CHAPTER 8 LESSON 1 Motion and Forces - Leon County Schools

Motion and Forces

CHAPTER 8 LESSON 1

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Describing Motion

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?

Describing Position

Think 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.

Reading Essentials

3TUDY#OACH

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 Check 1. Explain How do you describe an object's position?

Motion and Forces 125

Visual Check

2. Describe How would

you describe the position of point B without using the words center line?

Using Reference Points

Position 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 Displacement

See 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.

Distance and Displacement in a Soccer Game

A 10 m D

40 m 40 m

C 10 m B

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Key Concept Check

3. Relate Why does the

description of an object's motion depend on a reference point?

Motion

With 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.

Speed

A 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.

126 Motion and Forces

Reading Essentials

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Describing Speed

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Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

0 m

10 m

20 m

30 m

40 m

50 m

Constant Speed

In 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 Speed

How 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 Speed

The 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

=

_to_t_a_l_d_i_s_ta_n__ce_ 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.

Velocity

Describing 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.

60 m

70 m

80 m

Visual Check

4. Discover Between

which positions does the bus move the greatest distance?

Math Skills

The formula for determining average speed is

average speed = _d_ist_itm_an_e_ce_.

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 _kh_m_?

a. Change minutes to hours.

30 min = 0.5 h

b. Replace the terms in the

formula with the given terms.

average

speed

=

_1_0_k_m_ 0.5 h

c. Divide to get the answer.

_1_0_k_m_ 0.5 h

=

_2_0_k_m_ h

5. Use a Formula On a

hike, you travel 2,800 m in 2 h. What is your average speed?

Reading Essentials

Motion and Forces 127

Constant Velocity

Changing Velocity

Changing Speed

Changing Direction

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

4s

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Visual Check 6. Reason How do you know that the bikers in the first box are moving at a constant velocity?

ACADEMIC VOCABULARY

constant (adjective) not changing

Reading Check 7. Identify What are two ways in which the velocity of an object can change?

Constant Velocity

Suppose 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 Velocity

How 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.

128 Motion and Forces

Reading Essentials

Acceleration

Imagine that you are on a roller coaster. As your rollercoaster 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 Up

As 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 Down

As 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 Motion

Imagine 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.

Displacement Velocity Acceleration

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

Key Concept Check 9. Describe three ways an object can accelerate.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Reading Essentials

Motion and Forces 129

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