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Physics 114A - Mechanics Lecture 2 (Walker: 2.1-2.3) Position and Velocity January 7, 2014

Physics 114A - Mechanics Lecture 2 (Walker: 2.1-2.3) Position and Velocity January 7, 2014. John G. Cramer Professor Emeritus, Department of Physics B451 PAB jcramer@uw.edu. Write Down This Information!. Physics 114A Course Syllabus: http://courses.washington.edu/p114aw14

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Physics 114A - Mechanics Lecture 2 (Walker: 2.1-2.3) Position and Velocity January 7, 2014

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  1. Physics 114A - MechanicsLecture 2 (Walker: 2.1-2.3)Position and VelocityJanuary 7, 2014 John G. Cramer Professor Emeritus, Department of Physics B451 PAB jcramer@uw.edu

  2. Write Down This Information! Physics 114A Course Syllabus:http://courses.washington.edu/p114aw14 From the top of this web page, you will find links to the other web locations you will need for Physics 114A. Problems? See Susan Miller, Room C136 Physics-Astronomy Bldg.,Telephone: 543-4982. Physics 117 Labs: Information at:http://courses.washington.edu/phys117 Physics 114A - Lecture 2

  3. Announcements • Homework Assignments 1-3 are now online. HW#1 is due at11:59 PM on Thursday, January 16. You will need to log into your WebAssign account to do the Physics 114A homework. As of 12:30 PM today, 92/219 students have established WebAssign accounts. • Obtain a H-iTT clicker from the University Bookstore. We will begin using them next Monday, and we will have some test sessions later this week. Clickers should be set to Channel 01 (which is the default). Register your clicker, giving its 6-digit serial number and a 3-letter screen name of your choice, at: https://catalyst.uw.edu/webq/survey/jcramer/185653 . • Starting on Thursday of next week, I will have office hours in the Study Center on Tuesdays during the hour before class (12:30-1:20 PM) and Thursdays during the hour after class (2:30-3:20 PM). My TA, Ryan Hufschmidt, is in the Study Center on Mondays (11:30 AM-1:20 PM) and Thursdays (3:30-5:20 PM). Physics 114A - Lecture 2

  4. Lecture Schedule (Part 1) We are here. Physics 114A - Lecture 2

  5. Properties of Vectors Physics 114A - Lecture 2

  6. Position, Distance, & Displacement Before describing motion, you must set up a coordinate system – define an origin and select a positive direction. Physics 114A - Lecture 2

  7. Distance Distance is the total length of travel. If you drive from your house to the grocery store and back, you have covered a distance of 8.6 mi. Physics 114A - Lecture 2

  8. Displacement Displacement is the net change in position. If you drive from your house to the grocery store and then to your friend’s house, your displacement is -2.1 mi, while the distance you have traveled is 10.7 mi. Physics 114A - Lecture 2

  9. Average Speed and Velocity The average speed is defined as the distance traveled divided by the time the trip took: Average speed = distance / elapsed time Question: Is the average speed of the red car 40.0 mi/h, more than 40.0 mi/h, or less than 40.0 mi/h? Physics 114A - Lecture 2

  10. Average Speed and Velocity Average velocity = displacement / elapsed time If you return to your starting point, your average velocity is zero. 8 s 48 s Physics 114A - Lecture 2

  11. Average Speed and Velocity Graphical Interpretation of Average Velocity: The same motion, plotted one-dimensionally and as a two dimensional x-t graph: Average speed (0-4 s) = (8 m)/(4 s) = 2.0 m/s Average velocity (0-4 s) = (-2 m)/(4 s) = -0.5 m/s Physics 114A - Lecture 2

  12. Example:Distance & Displacement of a Dog You are playing a game of catch with your dog. The dog is initially standing near your feet. Then he runs 20 feet in a straight line to retrieve a stick and carries it 15 feet backtoward you before lying down on the ground to chew on the stick. • What is the total distance the dog travels? • What is the net displacement of the dog? • Show that the net displacement for the trip is the sum of the net displacements that make up the trip. Physics 114A - Lecture 2

  13. Instantaneous Velocity Definition: (2-4) This means that we evaluate the average velocity over a shorter and shorter period of time; as that time becomes infinitesimally small, we have the instantaneous velocity. Question: how is it possible to divide by zero in the above expression? Physics 114A - Lecture 2

  14. Instantaneous Velocity This plot shows the average velocity being measured over shorter and shorter intervals. The instantaneous velocity is tangent to the curve. Physics 114A - Lecture 2

  15. Calculating Instantaneous Velocity Physics 114A - Lecture 2

  16. Example:Speed & Velocity of the Dog The dog that you were playing within the previous example jogged 20 ftaway from you in 1.0 s to retrieve thestick and ambled back 15 feet in 1.5 s. • Calculate the dog’s average speed. • Calculate the dog’s average velocity for the total trip. Physics 114A - Lecture 2

  17. Graphical Interpretation of Average & Instantaneous Velocity Physics 114A - Lecture 2

  18. Velocity & Slope The position vs. time graph of a particle moving at constant velocity has a constant slope. 4.5 m The position vs. time graph of a particle moving with a changing velocity has a changing slope. 3.0 s slope = velocity = 4.5 m/3.0 s = 1.5 m/s Physics 114A - Lecture 2

  19. Constant Acceleration Acceleration characterizes the change in velocity with time: Dv/Dt. If the acceleration is constant, then the velocity is changing at a constant rate. Graphically, if we plot the velocity vs. time, it will fall on a straight line with a slope determined by the acceleration. Physics 114A - Lecture 2

  20. Position, Velocity, and Acceleration Velocity positive;acceleration zero. Velocity negative;acceleration negative. Velocity positive;accelerationnegative. Velocity zero;acceleration zero. Velocity positive;acceleration positive. Physics 114A - Lecture 2

  21. Before Thursday, read Walker, Chap. 2.4 to 2.7. • Homework Assignments are now on WebAssign. HW #1 is due at 11:59 PM on Thursday, January 16. So far, 92/219 students have established WebAssign accounts. • Obtain a H-iTT clicker from the University Bookstore. We will begin using them next week. Clickers should be set to Channel 01. Register your clicker, giving its 6-digit serial number and a 3-letter screen name of your choice (mine would be “JGC”), at: https://catalyst.uw.edu/webq/survey/jcramer/185653 .Note: it is not necessary to register your clicker before using it, but it is advisable to register it soon after to get credit for your class participation and accuracy to be applied and credited to you. End of Lecture 2

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