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1. The rapid back and forth motion of a particle about its mean position is called.
 
2. If the energy in a longitudinal wave travels from south to north, the particles of the medium would be vibrating in .
 
3. A whistle giving out a sound of frequency \(450\) \(Hz\), approaches a stationary observer at a speed of \(33\) \(m/s\). The frequency heard by the observer is (speed of sound = \(330\) \(m/s\)) .
 
4. A source of sound is travelling with a velocity \(40\) \(km/h\) towards an observer and emits a sound of frequency \(2000\) \(Hz\). If the velocity of sound is \(1220\) \(km/h\), then the apparent frequency heard by the observer is .