2 Representing Motion4 How Fast?

MAIN IDEAWrite the Main Idea for this section.

Recall and write the definition of the Review Vocabulary term.

absolute value

Use your book to answer the following questions.

1. What is the difference between average velocity and instantaneous velocity?

2. Define the term average speed.

REVIEW VOCABULARY

absolute value

NEW VOCABULARY

average velocity

average speed

instantaneous velocity

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Student Edition, pp. 46–

Velocity and Speed

Summarize the information about velocity and speed using a few sentences.

GET IT? Explain how average velocity is different from velocity.

Identify what the lengths of velocity vectors mean.

Complete the table showing how general variables compare to specific motion variables.

General Variable Specific Motion

Variable

y

m

x

b

Student Edition, pp. 46–50

Velocity and Speed

Equation of Motion

4 How Fast? (continued)

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GET IT? Explain the meaning of a position-time graph slope that isupward or downward, and above or below the -axis.

Student Edition, p. 50

Use with Example Problem 4

Use this column for

scratch work and

sketches.

4 How Fast? (continued)

TRY IT!

ProblemPosition The figure shows a motorcyclist traveling east along a straight road. After passing point B, the cyclist continues to travel at an average velocity of 18 m/s east and arrives at point C 2.8 s later. What is the position of point C?

A B Cxi = 46 m east

x = ?

1. ANALYZE AND SKETCH THE PROBLEM

Choose a coordinate system with the origin at A.

KNOWNS UNKNOWN

− v  =

x i  =

t =

= ?

2. SOLVE FOR THE UNKNOWN

Use the equation for position.

3. EVALUATE THE ANSWER

• Are the units correct?

• Does the direction make sense?

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SUMMARIZE

How does the MAINIDEA for this section relate to the chapter’s BIGIDEA?

REVIEW IT!

37. MAINIDEA How is an object’s velocity related to its position?

For problems 38–40, refer to Figure 25.

38. Ranking Task Rank the position-time graphs according to the average speed, from greatest average speed to least average speed. Specifically indicate any ties.

39. Contrast Average Velocities Describe differences in the average velocities shown on the graph for objects A and B. Describe differences in the average velocities shown on the graph for objects C and D.

40. Ranking Task Rank the graphs in Figure 25 according to each object’s initial position, from most positive position to most negative position. Specifically indicate any ties. Would your ranking be different if you ranked according to initial distance from the origin?

4 How Fast? (continued)

Time (s)

C

DB

A

Posi

tion

(m)

Figure 25

Position v. Time

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41. Average Speed and Average Velocity Explain how average speed and average velocity are related to each other for an object in uniform motion.

42. Position Two cars are traveling along a straight road, as shown in Figure 26. They pass each other at point B and then continue in opposite directions. The red car travels for 0.25 h from point B to point C at a constant velocity of 32 km/h east. The blue car travels for 0.25 h from point B to point D at a constant velocity of 48 km/h west. How far has each car traveled from point B? What is the position of each car relative to the origin, point A?

43. Position A car travels north along a straight highway at an average speed of 85 km/h. After driving 2.0 km, the car passes a gas station and continues along the highway. What is the car’s position relative to the start of its trip 0.25 h after it passes the gas station?

44. Critical Thinking In solving a physics problem, why is it important to create pictorial and physical models before trying to solve an equation?

4 How Fast? (continued)

D A Origin B C

xi = 6.0 km east

Figure 26

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