Name ____________________________ Date ____________________ Class ____________ Motion

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Guided Reading and Study

Acceleration This section describes what happens to the motion of an object as it accelerates, or changes velocity. It also explains how to calculate acceleration.

As you read the section, locate the main idea. Write the main idea in the top section of the graphic organizer. Then, as you read, look for details that give examples of the main idea. Look for examples in the text, the figures, and the captions. Record the details in the lower portion of the graphic organizer. Main Idea In science, acceleration refers to . . . Detail

Detail

Detail

What Is Acceleration? 1. What is acceleration? ____________________________________________________ ________________________________________________________________________ 2. Acceleration involves a change in either________________ or _________________. 3. Any time the speed of an object increases, the object experiences ________________________. 4. Is the following sentence true or false? Acceleration refers to increasing speed, decreasing speed, or changing direction. ________________________ 5. Deceleration is another word for negative ________________________. 6. Is the following sentence true or false? An object can be accelerating even if its speed is constant. ________________________

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Motion

Use Target Reading Skills

Name ____________________________ Date ____________________ Class ____________ Motion

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Guided Reading and Study

Acceleration (continued) 7. Circle the letter of each sentence that describes an example of acceleration. a. A car follows a gentle curve in the road. b. A batter swings a bat to hit a ball. c. A truck parked on a hill doesn’t move all day. d. A runner slows down after finishing a race. 8. The moon revolves around Earth at a fairly constant speed. Is the moon accelerating? ________________________________________________________________________ ________________________________________________________________________ 9. Use the table below to compare and contrast the meanings of acceleration. Acceleration In Everyday Language

In Scientific Language Increasing speed

Slowing down Turning

Calculating Acceleration 10. What must you calculate to determine the acceleration of an object? ________________________________________________________________________ ________________________________________________________________________ 11. What is the formula you use to determine the acceleration of an object moving in a straight line? ________________________________________________________________________ 12. Is the following sentence true or false? To calculate the acceleration of an automobile, you must first subtract the final speed from the initial speed. ________________________

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Name ____________________________ Date ____________________ Class ____________ Motion

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Guided Reading and Study

Graphing Acceleration 35

Speed (m/s)

30 25 20 15 10 5 0

1

2 3 Time (s)

4

5

14. The graph above shows the motion of an object that is accelerating. What happens to the speed of the object over time? ________________________________________________________________________ 15. The graph line is slanted and straight. What does this line show about the acceleration of the object? ________________________________________________________________________

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Motion

13. Circle the letter of each sentence that is true about calculating the acceleration of a moving object. a. If an object is moving without changing direction, then its acceleration is the change in its speed during one unit of time. b. If an object’s speed changes by the same amount during each unit of time, then the acceleration of the object at any time is the same. c. To determine the acceleration of an object, you must calculate the change in velocity during only one unit of time. d. If an object’s acceleration varies, then you can describe only average acceleration.

Name ____________________________ Date ____________________ Class ____________ Motion

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Section Summary

Acceleration Guide for Reading ■ What kind of motion does acceleration refer to? ■

How is acceleration calculated?

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What graphs can be used to analyze the motion of an accelerating object?

Acceleration is the rate at which velocity changes. Recall that velocity has two components—direction and speed. Acceleration involves a change in either of these components. In science, acceleration refers to increasing speed, decreasing speed, or changing direction. Any time the speed of an object changes, the object experiences acceleration. That change can be an increase or decrease. A decrease in speed is sometimes called deceleration, or negative acceleration. An object that is changing direction is also accelerating, even if it is moving at a constant speed. A car moving around a curve or changing lanes at a constant speed is accelerating because it is changing direction. Many objects continuously change direction without changing speed. The simplest example of this type of motion is circular motion, or motion along a circular path. The moon accelerates because it is continuously changing direction as it revolves around Earth. Acceleration describes the rate at which velocity changes. To determine the acceleration of an object moving in a straight line, you must calculate the change in speed per unit of time. This is summarized by the following formula. Final velocity!Initial velocity Acceleration = ------------------------------------------------------------------------------Time If velocity is measured in meters/second and time is measured in seconds, the unit of acceleration is meters per second per second, which is written as m/s2. You can use both a speed-versus-time graph and a distance-versustime graph to analyze the motion of an accelerating object. When a graph shows speed versus time as a slanted straight line, the acceleration is constant. You can find acceleration by calculating the slope of the line. If an object accelerates by a different amount each time period, a graph of its acceleration will not be a straight line. A graph of distance versus time for an accelerating object is curved.

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Name ____________________________ Date ____________________ Class ____________ Motion

Review and Reinforce

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Acceleration Understanding Main Ideas If the statement is true, write true. If it is false, change the underlined word or words to make the statement true. ____________________ 1. If a train is slowing down, it is accelerating. ____________________ 2. To find the acceleration of an object moving in a straight line, you must calculate the change in distance during each unit of time. ____________________ 3. A Ferris wheel turning at a constant speed of 5 m/s is not accelerating. ____________________ 4. An airplane is flying west at 200 km/h. Two hours later, it is flying west at 300 km/h. Its average acceleration is 100 km/h2. ____________________ 5. Graph A plots a race car’s speed for 5 seconds. The car’s rate of acceleration is 6 m/s2. ____________________ 6. Graph B plots the same car ’s speed for a different 5-second interval. The car ’s acceleration during this interval is 12 m/s2. Graph B

Graph A 24

21

21

18 15

18 15

Speed (m/s)

Speed (m/s)

24

12 9 6

12 9 6 3

3 0

0 0

1

3 2 Time (s)

4

0

1

2 3 Time (s)

4

Building Vocabulary From the list below, choose the term that best completes each sentence. Write your answers on the lines provided. acceleration

velocity

speed

distance

7. ________________________ occurs when the velocity of an object changes. 8. When you say that a race car travels northward at 100 km/h, you are talking about its ________________________.

