UNC-Chapel Hill PHYS 104 - Referential Frames (2 pages)

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Referential Frames

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Referential Frames


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Lecture Note
University of North Carolina at Chapel Hill
Phys 104 - General Physics I
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PHYS 104 1st Edition Lecture 25 Outline of Last Lecture I Standing Waves II Doppler Effect Outline of Current Lecture III Referential Frames IV Galilean Relativity V Einstein s Principle of Relativity Current Lecture An object does not have a true speed or velocity The definition of velocity x t assumes the existence of a coordinate system We must specify an object s velocity relative to or with respect to the coordinate system in which it is measured A reference frame is a coordinate system in which observers experimenters equipped with meter sticks stopwatches or any other needed equipment make position and time measurements on moving objects Three ideas are implicit o A reference frame extends infinitely far in all directions o The experiments are at rest in the reference frame o The number of experimenters and the quality of their equipment are sufficient to measure positions and velocities to any level of accuracy needed All the laws of physics not just Newton s laws of mechanics hold in any inertial reference frame Principle of Relativity Electromagnetic waves travel at a speed c 3 00 108 m s in vacuum Light travels at the SAME speed c in all inertial reference frames The definition of velocity is v x t In order for the speed of light to be measured as c in both frames the time t as measured by Laura cannot be the same amount of time as measured by Dan and Eric t This means that our assumptions for the nature of time must be reevaluated During an event t is the time when the event actually happens Light takes time to travel so the event is observed by you at a later time when the light waves reaches the your eyes Events are said to be simultaneous if they take place at different positions x1 and x2 but at the same time t1 t2 These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute In general simultaneous events are not seen at the same time because of the difference in light travel times from the event to an observer Spacetime diagram o Applies to an inertial frame o An event is a point on it defined by coordinates x t o History of an object is represented by world line One of the most striking conclusions of relativity is that two events occurring simultaneously in reference frame S are not simultaneous in any reference frame S that is moving relative to S o This is called the relativity of simultaneity

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