Section 3 Acceleration Review velocity Scan Use the checklist below to preview Section 3 of your book. Read all section titles. Read all boldfaced words. Read all graphs and equations. Look at all the pictures and read their captions. Define velocity in a sentence to show its scientific meaning. Student responses will vary. Sentences should include speed and direction. New acceleration centripetal acceleration Academic constant Use your book to define the terms below. rate of change in velocity acceleration of an object toward the center of a curved or circular path Analyze why we say an object is accelerating, when we usually mean that it is speeding up. An object that is slowing down also is accelerating. In the everyday sense, acceleration implies increasing in speed. In the scientific sense, it describes changing velocity increasing or decreasing speed, or changing direction. Use a dictionary to define constant to its scientific meaning. not varying or changing over time; a quantity that does not vary 24
Section 3 Acceleration (continued) Velocity and Acceleration SE, pp. 57 58 RE, p. 36 Synthesize Create a graph titled Speed Changing Over Time to show the acceleration of the car traveling around your course (above). Place the labels A, B, C, and D along the horizontal axis to represent the time when the car travels each part of the course. Draw a line on the graph to show how the speed of the car changes with time. Label each of the four parts of the graph with either a plus sign, a minus sign, or a zero to indicate where the car s acceleration is positive, negative, or zero. Speed Changing Over Time Accept all reasonable responses. Speed Changing Over Time Speed 0 0 in Two Dimensions SE, pp. 58 60 RE, pp. 36 37 1 2 3 4 5 6 7 8 9 10min Time Describe the relationship between speed and acceleration as shown in your graph. Acceleration is positive when speed is increasing, negative when speed is decreasing, and zero when speed is constant. Compare the results of applying the acceleration equation in the following two cases: (1) an object that goes from 0 to 10 m/s in 4 s, and (2) then goes from 10 m/s to 30 m/s in 8 s. (1) a (v f v i )/t (10 m/s 0 m/s)/4 s 2.5 m/s 2 (2) a (v f v i )/t (30 m/s 10 m/s)/8 s 2.5 m/s 2 The results are the same. Calculations show that if an object went from 0 to 10 m/s in 4 s and continued the same acceleration, it would go from 10 m/s to 30 m/s in another 8 s. 25
Section 3 Acceleration (continued) Analyze why the SI unit of acceleration is m/s 2. Acceleration is final velocity minus initial velocity, divided by time. The difference of two velocities, with SI units m/s, also must have units m/s. If this difference is divided by time, with SI unit s, the result has the units m/s/s, or m/s 2. Projectile SE, p. 59 RE, p. 38 Model a ball thrown horizontally. Sketch the path of the ball and draw arrows showing its horizontal and vertical velocity at three points along the path. Vary the length of your arrows to show the magnitude of the velocities. Circular SE, p. 59 RE, p. 38 SYNTHESIZE IT Create a top view of an object Velocity moving in a circle at constant speed, such as a ball on a string. Show at least two positions of the object. At each position, draw an arrow for the object s velocity and another arrow for the centripetal acceleration of the object. Centripetal acceleration Distinguish between average acceleration and instantaneous acceleration. Be sure to explain how the acceleration equation calculates average acceleration, instantaneous acceleration, or both. Average acceleration describes the acceleration of motion when acceleration is changing. Instantaneous velocity is the total change in velocity divided by the total time of travel. Velocity Instantaneous acceleration describes the acceleration of motion at a given point in time. The acceleration equation defines average acceleration, because it is the change in velocity, or v f v i, divided by the time traveled. 26
Tie It Together Use this activity with page 25. Draw a closed racecourse with parts labeled A, B, C, and D, where the following occurs: [Hint: the path can cross itself once.] A. The car is speeding up while traveling in a straight line. B. The car is curving left at constant speed. C. The car is traveling in a straight line at a constant speed. D. The car is curving right while slowing down. B C A D Accept all reasonable responses. A figure-8 track is the simplest answer but not the only possible one. Use your book to place the terms distance, displacement, speed, and velocity in the table below. Does not involve Involves direction direction Units do not involve time distance displacement Units involve time speed velocity 27
Chapter Wrap-Up Now that you have read the chapter, think about what you have learned and complete the table below. 1. Write an A if you agree with the statement. 2. Write a D if you disagree with the statement. After You Read Distance and displacement are the same thing. D SE p. 45 RE p. 27 Velocity and speed are the same thing. D SE p. 50 RE p. 30 Whenever an object accelerates, its speed or A SE p. 53 direction changes. RE p. 34 Compare your previous answers to these. Review Use this checklist to help you study. Review the information you included in your Foldable. Study your Science Notebook on this chapter. Study the definitions of vocabulary words. Review daily homework assignments. Re-read the chapter and review the charts, graphs, and illustrations. Review the Self Check at the end of each section. Look over the Chapter Review at the end of the chapter. SUMMARIZE IT After reading this chapter, list three things you have learned about motion. Accept all reasonable responses. 28