequency of the vibrating air column becomes equal to the natural frequency of tuning fork. These frequencies reinforce each other and hence, produce loud sound or resonance.

(a) Damped vibrations.
(b) (i) Light waves do not need material medium for their propagation, whereas, sound waves need material medium for their propagation.
(ii) Light waves are electromagnetic waves in nature, whereas sound waves are mechanical waves.

Bats can produce and detect the sounds of very high frequency. The sound produced by the bats gets reflected back from obstacles. By detecting the echoes, bats come to know the position of the obstacles, even in the dark. This helps them to fly skilfully, and locate their prey as well.

(a) Echo is the sound heard after reflection from a distant object, such as a hill or a high building, after the original sound has died off.
(b) Conditions necessary for echo formation:
(i) The minimum distance between the source of sound and reflecting object/obstacle should be 17 metres.
(ii) The size of the obstacle must be larger than the wavelength of sound.

The reflection of sound waves is utilized in megaphone and hearing aid (ear trumpet).
(a) Megaphone : Megaphone is a horn-shaped metal tube. This type of opening prevents sound energy from spreading out by successive reflections. Megaphones are used in addressing a group of people in fairs or tourist spots.
(b) Hearing aid : Its shape is like a trumpet, the narrow end of the trumpet is kept in the ear hole of a person, who is hard of hearing. The wider end of the trumpet faces towards the speaker. The sound waves are received by the wider end of the trumpet, which are then reflected into the narrow end. This in turn increases the intensity of sound energy. The person who is hard of hearing can hear easily.

Radar is a short form of "Radio detection and ranging". In this instrument radio waves of very short wavelength are used to locate an enemy aircraft or ship. A concave transmitter mounted on a rotating platform sends radio waves in all directions. The radio waves on striking the aircraft or ship are reflected back. The reflected radio waves are received by a concave receiver, which is mounted at a small angle with the transmitter, On receiving the radio waves, the position of the enemy aircraft can be located on a monitor screen as a bright spot.

Resonance : When the frequency of the applied periodic force is equal to the natural frequency of the body on which the force is applied such that the body readily takes up the vibrations and begins to vibrate with an increased amplitude, the phenomena is known as resonance.
Conditions for resonance : (i) The natural frequency of the given body must be equal to or an integral multiple of the frequency of the applied force (or vibrating body) (ii) The vibrating body must have sufficient force, so as to set the other body into vibrations

(a) At a particular speed, the frequency of the piston of the engine becomes the same as the natural frequency of the frame of the vehicle. Thus, the vehicle starts to vibrate violently, due to resonance.
(b) When a note of some particular frequency is played, its frequency matches with the natural frequency of an odd piece of cutlery. Thus, the cutlery starts vibrating violently due to resonance

(a) The periodic vibrations of a body of constant amplitude in the absence of any external force are called free vibrations of a body.
(b) The amplitude of a vibrating body continuously decreases during damped variations due to the presence of resistive force. So, the vibrating body loses energy with the result that its amplitude gradually decreases till it comes to rest.

(a) If two pendulums of same length are suspended from a string and one pendulum is made to vibrate, the other pendulum also starts vibrating with a large amplitude and in same phase because of resonance.
(b) Frequency and amplitude are acoustically measurable quantities related to pitch and loudness.

(a) When the handle of vibrating tuning fork is placed on a table top, the table produce forced vibrations.
(b) (i) Frequency is inversely proportional to the length of string.
(ii) Frequency is directly proportional to the square root of tension.