Moozy Posted December 31, 2014 Share Posted December 31, 2014 About the Doppler effect: We know that when a vehicle passes at high speed in front of a stationary observer, the sound of the horn is high frequency when the vehicle approaches, then low frequency when the vehicle moves away. My question: what happens when, on the contrary, a moving observer passes at high speed a stationary vehicle honking? Do we get the same Doppler effect? My guess: the effect is the same ... Link to comment Share on other sites More sharing options...
Robittybob1 Posted December 31, 2014 Share Posted December 31, 2014 About the Doppler effect: We know that when a vehicle passes at high speed in front of a stationary observer, the sound of the horn is high frequency when the vehicle approaches, then low frequency when the vehicle moves away. My question: what happens when, on the contrary, a moving observer passes at high speed a stationary vehicle honking? Do we get the same Doppler effect? My guess: the effect is the same ... Try it out. It seems a simple enough experiment. Link to comment Share on other sites More sharing options...
Moozy Posted December 31, 2014 Author Share Posted December 31, 2014 (edited) Thanx robittybob1, but I was expecting more elaborate answers... The question is not as 'shallow' as it may seem Edited December 31, 2014 by Moozy Link to comment Share on other sites More sharing options...
Janus Posted December 31, 2014 Share Posted December 31, 2014 (edited) The Doppler shift formula is[math]f = \left ( \frac{C+V_r}{C+v_s} \right ) f_o[/math]where fo is the original frequency of the sourceC is the speed of waves in the mediumVr is the velocity of the receiver relative to the medium (Positive if moving towards the source)Vs is the velocity of the source relative to the medium (positve if moving away from the receiver)The speed of sound is ~340 m/s, and a car driving at 60 mph moves at ~27 m/sso when the car is moving toward you, you get a frequency shift of 1.08 and when going away you get a shift of 0.92.If you where moving towards the car (car at rest with respect to the air), you get1.09 and moving away gives you 0.93.So you get slightly more shift when approaching and slightly less when moving away than when it is the car moving.If we bump the speed of the car up to that up to say, 1/2 the speed of sound, we get1.5 and 0.5 for the car moving and2 and 0.667 for you movingYou get a shift in both cases, but it isn't exactly the same.Now for light,traveling in a vacuum which does not require a medium and adjusting for Relativity, we use[math] f = f_o \sqrt { \frac{1+ \frac{v}{c}}{1- \frac{v}{c}}} [/math]where v/c is the relative speed of source and receiver measured as a fraction of the speed of light and is positive when they are approaching each other.With this formula, it doesn't matter which(source or receiver), is considered to be moving. Edited December 31, 2014 by Janus 3 Link to comment Share on other sites More sharing options...
studiot Posted December 31, 2014 Share Posted December 31, 2014 Impressive answer, Janus. +1 1 Link to comment Share on other sites More sharing options...
Robittybob1 Posted December 31, 2014 Share Posted December 31, 2014 Thanx robittybob1, but I was expecting more elaborate answers... The question is not as 'shallow' as it may seem I wonder if your ears could have picked up the mathematical relationship between speed and sound as Janus has just explained. When I look at the formula for light I do wonder how anyone ever comes to those sort of results, I'd like to know. Link to comment Share on other sites More sharing options...
Sensei Posted January 1, 2015 Share Posted January 1, 2015 (edited) When I look at the formula for light I do wonder how anyone ever comes to those sort of results, I'd like to know. Place laser on fast moving vehicle and turn it on. Laser beam send to receiver. Compare with exactly the same stationary laser. I wonder if your ears could have picked up the mathematical relationship between speed and sound as Janus has just explained. Don't you have ambulance/police/fire department in your area? Difference between sound from vehicle that is approaching/receding is clearly noticeable. Edited January 1, 2015 by Sensei 1 Link to comment Share on other sites More sharing options...
Robittybob1 Posted January 1, 2015 Share Posted January 1, 2015 ...Don't you have ambulance/police/fire department in your area? Difference between sound from vehicle that is approaching/receding is clearly noticeable. The question was whether the effect was reversible. Stationary siren does it have a different sound when one is approaching or receding? Link to comment Share on other sites More sharing options...
