the doppler effect
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
The Doppler EffectThe Doppler Effect
Ch. 25.9-25.11
Objectives
1. Describe doppler effect2. Describe bow waves3. 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!!!!
AA 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
Doppler Effect
• Bounce radar waves off of moving vehicles
• Compares f of radar waves from gun with the f of reflected waves
RadarRadar
• 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 pleaseDraw these please
F-14 Tomcat
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