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Name ____________________________ Date ____________________ Class ____________ Motion

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Enrich

Exploring Changing Directions

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Motion

When an object speeds up or slows down, it is easy to understand that it has accelerated. But when an object moves at a constant speed and changes direction, it is harder to understand why this is also called an acceleration. Look at the figure below. Suppose an object is moving in a circle at a constant speed of 2.0 cm/s. When the object is at point A, it is moving to the right and if it continued to go in a straight line, it would move to point B. Instead, it moves along the circle to point C. Examine the figure and answer the following questions on a separate sheet of paper. 1. Measure the distance in An object is moving centimeters from point A to point in a circle at 2.0 cm/s B. How far is it? 2. Measure the distance along the circle in centimeters from point A to point C. How far is it? 3. How far is the object from where it would have been if it had not been accelerated? (How far is it from B to C?) 4. Suppose the object at point A were moving at 4.0 cm/s. If it were not accelerated, where would it be after 1.0 second? 5. If the object moved along the circle at 4.0 cm/s where would it be after 1.0 second? C 6. Measure the distance in centimeters between the two A B points you identified in speed = 2.0 cm/s Questions 4 and 5. How does this compare to the distance you measured in Question 3? 7. Suppose the circle has a circumference of 25.1 cm. Compare where the object would have been if it had gone in a straight line at 4.0 cm/s for 6.28 s (no acceleration) to where it is after it has gone around the circle for 6.28 s at 4.0 cm/s.

E

D

Name ____________________________ Date ____________________ Class ____________ Motion

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Skills Lab

Stopping on a Dime Problem What is the distance needed between an out-of-bounds line and a wall so that a player can stop before hitting the wall? Skills Focus measuring, calculating, inferring Materials wooden meter stick tape measure 2 stopwatches or watches with second hands Procedure Part I Reaction Time 1. Have your partner suspend a wooden meter stick, zero end down, between your thumb and index finger. Your thumb and index finger should be about 3 cm apart. 2. Your partner will drop the meter stick without giving you any warning. You will try to grab it with your thumb and index finger. 3. Note the level at which you grabbed the meter stick and use the chart shown to determine your reaction time. Record the time in the class data table. 4. Reverse roles with your partner and repeat Steps 1 through 3.

Reaction Time Distance (cm)

Time (s)

Distance (cm)

Time (s)

15

0.175

25

0.226

16

0.181

26

0.230

17

0.186

27

0.235

18

0.192

28

0.239

19

0.197

29

0.243

20

0.202

30

0.247

21

0.207

31

0.252

22

0.212

32

0.256

23

0.217

33

0.260

24

0.221

34

0.263

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Name ____________________________ Date ____________________ Class ____________ Motion

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Skills Lab

Part II Stopping Distance

Class Data Table Student Name

Reaction Time (s)

Running Time (s)

Stopping Distance (m)

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Motion

5. On the school field or in the gymnasium, mark off a distance of 25 m. CAUTION: Be sure to remove any obstacles from the course. 6. Have your partner time how long it takes you to run the course at full speed. After you pass the 25-m mark, come to a stop as quickly as possible and remain standing. You must not slow down before the mark. 7. Have your partner measure the distance from the 25-m mark to your final position. This is the distance you need to come to a complete stop. Enter your time and distance into the class data table. 8. Reverse roles with your partner. Enter your partner’s time and distance into the class data table.

Name ____________________________ Date ____________________ Class ____________ Motion

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Skills Lab

Stopping on a Dime (continued) Analyze and Conclude Answer the following questions in the space provided. 1. Calculating Calculate the average speed of the student who ran the 25-m course the fastest. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 2. Interpreting Data Multiply the speed of the fastest student (calculated in Question 1) by the slowest reaction time listed in the class data table. Why would you be interested in this product? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 3. Interpreting Data Add the distance calculated in Question 2 to the longest stopping distance in the class data table. What does this total distance represent? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 4. Drawing Conclusions Explain why it is important to use the fastest speed, the slowest reaction time, and the longest stopping distance in your calculations. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 5. Controlling Variables What other factors should you take into account to get results that apply to a real basketball court? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________

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Name ____________________________ Date ____________________ Class ____________ Motion

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Skills Lab

6. Communicating Suppose you calculate that the distance between the out-of-bounds line and the wall in a playground or gymnasium is too short for safety. Write a proposal to the school that describes the problem. In your proposal, suggest a strategy for making the court safer. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________

More to Explore Visit a local playground and examine it from the viewpoint of safety. Use what you learned about stopping distance as one of your guidelines, but also try to identify other potentially unsafe conditions. Write a letter to the Department of Parks or to the officials of your town informing them of your findings.

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Motion

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