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Physics Lecture 2.1: Motion and Definitions

This lecture discusses the concepts of motion, position, velocity, and acceleration. It covers graphs, equations, free fall, and examples including baseball and the Golden Gate Bridge. Homework and concept questions are included.

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Physics Lecture 2.1: Motion and Definitions

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  1. PHYSICS 50: Lecture 2.1 RICHARD CRAIG

  2. Plan for the day • Motion • Definitions • Position, Velocity and Acceleration • Graphs • Equations • Free Fall • Examples • Baseball • Quiz: GG Bridge • Sprinter • Concept Questions

  3. Homework #1 • Read Chapter 1(all), Chapter 2 (not 2.6) • Exercises and Problems:1.3, 1.49, 1.99, 2.14, 2.39, 2.59 • Due Thursday, 1/31

  4. Velocity (1-D) Average Velocity (1-D) Instantaneous Velocity (1-D)

  5. Acceleration (1-D) Average Acceleration (1-D) Instantaneous Acceleration (1-D)

  6. Example:Velocity Graph Where is the acceleration largest What is the acceleration between 15s and 25s?

  7. Equations of Motion:Constant Acceleration

  8. The Car and the Cop

  9. Car and the Cop • What is the acceleration of the car? • When do the car and the cop have same speed? • When does the car catch the cop? • Where does the cop catch the car? • What would the answer be if the car had a 5s head start?

  10. Equations of motion:Free Fall • Gravity near the surface of the earth acts as a constant acceleration • The value of “g” (the acceleration due to gravity) is about 9.8m/s2 • “g” points down (- if your coordinate system has up as +)

  11. Example: Throw a ball in the air If you throw a baseball straight up with a speed of 30m/s (about 65 MPH) and ignore air resistance: How high will it go? At it’s highest point what is it’s velocity? At it’s highest point what is it’s acceleration? Does it matter how heavy the ball is?

  12. Example: Sprinter Example where acceleration is not constant In a 100m dash a runner accelerates (assume constant acceleration) for the first 3.0 sec then runs at a constant speed for the next 7.0 sec to the finish What is his acceleration during the first period? What is his speed during the second period?

  13. Concept Question If you drop an object in the absence of air resistance, it accelerates downward at 9.8 m/s2. If instead you throw it downward, its downward acceleration after release is 1. less than 9.8 m/s2. 2. 9.8 m/s2. 3. more than 9.8 m/s2.

  14. The graph shows position as a function of time for two trains running on parallel tracks. Which is true? 1. At time tB, both trains have the same velocity. 2. Both trains speed up all the time. 3. Both trains have the same velocity at some time before tB. 4. Somewhere on the graph, both trains have the same acceleration.

  15. Concept Test You are throwing a ball straight up in the air. At the highest point, the ball’s 1. velocity and acceleration are zero. 2. velocity is nonzero but its acceleration is zero. 3. acceleration is nonzero, but its velocity is zero. 4. velocity and acceleration are both nonzero.

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