The Doppler Effect Ch. 25.9-25.11
Objectives • Describe doppler effect • Describe bow waves • Describe sonic booms
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.
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
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
Radar • Bounce radar waves off of moving vehicles • Compares f of radar waves from gun with the f of reflected waves
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.
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
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.
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
Patterns made by an object moving at successive speeds Draw these please
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!!
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
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
Shock Waves • Shock Waves spread until they reach the ground.
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