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The Doppler Effect

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The Doppler Effect

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  1. The Doppler Effect Ch. 25.9-25.11

  2. Objectives • Describe doppler effect • Describe bow waves • Describe sonic booms

  3. Stationary Source • Bug bobbing at constant frequency on surface of water • What do you notice about f & ? • Bug forms concentric circles. • Waves encounter point A as frequently as point B.

  4. The Doppler EffectMoving Source • Bug begins to move across surface @ less than wave speed. • Which observer (A or B) will encounter a higher frequency of waves? • Answer: B • Doppler Effect: • change in FREQUENCY (f) due to the motion of the source • What about wave speed, does it change? • NO!!!! A A B

  5. Doppler EffectMoving Source • Water waves spread over flat surface, but Sound waves travel in 3-D and expand like balloon • What do you notice about the waves in front of train compared to those in back when train is stationary? In motion? • Wave crests ahead of moving source are closer together than behind source • What does this mean? • Higher frequency in front, lower frequency in back. • i.e. car/train horn • Demo with string and noise

  6. Doppler Effect

  7. Radar • Bounce radar waves off of moving vehicles • Compares f of radar waves from gun with the f of reflected waves

  8. Checkpoint • Q: When a source moves towards you, is there an increase or decrease in wave speed? • A: Neither! It is the frequency of the wave that undergoes a change, NOT the wave speed.

  9. What happens when the speed of the source is is EQUAL to the speed of waves it produces? • The waves pile up and superimpose on each other • This is a barrier wave: • Source and waves are equal speeds

  10. What happens when the speed of the source is is GREATER than the speed of waves it produces? • Produces a Bow Wave: • Speed of source is greater than wave speed • The wave crests overlap at edges creating a V shape.

  11. Bow Waves • This can be seen by speedboats • Increased speed produces a narrower V shape. • Notice that the boat down below is outrunning the waves it produces

  12. Patterns made by an object moving at successive speeds Draw these please

  13. F-14 Tomcat

  14. Do Planes actually “break the sound barrier?”Here’s what happens. . . • Overlapping wave crests disrupt flow of air over wings --> harder to control plane when flying close to speed of sound (761mph) • But, the barrier is not real • Think of it this way… • Just like a boat travels faster than speed of water waves…a plane can travel faster than the speed of sound • Supersonic!!

  15. Shock Waves • Shock Wave: • produced from overlapping spheres that form a cone • Jet “pushes” sound waves in front of it • Sound waves must “obey” the speed limit--> pile up against each other • These “piled up” waves are called shock waves

  16. Shock Waves • Sonic Boom: • the “crack” you hear when the compressed air of a supersonic object reaches the ground • Sound waves hit you all at once (not one at a time) • Boat (one wave at at time) vs. jet (all waves at once) • A bullet passing overhead produces a “crack” which is a small sonic boom • Bigger the object --> more air is displaced --> louder the boom • Sonic Boom Description

  17. Shock Waves • Shock Waves spread until they reach the ground.

  18. Crests overlap and encounters listener in a single burst • So all listeners hear the boom at the same time? • Shock wave and sonic boom are swept continuously behind aircraft • Bullets, whips, and towels