Lecture10

Lecture10 - Physics 344 Foundations of 21st Century Physics...

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Physics 344 Foundations of 21 st Century Physics: Relativistic and Quantum Systems Instructor: Dr. Mark Haugan Office: PHYS 282 [email protected] TA: Dan Hartzler Office: PHYS 7 [email protected] Grader: Fan Chen Office: PHYS 222 [email protected] Office Hours: If you have questions, just email us to make an appointment. We enjoy talking about physics! Help Session: Thursdays 2:00 – 4:00 in PHYS 154 Reading: Chapters 1 through 8 in Six Ideas that Shaped Physics, Unit R. Exam 1: Wednesday, October 5 at 8:00pm in WTHR 104
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Recitation Recap The relativity of simultaneity, Lorentz contraction and time dilation are the most basic relativistic phenomena relating measurements of the space and time separations between pairs of events made by observers in different inertial frames. To understand them and to understand how they are built into and determine the form of the Lorentz transformation equations, we’ve had to be extremely careful about defining procedures that inertial observers use to make their measurements of the space and time separations between event pairs. The homework problems I’ve asked you to do have been opportunities to sharpen your understanding of those measurement processes and basic relativistic effects by using them to predict and explain what happens in (sometimes fanciful) physical situations. We will wrap up our analysis of proper time measurements as consequences of time dilation this week. Yesterday, we examined a variation on a homework problem to emphasize the way that what you know about measurement procedures, about physical laws, about Lorentz transformations and about spacetime diagrams all needs to be integrated to predict and explain what happens in a situation involving Lorentz contraction.
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t t x x In problem R7R.3 two astronauts, A1 and A2, are riding at the front and back ends of a spaceship which they measure to be L R long. The ship is traveling in the x direction at 0.8 c relative to a frame S in which a long measuring stick lies at rest along the x axis. The astronauts us their synchronized clocks to make simultaneous cuts through the stick. In this spacetime diagram, the cuts are shown as being made at t’ = 0. The vertical worldlines that begin at those events are those of the front and back ends of the section they cut from the long measuring stick. You determined that the length of that section measured in S is 5 L R /3. A1 worldline A2 worldline Yesterday, we added to the situation a second long measuring stick made of the same material as the first and lying parallel to it but at rest in S’ . When the astronauts make their cuts, they cut sections from both sticks. The worldlines of the ends of the cut section of the second stick are essentially identical with the worldlines of A1 and A2.
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t t x x A1 worldline A2 worldline How long do the Astronauts measure each of the cut sections to be? The cuts are made simultaneously, thus, marking
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Lecture10 - Physics 344 Foundations of 21st Century Physics...

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