The adjacent diagram shows three different modes of vibrations P, Q and R of the same string.
i) Which vibrations will produce a louder sound and why?
ii) The sound of which string will have maximum shrillness?
iii) State the ratio of wavelengths of P and R.
i) R will produce louder sound because its amplitude is more than P and Q.
ii) The sound of P string will have maximum shrillness.
iii) The ratio of wavelength of P and R i.e., 
i) What is meant by the term:
1. Amplitude 2. Frequency, of a wave?
In the diagram alongside, A, B, C, D are four pendulums suspended from the same elastic string PQ. The length of A and C are equal to each other while the length of the pendulum B is smaller than that of D. Pendulum A is set into a mode of vibrations.
i) Name the type of vibrations taking place in pendulum B and D?
ii) What is the state of the pendulum C?
iii) State the reason for the type of vibrations in pendulum B and C.i) B and D would be in a state of forced vibration.
ii) Since the length of the pendulum, C is same as that of A, therefore, it is in the state of resonance.
iii) The pendulum C has its length same as that of A, therefore, its natural time period is also same that of a and both remain in the same phase so resonance takes place. But, B’s length is lesser than the length of A, hence B would vibrate with smaller amplitude.
The diagram below shows the displacement time graph of a vibrating body.
i) Name the type of vibrations produced by the vibrating body.
ii) Give one example of a body producing such vibrations.
iii) Why is the amplitude of the wave gradually decreasing?
iv) What will happen to the vibrations of the body after some time?i) Damped vibrations.
ii) Tuning fork vibrating in air produces damped vibrations.
iii) Due to friction, energy is continuously lost. Hence, the amplitude of the wave is gradually decreasing.
iv) After some time, the amplitude gradually decreases and finally stops.
i) State one important property of waves used for echo depth sounding.
ii) A radar sends a signal to an aircraft at a distance of 30 km away and receives it back after 2 × 104 second. What is the speed of the signal?i) The ultrasonic waves travel undeviated through long distances and they can be confined to a narrow beam.
ii) Given:
Distance of aircraft = 30 km = 30,000 m
Total distance= 2 30,000 = 60, 000 m
Time taken = 2 
Hence, the speed of sound (v) is,
= 3 x 108 m/s
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