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# Relativity/Relative Motion

Question
Hello !!
My Question is about Relative Motion;
When traveling in any automobile like car or bike, if any airplane is seen in the sky flying in the opposite direction to that of our direction of motion it seems to stand still in the sky whereas if the airplane is moving in the same direction as that of our direction of motion, the speed of aircraft seems faster than usual.
However, in the same case, if we are traveling in a car, the cars in the opposite lane having direction of motion opposite to that of ours seem to be faster than usual, same goes for trains.
This observation is enigmatic in my belief as airplanes flying with opposite direction of motion to that of ours (when in a moving car) seem to be stand still or move very slowly whereas cars and trains having opposite direction of motion to that of ours feel faster than usual.
Please explain this phenomenon as why it happens and release me from the conundrum of relative motion.

Danny, I am sorry to be late in responding.  I have been travelling.

What you describe is nothing that I have seen.  I suppose it is an illusion.

My only comment is that relative motion is well defined and should be always measurable, not just based on appearances.  The relative velocity of one object with respect to another is the difference of their velocities both measured with respect to the same inertial reference frame. This can be a frame in which one of them is stationary, but this is not a requirement.

The result may be different according to what measurement frame is chosen. That is a relativistic effect.  In classical physics, the result would not depend on the frame.

Ben

Relativity

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#### Uncle Ben

##### Expertise

I can answer questions regarding Einstein's Theory of Relativity, particularly in Special Relativity. I will not answer homework questions or mathematical problems that require special symbols.

##### Experience

I have taught physics at the college level, undergraduate and graduate, for many years including Special Relativity. I have taught at Johns Hopkins, Case-Western, and MIT. I have also served as a staff member of the Commission on College Physics, which was supported by the National Science Foundation to recommend improvements in the curriculum of college physics departments in the US. I am also the author of a textbook titled Vector Calculus, which was used at MIT in the teaching of electromagnetic theory and relativity. My research interests were mainly in solid state physics, especially the properties of metals at low temperatures. I am listed in the publication known as American Men of Science.

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I have dozens of papers published in the Physical Review and in the American Journal of Physics.

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I hold a Ph.D. degree in physics from the Johns Hopkins University.

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