Lect17_[Compatibility_Mode]

Lect17_[Compatibility_Mode] - Physics 344 Foundations of...

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Physics 344 Foundations of 21 st Century Physics: Relativity, Quantum Mechanics and heir Applications Their Applications Instructor: Dr. Mark Haugan Office: PHYS 282 haugan@purdue.edu TA: Dan Hartzler Office: PHYS 7 dhartzle@purdue.edu Grader: Shuo Liu Office: PHYS 283 liu305@purdue.edu Office Hours: If you have questions, just email us to make an ppointment. e enjoy talking about physics! appointment. We enjoy talking about physics! Reading: Re-read chapters 1 through 10 as necessary. Notices: Exam I will be at 8:00pm Thursday, October 7 in MTHW 210
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Recitation Recap and Review The first problem we worked on yesterday dealt with three events and the time and spatial separations between them measured by observers in two different inertial frames. It was intended as a review of much of what we studied before turning our attention to energy, momentum and mass. t The events A and B were lights turning on beside a straight section of train track. The information given in the problem is depicted in this track- frame spacetime diagram. Is there an inertial frame in which these events t=5μs B occur at the same location? Answer 1: The interval between these events is 222 2 2 (25 100) 75 BA stx s s μ Δ= Δ = x x = 10μs A and, therefore, spacelike. So, there is no frame in which the events occur at the same location. nswer 2: To be at both events an observer would have to travel 10 s 5 s s Answer 2: To be at both events an observer would have to travel 10 μs in 5 μs as measured in the track frame, a speed greater than c . This is not possible. Either way, explain your reasoning!
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Is there an inertial frame in which these events occur at the same time? Answer 1: The interval between these events is 222 2 2 (25 100) 75 BA stx s s μ Δ= Δ = and, therefore, spacelike. So, there is a frame in which the events occur at the t same time. Answer 2: We show that there is such a frame by constructing it. t pproach a: The time separation between the Approach a: The time separation between the events in an Other frame is ( ) tt x γβ Δ=Δ− Δ etting this equal to zero we conclude that t=5μs B x Setting this equal to zero we conclude that /5 / 1 0 1 / 2 tx β = ΔΔ = = which is a possible speed. pproach b: We know that the tangent of the x x = 10μs A Approach b: We know that the tangent of the angle θ between the x axis and the x’ axis of the Other frame in which the events at the same time is β . So, β = 5/10 = ½. θ Whichever way, explain your reasoning! It is how you get an answer that indicates whether or not you’ve understood the physics and can use it to predict and explain what will happen in new situations.
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Determine the speed at which the train must travel relative to the track in order for the times of events A and B to be equal measured in the train frame. When
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This note was uploaded on 11/26/2010 for the course PHYS 344 taught by Professor Garfinkel during the Spring '08 term at Purdue University-West Lafayette.

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Lect17_[Compatibility_Mode] - Physics 344 Foundations of...

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