Doppler Shift

 

Subject Area

 Physics - waves
Age or Grade

 9th grade
Estimated Length

 50 minutes
Prerequisite knowledge/skills

 Not much. Knowledge that sound is a compressional wave is good though.
Description of New Content

 The Doppler shift happens when an object emitting waves (sound in this case) is moving towards (higher pitch) or away (lower pitch) from you.
Goals

 Give the kids a hands-on demonstration of the Doppler effect.
Materials Needed


 String and a bunch of rulers with a hole in one end. Tie a one meter (or more) piece of string through the hole in the end of the ruler.

Procedure

 

 

 

Opener

Introductory Question: What property of waves changes when a pitch goes up? The frequency, or how fast the wave crests are passing by you.

Development

Lecture: Waves
How could you make the wave crests pass by you faster? If you were moving towards the sound, or if the thing making the sound were moving towards you.
What would you hear then? Pitch goes up.
Has anyone ever heard this? They probably don't think they have. Play a video on Doppler shift:
Video shows car with horn blaring driving past camera. Pitch goes up, then down.
Ask kids what they thought happened. Explain that the waves became bunched up, frequency went up (pitch went up). As it passed, waves spread out, pitch went down.

Lab: Give each group a ruler with the string and have them twirl it above their heads, like a helicopter. This is obviously best done where there is a lot of space. The ruler must be spinning for it to make the humming sound. They should notice that as the ruler is coming towards them, the pitch increases and as it is going away from them. It is important for each kid to try it himself. We did 1m pieces of string, but it may work better with slightly longer pieces.

Closure

Write-up: The assignment is to explain the experiment, what they observed and why they observed it.
Evaluation

 Their write-ups will tell you how well they understood it
Extensions

 Reflection question: What other waves have we learned about? Do you think the Doppler Effect applies to them? If so, what would you observe?

Astronomers use the Doppler shift all the time, but with light. Light is of course also a wave and the faster something is moving away from you, the more the light is shifted. This proves the expansion of the Universe and other cool things.
References There is a worksheet on the internet, but for the life of me I can't find it.
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