chapter 8 lesson 1 motion and forces

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Describing PositionThink about calling a friend on his or her cell phone.

Imagine that you want to get together. One of the first questions you might ask is, “Where are you?” Your friend might answer, “I’m at the mall” or “I’m two blocks north of school” or “I’m 3 m away from you. Look down the hall.”

What do all these possible answers have in common? Each one has a reference point that helps describe your friend’s position. A reference point is the starting point you choose to describe the location, or position, of an object.

When your friend describes his or her location, he or she chooses a reference point. This reference point could be the mall, your school, or even you. Then your friend compares his or her exact location to that reference point.

Your friend also gives you other information to describe his or her location. Your friend might tell you a distance, such as 2 blocks or 3 m. Your friend might also mention a direction, such as north or down the hall.

Your friend just described his or her position. Position describes an object’s distance and direction from a reference point. Position always includes a distance, a direction, and a reference point.

What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you’ve read this lesson, reread the statements to see if you have changed your mind.

Before Statement After

1. If an object’s distance from a starting point changes, the object is in motion.

2. Speed describes how fast something is going and the direction in which it is moving.

Key Concepts • How do you describe an

object’s position?• How do you describe an

object’s motion?• How do speed and velocity

differ?• What is acceleration?

Building Vocabulary Write each vocabulary term in this lesson on an index card. Shuffle the cards. After you have studied the lesson, take turns picking cards with a partner. Each of you should define the term using your own words.

Key Concept Check1. Explain How do you describe an object’s position?

Motion and Forces

Describing Motion

LESSON 1

CHAPTER 8

Reading Essentials Motion and Forces 125

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Using Reference PointsPosition depends on a reference point. If the reference

point in the figure below is the goal, or point A, the player’s position is 10 m in front of the goal. If the reference point is center field, point B, the player’s position is 40 m toward the goal. The location of the player has not changed. Only the description of the position changed because the reference point changed.

Distance and DisplacementSee the figure below. During one play in the soccer game,

the player runs 41.2 m from position D to position C. Then she runs 10 m to position B. The dotted lines show her path. The total distance she travels is 40 m + 10 m = 51.2 m. The solid arrow that completes the triangle shows the player’s displacement. Displacement is the difference between the initial, or starting, position and the final position. The player starts at point D and finishes at point B. Her displacement is 40 m in front of her initial position. As you can see, an object’s displacement and the distance it travels are not always equal.

MotionWith 5 s left on the clock, the soccer ball is 50 m from

the goal. When the game ends, the ball is in the goal. What happened to the ball during the last 5 s of the game? The ball was in motion. Motion is the process of changing position.

SpeedA bus moves fast on the highway. In heavy traffic, it

moves slowly. Speed is the distance an object moves in a unit of time. Any unit of time, such as 1 s, 1 min, 1 h, or 1 y, may be used to calculate average speed. For example, you can say that a bus travels 15 km/h or 0.25 km/min.

Key Concept Check 3. Relate Why does the description of an object’s motion depend on a reference point?

Visual Check2. Describe How would you describe the position of point B without using the words center line?

Distance and Displacement in a Soccer Game

10 m

10 m

40 m

40 m

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D

C

126 Motion and Forces Reading Essentials


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Constant SpeedIn the figure above, the bus moves from positions 1 to 2

to 3 to 4 at the same speed of 10 m/s. When an object moves the same distance over a given unit of time, it is said to have a constant speed. The bus has a constant speed from positions 1 to 4.

Changing SpeedHow is the motion of the bus between positions 4 and 7

different from its motion at the earlier positions? The bus moves a greater distance each second. When the distance an object covers increases or decreases over a given unit of time, the object is said to be changing speed.

Average SpeedThe speed of most moving objects is not constant, which

is why the speedometer in a car is always slightly changing. When you describe your speed over an entire trip, you are usually describing average speed. Average speed is equal to the total distance traveled divided by the total time.

average speed = total distance ____________ total time

The bus above travels 80 m from second 1 to second 7 on the stopwatch. Therefore, the average speed of the bus is 80 m/6 s or 13.2 m/s.

VelocityDescribing speed is not the only way to describe motion.

You can also talk about direction. Velocity is the speed and direction of a moving object. For example, the speed of an object might be 5 m/s, while its velocity is 5 m/s to the east. The words speed and velocity have different meanings, just as distance and displacement have different meanings.

You can use arrows to show the velocity of the object. The length of an arrow represents the speed of an object. The longer the arrow is, the faster the object is moving. The head of an arrow points in the direction the object moves.

