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| Waves and Wave Motion |
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| Most of us have seen the formation of ripples when a pebble is dropped into a pond. The ripples are produced due to the disturbance created by the pebble. The ripples travel in concentric circles of ever-increasing radius till they strike the boundary of the pond. Place a toy boat near the disturbance. You will observe that the toy boat does not move towards or away from the source of disturbance, but just moves up and down at the same point. This indicates that the water does not move forward but the particles of water move up and down about its original position and it is the disturbance, which moves forward. The formation of ripples on the surface of water is an example of wave motion. |
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| Propagation of sound waves through air |
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| Another example of wave motion is the propagation of sound from one place to another. When a person speaks, the molecules of air near the mouth of the speaker are disturbed resulting in vibration of these molecules about their mean positions. These vibrating molecules in turn push the nearby molecules and the process continues until the molecules of the air near the listener's ear start vibrating. These disturbances vibrate the eardrum and these vibrations of the eardrum in turn sends the message to the brain through nerve connections. In this case also, it is the disturbance, which moves forward and not the particles of air. |
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| From the above examples, we can conclude that in a wave motion, the particles of the medium do not move from one point to another, but simply vibrate about their mean positions. |
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| Thus a wave may be defined as a form of disturbance which travels through a material medium due to the repeated periodic motion of the particles of the medium about their mean position, the disturbance is transmitted from one particle to the next particle. |
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| Seven pendulum bobs suspended from a meter scale |
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| Let us now suspend seven pendulum bobs from a meter scale as shown in the figure. The bobs 2,3,4,5 and 6 are suspended by strings of equal lengths and are at equal distances from each other. The bobs 1 and 7 are suspended by slightly longer strings and the distance between the bobs 1 and 2 and so also between 6 and 7 is slightly larger than that between the other bobs. Now pass a string horizontally through all the seven strings, tying firm knots with each string so that all the seven bobs are at the same level and at equal distances from each other. Slightly push the first bob so that it starts oscillating without colliding with its neighboring bob. After some time the second bob starts oscillating, then bob 3 and so on until the last bob starts oscillating. How did the disturbance travel from the first to the seventh bob? When the first bob is pushed it gains some energy and starts oscillating. This oscillation produces a disturbance in the horizontal string and this disturbance in turn is transferred to the other bobs. That is the energy given to the first bob is transferred to the last bob but matter is not transferred. The above examples helps us in identifying the main characteristics of wave motion. |
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| The main characteristics of wave motion are: |
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 Particles of the medium vibrate about their mean position while the wave moves forward. |
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 Each particle of the medium vibrate, takes energy from its preceding particle and transmits it to the next particle. |
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 During a wave motion, the medium does not move as a whole. Only the disturbance travels through the medium. |
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