Moozy Posted January 1, 2015 Author Share Posted January 1, 2015 (edited) Thank you Janus for this detailed answer. I am reading a book on the "run away" of galaxies (in french, "la fuite des galaxies") and I wanted this point to be clarified (a Doppler effect can be detected on a stationary object provided the receiver is moving). By the way, Happy New Year 2015 to all the distinguished members of this forum Edited January 1, 2015 by Moozy Link to comment Share on other sites More sharing options...
Sensei Posted January 1, 2015 Share Posted January 1, 2015 (edited) The question was whether the effect was reversible. Stationary siren does it have a different sound when one is approaching or receding? Yes. Record sound using f.e. digital camera video recorder while police/ambulance/fire fighters will be going.. Then compare sound, when they're approaching, and when they're receding you. If you will be following car with siren, you won't see difference, as both you and that car have similar/equal velocity. Somebody driving car with 70 km/h has speed [math]19.44\frac{m}{s}[/math]. That's 5.7% of speed of sound (340 m/s). Somebody driving car with 100 km/h has speed [math]27.77\frac{m}{s}[/math]. That's 8.17% of speed of sound. Edited January 1, 2015 by Sensei 1 Link to comment Share on other sites More sharing options...
Robittybob1 Posted January 1, 2015 Share Posted January 1, 2015 Yes. Record sound using f.e. digital camera video recorder while police/ambulance/fire fighters will be going.. Then compare sound, when they're approaching, and when they're receding you. If you will be following car with siren, you won't see difference, as both you and that car have similar/equal velocity. Thanks and when you pass the vehicle with siren? Link to comment Share on other sites More sharing options...
Sensei Posted January 1, 2015 Share Posted January 1, 2015 Check by yourself on YouTube https://www.youtube.com/results?search_query=sirens+doppler+effect Link to comment Share on other sites More sharing options...
Strange Posted January 2, 2015 Share Posted January 2, 2015 Thank you Janus for this detailed answer. I am reading a book on the "run away" of galaxies (in french, "la fuite des galaxies") and I wanted this point to be clarified (a Doppler effect can be detected on a stationary object provided the receiver is moving). Note: if this is about the expanding universe (big bang) model, then this is not the Doppler effect. 1 Link to comment Share on other sites More sharing options...
Robittybob1 Posted January 2, 2015 Share Posted January 2, 2015 Check by yourself on YouTube https://www.youtube.com/results?search_query=sirens+doppler+effect All the YT videos has the source of the sound moving. None with the situation being examined. Link to comment Share on other sites More sharing options...
Moozy Posted January 3, 2015 Author Share Posted January 3, 2015 @ Strange : Question: Why cosmoslogists have come to the conclusion that the universe is expanding ? Answer: The redshift (i.e. the Doppler effect) of galaxies. Link to comment Share on other sites More sharing options...
Strange Posted January 3, 2015 Share Posted January 3, 2015 @ Strange : Question: Why cosmoslogists have come to the conclusion that the universe is expanding ? Answer: The redshift (i.e. the Doppler effect) of galaxies. That was one of the first pieces of evidence to confirm the prediction, but not the most convincing. The CMB was the thing that convinced nearly everybody and destroyed the steady state models. But it isn't the Doppler effect. If you use the Doppler effect to work out the rate at which galaxies are receding, you get the wrong result. For example, we can see galaxies that are receding faster than light. If it were the Doppler effect, then they would not be visible. 1 Link to comment Share on other sites More sharing options...
Mordred Posted January 3, 2015 Share Posted January 3, 2015 As Strange mentioned its not Doppler effect as per se. It's a similar effect however. For expansion the correct term is cosmological redshift. When gravity is involved ie gravity wells. You have gravitational redshift. Each has its own formula as the cause and therefore amount of influence on the 3 is different Doppler is motion. 1 Link to comment Share on other sites More sharing options...
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