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Monday, November 30, 1998

Lecture 35. Results from Exam #3 Overall Grades Thus Far Sketch of Remainder of the Semester Chapter 13: Hooke’s Law (again!) Simple Harmonic Motion. Monday, November 30, 1998. Physics 111. Exam 3. Generally, most of you did WAY better this time!.

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Monday, November 30, 1998

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  1. Lecture 35 Results from Exam #3 Overall Grades Thus Far Sketch of Remainder of the Semester Chapter 13: Hooke’s Law (again!) Simple Harmonic Motion Monday, November 30, 1998 Physics 111

  2. Exam 3 Generally, most of you did WAY better this time! I guess it really was a pretty EASY test… As you’ll be able to tell from the grade distribution which sits about 14 points higher than either of the two previous exams! You should have been able to really ENJOY your turkey dinner!

  3. D C B A If I had to put letter grades on this exam...

  4. Overall Grades (As seen on the Physics 111 web site…) I have compiled overall grades up through and including Exam #3. I have dropped your lowest homework grade (as promised) but have NOT dropped your lowest exam score(yet). I will do the latter at the end of the semester. You will find your current overall average marked on your test in purple crayon.

  5. If your average is below 70, you might like to stop by and visit me.

  6. Please, do not leave town early…you’ll just make my life …ahem… ….miserable???... Upcoming Exams Our final exam period is scheduled for the very last possible day of finals’ week: Friday, December 18, 1998! (Is that pathetic enough?)

  7. Upcoming Exams This 2-hour exam period will contain TWO exams. The first 50 minutes will be used for an exam on the material that we cover over the next two weeks. It will be structured very much like Exam #3 (three sections, one of which will be multiple choice---you do two of the three). You will be permitted to use the usual 3”X5” note card.

  8. Upcoming Exams Final Exam: 1 hour plus comprehensive 5 sections, you do 4 (one will be multiple choice) Each section somewhat shorter (25 points)] 8.5”X11” crib sheet + 3”X5” notecard from Exam #4 I will give you math & constants but not physics formulae

  9. Upcoming Exams Exam #4 (during the first 50 minutes of our final exam period) counts the same as the previous 3 exams (the best 3 scores from Exams #1 - 4 count 15% each). If you are REALLY happy with your performance on the first 3 exams, simply notify me and you will be permitted to SKIP Exam #4. In this case you may show up 50 minutes into the exam period. (I’m guessing there won’t be too many of YOU!)

  10. Upcoming Exams Everyone MUST take the Final Exam. No Exceptions! You cannot pass this class without completing the final exam. The final exam counts for everyone and is worth 20% of your final grade.

  11. There will be TWO more homework assignments: - due Monday, December 7 - due Friday, December 11 Other Notes There will be two more meetings of Lab: - Thursday, December 3 (Lab 10) - Thursday, December 10 (Lab Final=Party?) We are going to skip Chapter 12 -- not that the material in Chapter 12 isn’t important, but I think the time is better spent on waves and sound (Chapters 13 and 14).

  12. Any Questions?

  13. Chapter 13 Vibrations and Waves We’ve already studies some vibrational motion, when we we examining the curious behavior of springs and objects that interact with them. We will expand our studies to objects that behave similarly to our spring, such as the pendulum and rotating objects.

  14. We’re going to start by reviewing some of the basic properties of springs. You may recognize this stuff…These are the same notes I used back in Chapter 5! So let’s go through them quickly just to refresh our memories -- especially after that long Thanksgiving Break!

  15. x l Springs! Springs! However, if we compress or stretch the spring by some amount x, then the spring is observed to exert a Force in the opposite direction. Hook discovered this force could be modeled by the mathematical expression F = - kx Notice that this force operates along a linear line!

  16. Force x Slope of this line is -k, where k is the spring constant. Springs! Springs! Which means that if we looked at the plot of Force versus compression/stretching x...

  17. Force x Springs! Springs! Notice that the force is always in the OPPOSITE direction of the displacement. We call such a force a Restoring Force Because the force acts to “restore” the particle to its original position.

  18. Force F1 x -x1 Springs! Springs! If we look at the work done by an applied force which compresses the spring through a distance (-x1)... Work done BY the external force ON the spring. This energy is stored in the spring...

  19. Springs! Springs! Potential Energy of a spring is So, for spring problems, we have a new TOTAL MECHANICAL ENERGY given by And it is THIS quantity which will be conserved absent other, outside forces.

  20. Concept Quiz! Springs & Energy

  21. A wealthy socialite, bored of counting his gold coins, decides to play with his new spring toy. Predict the motion of the mass at the bottom of this spring as explicitly as possible. Assume no friction and no air resistance.

  22. h Now, sketch a plot of the height of the block above the floor as a function of time. What kind of mathematical functions (with which you’re familiar) result in such a pattern?

  23. Equilibrium position Amplitude Amplitude period

  24. This type of oscillatory behavior is known as Simple Harmonic Motion NO! Not simple harmonica music! An object in simple harmonic motion displays an acceleration that is proportional to the displacement and in the opposite direction.

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