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Physics 114A - Mechanics Lecture 3 (Walker: 2.4-5) Velocity & Acceleration January 9, 2014

Physics 114A - Mechanics Lecture 3 (Walker: 2.4-5) Velocity & Acceleration January 9, 2014. John G. Cramer Professor Emeritus, Department of Physics B451 PAB jcramer@uw.edu. Announcements.

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Physics 114A - Mechanics Lecture 3 (Walker: 2.4-5) Velocity & Acceleration January 9, 2014

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  1. Physics 114A - MechanicsLecture 3 (Walker: 2.4-5)Velocity & AccelerationJanuary 9, 2014 John G. Cramer Professor Emeritus, Department of Physics B451 PAB jcramer@uw.edu

  2. Announcements • Homework Assignments #1 is now available on WebAssign and is due at 11:59 PM on Thursday, January 16. Assignment #2 is also posted and is due at 11:59 PM on Thursday, January 23. So far 153/219 students have established WebAssign accounts. • Obtain a H-iTT clicker from the University Bookstore. We will begin using them for credit next Monday. RF Clickers should be set to Channel 01. Register your clicker using the “Clicker” link on the Physics 114A Syllabus page. So far 86/219 clickers have been registered. • 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 3

  3. Lecture Schedule (Part 1) We are here. Physics 114A - Lecture 3

  4. Velocity & Acceleration

  5. Graphical Interpretation of Average & Instantaneous Velocity Physics 114A - Lecture 3

  6. 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 3

  7. 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 3

  8. Acceleration Average acceleration: Instantaneous acceleration: Physics 114A - Lecture 3

  9. Position, Velocity, & Acceleration Velocity positive;acceleration zero. Velocity negative;acceleration negative. Velocity positive;accelerationnegative. Velocity zero;acceleration zero. Velocity positive;acceleration positive. Physics 114A - Lecture 3

  10. Acceleration Average acceleration: Eqn. (2-5) Physics 114A - Lecture 3

  11. Acceleration Graphical Interpretation of Average and Instantaneous Acceleration: Physics 114A - Lecture 3

  12. Example: An Accelerating Train A train moving in a straight line with an initial velocity of 0.50 m/s accelerates at 2.0 m/s2 for 2.0 s, coasts with zero acceleration for 3.0 s, and then accelerates at -1.5 m/s2 for 1.0 s. (a) What is the final velocity vf of the train? (b) What is the average acceleration aavof the train? Physics 114A - Lecture 3

  13. Acceleration vs. Deceleration Acceleration (increasing speed) and deceleration (decreasing speed) should not be confused with the directions of velocity and acceleration: Decelerating Accelerating Accelerating Decelerating Physics 114A - Lecture 3

  14. ChangingAcceleration ConstantAcceleration Motion with Constant Acceleration If the acceleration is constant, the velocity changes linearly: (2-7) Slope Constant Slope Changing Physics 114A - Lecture 3

  15. Motion with Constant Acceleration Velocity vs. time: (2-7) Average velocity: (2-9) Position as a function of time: (2-10) (2-11) Velocity as a function of position: (2-12) Physics 114A - Lecture 3

  16. Motion with Constant Acceleration The relationship between position and time follows a characteristic curve. Parabola Physics 114A - Lecture 3

  17. Motion with Constant Acceleration Physics 114A - Lecture 3

  18. Example: Hit the Brakes! A park ranger driving at 11.4 m/s in back country suddenly sees a deer “frozen” in the headlights. He applies the brakes and slows with an acceleration of -3.80 m/s2. (a) If the deer is 20.0 m from the ranger’s car when the brakesare applied, how close doesthe ranger come to hittingthe deer? (b) What is the stoppingtime? Physics 114A - Lecture 3

  19. Before Friday, read Walker Chapter 2.6 and 2.7. • Register for WebAssign to do homework (so far, 177/220). Homework Assignments #1 is now available on WebAssign. Assignment #1 is due at 11:59 PM on Thursday, January 11. Assignment #2 is also posted and is due at 11:59 PM on Thursday, January 18. • Obtain a H-iTT clicker from the University Bookstore. We will begin using them for credit next Monday. RF Clickers should be set to Channel 01. Register your clicker using the “Clicker” link on the Physics 114A Syllabus page (so far 86/219 students are registered). We will test clickers now. End of Lecture 3

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