THE DOPPLER EFFECTPHYSICS 101 LO5 BY CALLEE WAH YAN YUEN

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

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

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

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

FORMULAS

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?

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 = ?

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.

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!

WATCH IT AGAIN!DOES IT MAKE SENSE NOW? : )

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Thank you!