How is energy transmitted in wave motion? Ans: During wave motion, the particles of the medium are set into oscillations about their mean positions. An oscillating particle of the medium hands over its motion to a particle just ahead of it with some constant phase difference and it results in transmission of energy.

What are the properties of the medium for the propagation of a mechanical wave? Ans: For the propagation of a mechanical wave, the medium must have two properties: modulus of elasticity so that particles can return to equilibrium position after disturbance, and the inertia so that particles can store energy and overshoot their mean positions.

Why is the sound produced in air not heard by a person deep inside the water? Ans: The water surface reflects the sound back into the air and only a little fraction of incident intensity (= 0.1%) is refracted through water. Hence, a person deep inside the water cannot hear the sound produced in air.

Why is a sound in a hall is louder than in the open space? Ans: In a hall, from the walls and ceiling the multiple reflection of sound takes place. The reflected sound mix up with the direct sound. Due to this the intensity of sound in the hall increases. But in the open space, a faint sound is heard because no such reflection of sound takes place. Hence, sound is louder in a hall than in the open space.

Why holes are made in the wooden box of the sonometer? Explain. Ans: The vibrations of tuning fork is transmitted to the wooden platform and then to the air inside the box. The air inside the box vibrates and comes in contact with the air outside the box through the holes, as a result we hear the vibrations.

When the stem of a vibrating tunning fork is gently pressed agains the top face of sonometer box, the air enclosed in the box also vibrates. This increases the intensity of sound. The holes bring the inside air in contact with the outside air and check the effect of elastic fatigue.

To measure the frequency of a.c. mains, a magnet is used. Why? Ans: For the vibration of a string, an external periodic force is needed. A conductor carrying a current experience a force when it is placed at right angles to the lines of force of a magnetic field. If there is no magnet, there will be no external periodic force. Due to this reason magnet is used for this experiment.

A tuning fork produces a feeble sound. But when it is pressed against a table, a loud sound is heard. Why? Ans: Feeble sound is heard due to vibration of small air mass around the prongs of the tuning fork. But when the tuning fork is pressed against the table large mass of air in contact to table comes into vibration producing a load sound. That’s why, a loud sound is heard.

Why does an empty vessel produce loud sound than that filled one? Ans: When an empty vessel is struck, it begins to vibrate and the air column in it also set in vibrations with larger amplitude. On the other hand, when the filled vessel is struck, the liquid molecules are set in vibrations (instead of air molecules) which have smaller amplitude than that of the air molecules. As intensity (amplitude)2, so the empty vessel produces louder sound than a filled one.

In order to hear distinct beats, why should the difference in the frequencies of the two sources be smaller than 10 Hz? Ans: This is due to persistence of hearing. The impression of sound heard by ears persists on our mind for 1/10th of a second. If another sound is heard before 1/10 th second, we cannot distinguish between the two. Therefore, in order to hear distinct beats, the best period (i.e. time interval between two successive beats) must be greater than 1/10 second. In other words, the frequencies between the two sources should be less than 10 Hz.