Powerpoint for physics

1. THE DOPPLER EFFECT
HANTAO MAI
PHYSICS 101: SECTION 201

2. TABLE OF CONTENTS
1) WHAT IS THE DOPPLER EFFECT?
2) MOVING SOURCE, STATIONARY RECEIVER
3) MOVING RECEIVER, STATIONARY SOURCE
4) EXAMPLE QUESTION + STEP BY STEP SOLUTION
5) KEY EQUATIONS
6) CONCLUSION

3. WHAT IS THE DOPPLER EFFECT?
• IF THERE IS A RELATIVE MOTION BETWEEN THE SOURCE OF THE SOUND AND THE
RECEIVER, THE FREQUENCY AT THE RECEIVER IS DIFFERENT FROM THE FREQUENCY
THAT IS TRANSMITTED.
• THIS IS KNOWN AS THE DOPPLER EFFECT AND EXPLAINS THAT IF THE RECEIVER AND
THE SOURCE ARE MOVING TOWARDS EACH OTHER, THE RECEIVED FREQUENCY IS
HIGHER AND IF THEY ARE MOVING APART, THE RECEIVED FREQUENCY IS LOWER.
• IN GENERAL, SINCE THE WAVE SPEED IS THE SAME, IF A RECEIVER IS CLOSE TO THE
SOURCE, IT WILL DETECT MORE WAVES PER SECOND COMPARED TO A RECEIVER
THAT IS FARTHER AWAY.
• THE DOPPLER EFFECT CAN BE DESCRIBED BY THE EQUATION:
FR= FS(V + VR)
V + VS

4. WHAT IS THE DOPPLER EFFECT? (CONT.)
• THE EQUATION FOR THE DOPPLER EFFECT CAN EXPLAIN THREE DIFFERENT
SCENARIOS WHICH INCLUDE:
• A STATIONARY SOURCE, BUT MOVING RECEIVER
• A STATIONARY RECEIVER, BUT MOVING SOURCE
• A MOVING SOURCE AND RECEIVER
• IN THE EQUATION LISTED ABOVE, FS IS DENOTED TO BE THE FREQUENCY OF THE
SOURCE AND FR IS DENOTED TO BE THE FREQUENCY OF THE RECEIVER.

5. WHAT IS THE DOPPLER EFFECT
(EXPLANATION OF EQUATION)
• BELOW IS THE EQUATION OF THE DOPPLER EFFECT:
FR= FS(V + VR)
V + VS
• FOR THIS EQUATION:
• V IS THE SPEED OF SOUND IN AIR
• VR AND VS ARE THE SPEED OF THE RECEIVER AND THE SOURCE, RELATIVE TO THE AIR.
• THE NUMERATOR IS +/- WHILE THE DENOMINATOR IS -/+. FOR THE CASES WHERE
THE MOTIONS OF THE RECEIVER AND SOURCE ARE TOWARDS EACH OTHER, THE TOP
SIGN IS USED.

6. MOVING SOURCE, STATIONARY RECEIVER
• THIS IS THE CASE WHERE VR IS SET TO BE 0 WHICH RESULTS IN THE EQUATION:
FR= FS( V )
V - VS
• A RECEIVER AT ANY POINT AROUND THE SOURCE WILL DETECT THE SAME
FREQUENCY BECAUSE THE WAVE FRONTS ARE ALL EQUALLY SPACED AND ALL
TRAVEL AT THE SAME SPEED.
• HOWEVER, IF THE SOURCE WERE TO MOVE, IT WOULD BE CLOSER TO ONE
RECEIVER. SINCE THE WAVES ARE ALL TRAVELLING AT THE SAME SPEED, THE
RECEIVER THAT THE SOURCE IS CLOSER TOO WILL DETECT MORE WAVES PER
SECOND AS THE SOURCE IS CLOSER.

7. MOVING RECEIVER, STATIONARY SOURCE
• FOR THIS CASE, WE SET VS TO BE 0 SO THE EQUATION BECOMES:
FR= FS(V + VR)
V
• IN THIS CASE, IF THE RECEIVER IS MOVING AWAY FROM THE SOURCE, IT WILL
DETECT A REDUCTION IN THE WAVE SPEED, WHILE IF IT IS MOVING TOWARDS
THE SOURCE, IT WILL DETECT AN INCREASE IN THE WAVE SPEED.

8. EXAMPLE QUESTIONS
• QUESTION (PAGE 441):
• A HIGH-SPEED TRAIN TRAVELLING AT 250KM/H BLOWS ITS WHISTLE AS IT
APPROACHES A LEVEL CROSSING. THE WHISTLE HAS A FREQUENCY OF 660 HERTZ.
WHAT FREQUENCY WOULD BE PERCEIVED BY SOMEONE STANDING AT THE CROSSING
AS THE TRAIN:
• (A) APPROACHES THE CROSSING
• (B) MOVES AWAY FROM THE CROSSING?
• (A) FOR PART A, YOU CAN SET THE SPEED OF THE RECEIVER TO 0 AS THE
PERSON IS NOT MOVING. YOU ARE GIVEN THE SPEED OF THE TRAIN (69.4M/S)
AND YOU CAN APPROXIMATE THE SOUND OF AIR AS 343M/S.

9. EXAMPLE QUESTION (CONT.)
• PLUGGING IN THE VALUES, WE GET:
FR= 660(343 + 0) = 828 HZ
343 – 69.4
• NOTICE THE NEGATIVE SIGN ON THE DENOMINATOR AS THE SOURCE IS MOVING
TOWARDS THE RECEIVER SO THE TOP SIGN IS USED.
• (B) FOR THIS PART, THE TRAIN IS MOVING AWAY FROM THE RECEIVER SO THE BOTTOM SIGN
(+) IS USED.
FR= 660(343 + 0) = 549 HZ
343 + 69.4

10. KEY EQUATIONS (SUMMARY)
• FOR A CASE WHERE THE SOURCE IS MOVING BUT THE RECEIVER IS
STATIONARY:
FR= FS(__V )
V - VS
• FOR A CASE WHERE THE RECEIVER IS MOVING BUT THE SOURCE IS
STATIONARY:
FR= FS(V + VR)
V
• THE GENERAL EQUATION IS GIVEN BY:

11. CONCLUSION
• THERE ARE MANY ONLINE RESOURCES THAT CAN BE USED TO HELP YOU WITH
YOUR UNDERSTANDING OF THE DOPPLER EFFECT.
• FOR FURTHER INFORMATION ON THE DOPPLER EFFECT:
• HTTPS://WWW.YOUTUBE.COM/WATCH?V=H4ONBYRBCJY
• HTTPS://WWW.YOUTUBE.COM/WATCH?V=DC7L7QQA8XK