PHYS 101 LO5

1. THE DOPPLER EFFECT
PHYSICS 101 LO5
BY CALLEE WAH YAN YUEN

2. WATCH THIS VIDEO OF SHELDON IN HIS DOPPLER EFFECT
COSTUME. DO YOU UNDERSTAND WHAT HE WAS TRYING TO
EXPLAIN? COME REVISIT THIS AFTER THE PRESENTATION!
GO TO THIS LINK:
HTTPS://WWW.YOUTUBE.COM/WATCH?V=Y5KAECZ_AAY

3. WHAT IS THE DOPPLER
EFFECT?
• A change/shift in frequency or wave when the observer moves relative to
its source.
• This effect was named after the Austrian physicist, Christian Doppler
(1803-53).
• The frequency at the source and the receiver are different when there is
relative motion between the two.
• There are 2 exceptions to this effect:
1. Supersonic; When the object is moving very very fast (ex: airplane)
2. Light; because the speed of light is too fast for the doppler effect to
actually take place

4. HOW DOES IT WORK?
• When the source moves closer to observer A, the
frequency of the siren that observer A hears increases.
Source
frequency increases
for observer A

5. HOW DOES IT WORK?
• When the source moves father away relative from
observer B, the frequency of the siren that observer B
hears decreases.
Source
frequency decreases
for observer B

6. FORMULAS

7. QUESTION
An ambulance moving at 60m/s zooms past a kid that
sits waiting at the bus stop. The siren of the ambulance
is measured to have a frequency of 3kHz. What
frequency does the kid hear now that the ambulance
have moved farther away relative from where he is
sitting?

8. SOLUTIONS
To solve this problem, let’s first list down what we are given:
• source velocity (vs) = 60m/s
• source frequency (fs) = 3kHz
• observer velocity (vob) = 0m/s (assuming the kid is
stationary)
• speed of wave (v) = 343m/s (assuming the medium is the
open air which is a constant)
• observer frequency = ?

9. SOLUTIONS
• Make sure to convert all units to SI units first!
• 3kHz = 3000Hz
• We know that the the frequency source is moving away from
the observer.
• Now, let’s apply the formula that we know:
• fob = fs/[1+(vs/v)]
fob = (3000/[1+(60/343)]
fob = 2553Hz
• The frequency of the siren heard by the kid is 2553Hz.

10. FUN FACT:
THE DOPPLER EFFECT IS COMMONLY USED IN OUR DAILY
LIVES! THE RADAR GUN THE POLICE USE SENDS OUT
MICROWAVES THAT BOUNCE OFF MOVING VEHICLES TO
DETECT REFLECTED WAVES. THE BEAT FREQUENCY
BETWEEN THE WAVES CAN DETERMINE THE SPEED OF THE
CAR!

11. WATCH IT AGAIN!
DOES IT MAKE SENSE NOW? : )
HTTPS://WWW.YOUTUBE.COM/WAT
CH?V=Y5KAECZ_AAY

12. Thank you!