1. The Doppler Effect Ch

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 Effect Moving Source
Bug begins to move across 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 Effect Moving 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!!

18. Shock Waves Shock Wave: Jet “pushes” sound waves in front of it
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

19. 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

20. Shock Waves
Shock Waves spread until they reach the ground.

21. 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