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Describing Speed

Visual Check4. Discover Between which positions does the bus move the greatest distance?

The formula for determining average speed is

average speed = distance _______ time .

For example, a bus carrying students to a soccer game traveled 10 km in 30 min. What was the average speed

of the bus in km ___ h ?

a. Change minutes to hours.30 min = 0.5 h

b. Replace the terms in the formula with the given terms.

average speed = 10 km _____ 0.5 h

c. Divide to get the answer.

10 km _____ 0.5 h = 20 km _____ h

5. Use a Formula On a hike, you travel 2,800 m in 2 h. What is your average speed?

Math Skills



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Constant VelocitySuppose a road biker rides along a flat straight road, as

shown in the first panel in the figure above. He or she moves with constant velocity. Constant velocity means that an object moves with constant speed and its direction does not change. If the biker’s velocity is constant, then the arrows that represent the biker’s velocity are all the same length and point in the same direction.

Changing VelocityHow is the motion of the bikers in the second and third

panels of the figure above different from the motion of the bikers in the first panel? In the second and third panels, the arrows that represent velocity are different from each other. In the second panel, the arrows have different lengths. In the third panel, the arrows point in different directions. Because the arrows are not identical, you know that the velocity is changing. Velocity changes when either an object’s speed or direction of motion changes.

Change in Speed Imagine that the biker with the stripe on her shorts in the second panel wants to pass the other biker. She goes faster. Each second, the rider moves a greater and greater distance. Her speed changes. Therefore, her velocity also changes.

Change in Direction The biker in the third panel rides along a turn in the road. Even though the biker’s speed is constant, her direction changes, as shown by the different angles of the arrows. Because the rider’s direction changes, her velocity changes.

Visual Check6. Reason How do you know that the bikers in the first box are moving at a constant velocity?

Reading Check7. Identify What are two ways in which the velocity of an object can change?

ACADEMIC VOCABULARYconstant(adjective) not changing

Constant Velocity Changing VelocityChanging Speed Changing Direction

0s 1s 2s 3s 4s 0s 1s 2s 3s 4s



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AccelerationImagine that you are on a roller coaster. As your roller-

coaster car goes down a hill, you move faster and faster. You feel as if you are being pushed back against the seat. Because the speed of the car increases, the velocity of the car increases. Next, your car climbs a hill. The car moves slower and slower as it climbs. Because the speed of the car decreases, the velocity of the car decreases. Suddenly the roller-coaster cars enter a curve. The velocity changes again because the direction of the car’s motion changes. When the velocity of an object changes, it accelerates. Acceleration is a measure of how quickly the velocity of an object changes.

Speeding UpAs a roller-coaster car travels downhill, it covers a greater

distance each second. The velocity increases. The roller-coaster car’s acceleration is in the same direction as its motion. This is called positive acceleration.

Slowing DownAs a roller-coaster car climbs a second hill, it moves

a shorter and shorter distance each second. The velocity decreases. This means that the roller-coaster car’s acceleration is in the opposite direction of its motion. The action of slowing down is called negative acceleration, also known as deceleration.

Changing Direction of MotionImagine the roller-coaster car entering a loop in the track.

As you travel through the loop, your feet go up and your head is pointed toward the ground. The direction of the car’s motion constantly changes. If the car is changing direction, then its velocity is changing and the car is accelerating. Because the car is accelerating, there must be unbalanced forces acting on it. The track applies an unbalanced force on the roller-coaster car by pushing it toward the center of the loop. This creates the roller-coaster car’s circular motion.

Make a three-tab book. Label the tabs, define the terms, and explain how they are related.

Reading Check 8. Contrast What is the difference between positive and negative acceleration?

Key Concept Check9. Describe three ways an object can accelerate.

Displacement

Acceleration

Velocity



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Mini Glossary

Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind?

What do you think

END OF LESSON

Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.

ConnectED

acceleration: a measure of how quickly the velocity of an

object changes

displacement: the difference between the initial, or starting,

position and the final position

motion: the process of changing position

position: an object’s distance and direction from a reference

point

reference point: the starting point you choose to describe

the location, or position, of an object

speed: the distance an object moves in a unit of time

velocity: the speed and direction of a moving object

1. Review the terms and their definitions in the Mini Glossary. Write a sentence that shows how position and reference point are related.

2. Use the words acceleration, speed, and velocity to fill in the blanks in the word map below.

3. Compare and contrast velocity and speed.

A measure of how quickly

is distance an object moves over time.

changes over time is

in a given direction is



chapter 8 lesson 1 motion and forces

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