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41. If the velocity of sound in liquid is much greater than that in a gas, then why don't a person under water unable to listen to someone speaking in air?

As the water is a good absorber good reflector and good damper so when sound waves incident on the surface of water, some parts of waves are reflected by it and some is absorbed but a very small amount is left for the propagation which when propagated through water dies out quickly due to damping. Hence, a person under water unable to listen to someone speaking in air even if the velocity of sound in liquid is much greater than that in a gas.

42. Velocity of sound in hydrogen is greater than that in oxygen why?

For the constant pressure and temperature, the velocity of the sound varies inversely to the square root of its density i.e.

where, v is the velocity of the sound in air and   is the density of gas.
As the density of oxygen gas is greater than that of the hydrogen, the velocity of sound in hydrogen is greater than that of the oxygen.

43. Which is the most fundamental property of a wave? Why?

44. Why are stationary waves called so?

45. What causes the rolling sound of a thunder?

46. A closed earthen vessel (Surai) is gives. Can we distinguish when the vessel is filled or empty without lifting it and looking inside. Justify your answer.

47. Differentiate between forced vibration and free vibration.

48.