Doppler effect and Shock waves Lecture 10 Pre-reading : §16.8–16.9 Please take a clicker and an evaluation form

Doppler Effect Change in perceived frequency due to relative motion of a source (S) and listener (L) Stationary SourceMoving Source

Doppler Effect Change in perceived frequency due to relative motion of a source (S) and listener (L) Case 1: Source at rest, Listener moving f L = (1 + v L /v) × f S Case 2: Source and Listener moving Pay attention to sign of v L, v S ! (positive from L to S) For light waves f L = √[ (c–v) / (c+v) ] × f S c = 3.0 × 10 8 ms –1

Doppler Effect Change in perceived frequency due to relative motion of a source (S) and listener (L) Case 1: Source at rest, Listener moving f L = (1 + v L /v) × f S Case 2: Source and Listener moving Pay attention to sign of v L, v S ! (positive from L to S) For light waves f L = √[ (c–v) / (c+v) ] × f S c = 3.0 × 10 8 ms –1

Stationary SourceMoving Source

Clicker questions A bird flies in a circle centred on a person’s head and emits a continuous sinusoidal sound wave with frequency of f = 1000 Hz.

d Now there is another person standing a large distance away. A bird flies in a circle centred on a person’s head and emits a continuous sinusoidal sound wave with frequency of f = 1000 Hz.

Shock Waves What if speed of source is equal to or greater than the speed of sound? v S = v sound v S > v sound

This is exactly analogous to the bow wave produced by a swimmer moving faster than the speed of the waves on water. Bow wave produced by a platypus swimming

Shock Waves F/A-18 “Hornet”

Shock Waves What if speed of source is equal to or greater than the speed of sound? v S = v sound v S > v sound

Shock Waves The waves ‘pile up’ at surface When that surface hits your ears, you hear very loud sound: sonic boom If v S > v (sound), the surface has the shape of a cone with an opening angle α where v S /v is called the Mach number

The diagram shows the wavefronts generated by an airplane flying past an observer A at a speed greater than that of sound. Clicker questions

The carillon in the Quad is ringing. You are so excited to have finished your Physics lecture and get to lunch that you run towards the Quad at twice the speed of sound. As you approach the bells, they sound 1. higher pitched than normal 2. lower pitched than normal 3. like a sonic boom

That’s all, folks!

The amount of time Greg spent doing demos was: A) too long B) a bit long C) just right D) a bit short E) too short