Wave Motion and Sound

Wave Motion. - One of the most important phenomena in nature is the transformation of energy from one point to another by wave motion. This kind of motion is illustrated in many ways. When a stone is dropped into a pool of still water, the surface of the water is covered with circular wavelets which widen out from the central point where the stone fell. The water does not really move outward from the central point, but it rises and then falls again. One can see something alike observing a floating leaf or piece of wood. It does not move forward but return again to its former position. Hence, the water, on which the leaf rests, must have the same kind of upward motion rather than a forward motion.

When one end of a rope is fastened to a rigid wall and the free end moved up and down rapidly, each jerk travels along the rope, each portion of the rope communicating the jerk to the next portion. Each particle to the rope imparts its upward or downward motion to its neighbours. The jerk moves forward, but the particles of the rope move only up and down. Motions of this kind are wave motions. In all these cases it is evident that there is a vibrating center which produces motions in those portions of the medium immediately in contact with it, and that these portions impart their motions to the neighbouring portions.

Transverse Waves. - If part of a stretched string is drawn aside, the tension in the string tends to bring it back to its position of equilibrium. Since the string has inertia, the force that causes the displacement requires time to produce its full effect so that a wave can travel along the string with a definite velocity. Waves of this kind are easily produced in a rope fixed at one end and held in the hand at the other. If the rope is lightly stretched, a jerk imparted to the end B travels down the rope as a wave. The more tightly the rope is stretched, the more rapidly the jerk travels down it. If a series of to-and-fro movements is imparted to the end В a series of waves travels down the rope. Such waves are known as transverse waves, because the particles of the medium in which the waves travel move perpendicular to the direction of the wave motion. They can be easily represented by plotting the displacements on the vertical axis and the distance from the source in a given direction on the horizontal axis. Light and other forms of electromagnetic waves are excellent illustrations of transverse waves [2, С. 99].

Read the text, translate it and find one wrong statement in the list of the main statements below the text.

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