The Doppler Effect

Class the 7th

Did you complete at least 70% of the reading assignment (17.4)?

A.Yes B. No Bring Chapter Summaries

Think about this: a stationary observer hears a pitch from a source that moves towards him @ a large fraction of the speed of sound. If instead the source is stationary and the observer moves at the same speed towards the source, the observed frequency will be lower/higher/same?

Remember: the online schedule has precedence.

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2 When a frequency source & an observer are moving toward each other the frequency heard by the observer is:

A. lower than B. higher than or C.equal to the

frequency of the source.

Quiz 401103021

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A stationary observer hears a pitch from a source that moves towards him @ ½ of the speed of sound. If instead the source is stationary and the observer moves at the same speed towards the source, the observed frequency will be: (pp)

A. lower than B. higher than or C. equal to the frequency of the

source.

Animation of the Doppler Effect

• Please go to the computer resources on line: http://stokes.byu.edu/123-resources.html

• Find Doppler and run it. – Move observer (man) right and left– Move source (trumpet) right and left.– Are the clicks at the same rate when Washington

approaches the source as when the trumpet approaches Washington?

• Online demo• Video • Demo

f’ = (v±vo)/(v±vs)fO___________________S___Right is plus, left minusKnowing how to get the signs right is the tricky

part. Your book has a way. Here is another. The arrow ► on the line above points in the plus direction, to the right like it usually does. If source (S) is moving to the right (left) use the plus (minus) sign. The same thing with the observer (O).

But you should always check yourself by remembering that when observer and source approach each other the observed frequency is higher, while if they move away from each other it is lower.

3. An organ is playing a hymn. We move away from the organ at velocity v0. The speed of sound is v. Which expression gives the frequency we hear?

A. f′ = f(1 + v0/v)B. f′ = f(1 - v0/v)

Hear: “Come, Come Ye Saints”

• Please go to the computer resources on line: http://stokes.byu.edu/123-resources.html

• Find Bells and run it.• Key of G —►key of Gb —• Lower freq by 2 1/12 = 1.059• Move away from Organ f’ = f(1 - vo/v)• vo = v(1- f’/f) =343 m/s (1-1/1.059) =19 m/s• 43 mph

3. An organ is playing a hymn. We move away from the organ at velocity v0. The speed of sound is v. Which expression gives the frequency we hear?

A. f’ = f(1 + v0/v)B. f’ = f(1 - v0/v)

How fast was she going?

Bat of long scream

A. f’ = f(1+v0/v)/ [1+vS/v] B. f’ = f(1-v0/v)/ [1 +vS/v]C. f’ = f(1+v0/v)/ [1- vS/v]D. f’ = f(1-v0/v)/ [1 - vS/v]

Shock waves

• Please go to the computer resources on line: http://stokes.byu.edu/123-resources.html

• Find Boom and Boomray and run them. (with different velocities if you can figure that out.)

• Scripted version has narration.

This is picture of ripple tank

What frequency does the bat hear? (pp)

A. f’ = f(1+v0/v)/ [1+vS/v] B. f’ = f(1-v0/v)/ [1 +vS/v]C. f’ = f(1+v0/v)/ [1- vS/v]D. f’ = f(1-v0/v)/ [1 - vS/v]

What frequency does the bat hear? (pp)

A.f′ = f(1+v0/v)/ [1+vs/v] B.f′ = f(1-v0/v)/ [1 +vs/v]C.f′ = f(1+v0/v)/ [1- vs/v]D.f′ = f(1-v0/v)/ [1 - vs/v]