direction fill in the missing letters to form the word write your answer on the

1.move particular to the direction particles move up and down as they move forward.
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2. its particles of the medium are moved back to the forth parallel to the direction of the motion of energy.
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3. this kind of waves does not need any medium to propagate its can passes through to vacuum .
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4. this kind of waves need a medium crab and this can be solid liquid or gas .
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5. it's better to the number of waves passing through a point per second.
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