LESSON 1

Forces and Motion

Position and Motion

Key Concept
• How can you describe
What do you think? Read the two statements below and decide the position and motion
whether you agree or disagree with them. Place an A in the Before of a train outside the
column if you agree with the statement or a D if you disagree. After window?
you’ve read this lesson, reread the statements to see if you have
changed your mind.
Before Statement After
Displacement is the distance an object moves
along a path.
The description of an object’s position depends
on the reference point.

Mark the Text

Where are you right now? Create an Outline As you

read, make an outline to
How would you describe where you are right now? You might summarize the information
say that you are sitting one meter to the left of your friend. You in the lesson. Use the main
might explain that you are at home, which is two houses north headings in your outline.
Use your outline to review
of your school. the lesson.
Describing Position What do these descriptions have in
common? Each states your location relative to a certain point.
This point is called the reference point. A reference point is the
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starting point you choose to describe the location, or position, of

an object. The reference point in the first example is your
friend. In the second example, it is your school. Each description
of your location also includes your distance and direction from Academic Vocabulary
the reference point. Describing your location in this way defines specify
your position. In the first example, the distance is one meter. (verb) to indicate or
The direction is to the left, and the reference point is your identify
friend. In the second example, the distance is two houses. The
direction is north, and the reference point is your school. A
position is an object’s distance in a certain direction from a
reference point. A complete description of your position
includes a distance, a direction, and a reference point.

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 Scientific Vocabulary The Reference Direction Sometimes direction is described
reference direction using the words positive or negative. The reference direction is
(noun) a direction that the positive (+) direction. The opposite direction is the negative (-)
you choose from a starting
direction. Suppose east is the reference direction in the diagram
point to describe an
object’s position below. The museum’s entrance is 80 m east of a bus stop. The
library is 40 m west of the bus stop. You could say that the
museum is +80 m from the bus stop and the library is -40 m from
the bus stop. Using the words positive or negative to describe
direction can help explain changes in an object’s position.

East

Library Bus stop 20 m Museum

Scientific Vocabulary Moving in Two Dimensions Sometimes you need to use more
dimension than one reference direction to describe an object’s position.
(noun) a measurement of When you describe position using two directions, you are using
something, such as height two dimensions.
or width
Describing Position in Two Dimensions To describe a
position on a map, you might choose north and east or south
and west as the reference directions. Sometimes north, south,
Academic Vocabulary east, and west are not the most useful reference directions.
imagine Imagine that you are looking at a skyscraper. You might describe
(verb) to form ideas in a certain window as “up” and “to the left.”
your mind
Suppose you want to locate your classmate’s home on a map.

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To find a position in two dimensions, first choose a reference
point. You could choose your home as a reference point. Next,
give specific reference directions. For example, south and east.
Then, determine the distance along each reference direction. For
example, your classmate’s home might be one block south and
four blocks east of your home.

What is motion?
Sometimes you need to describe how an object’s position
changes. Suppose a boat is floating on a lake.

Motion How do you know whether the boat has moved

throughout the day? You know this when its position changes
relative to, or compared to, something else. Motion is the
process of changing position.
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 Observing Motion Is the man in the figure below in motion?
Use the fishing pole as the reference point. The positions of the Scientific Vocabulary
man and the pole do not change relative to each other. The man relative
does not move relative to the pole. When the buoy is the (adjective) compared to
reference point, the man’s distance from the buoy changes. The something or someone
else
man is in motion relative to the buoy.

Motion Using Reference Points How would you describe the

position of the soccer player in the figure? If the reference point 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 position is
40 m toward the goal. The actual location of the player does not
change. The description of the position changed because the
reference point changed.
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C 10 m
41.2 m
B
A 40 m
10 m D

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 Distance and Displacement Suppose a baseball player runs
the bases. Distance is the length of the path the player runs.
Suppose he runs 25 m to get to first base. When he gets to
second base, he has run 25 m + 25 m = 50 m. Displacement is
the difference between the initial (first) position and the final
position of an object. The initial position is home plate. At first
base, the player’s distance and displacement are the same. He
has run a distance of 25 m and he is 25 m from home plate,
where he started. At second base, distance and displacement are
different. The player has run a distance of 50 m, but his
displacement is 35 m from home plate. Once the player has runs
all the bases and is back at home plate, he has run 100 m
(25 m × 4), but his displacement is 0 m. His starting position
and ending position are the same—home plate. Distance
depends on the path taken. Only the starting and ending
positions matter in displacement. Notice that distance and
displacement are equal only if the motion is in one direction.

What do you measure to determine

motion?
How fast do you walk when you are hungry and you smell
good food in the kitchen? How fast do you move when you have
a difficult chore to do? Sometimes you move quickly. Other
times you might move slowly.

Changes Over Time One way you can describe how fast you
move is to determine your speed. Speed is the measure of the
distance an object travels per unit of time.

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 Constant and Changing Speed Speed is either constant or
changing. Look at the figure at below. The stopwatches show the
girl’s motion every second for 6 seconds. In the first 4 seconds,
she travels the same distance during each second. This means
that she was moving with constant, or unchanging, speed.
When the girl starts running, the distance she travels each
second gets larger and larger. The girl’s speed changes.

Average Speed Suppose you want to know how fast you

traveled from the park to the library. As you moved, your speed
changed from second to second. Therefore, in order to describe
your speed, you describe the average speed of the entire trip.
Average speed is a ratio. It is the distance an object moves
divided by the time it takes for the object to move that distance.
If you traveled the 1-km distance to the library in 15 min, or
0.25 h, your average speed was 1 km/0.25 h, or 4 km/h.
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Speed and Direction When you describe your motion to a

friend, you might say how fast you are traveling. You are
describing your speed. You could give your friend a better
description of your motion if you also state the direction in
which you are moving. Velocity is the speed and direction of an
object’s motion. Velocity is a vector. A vector is a quantity that
has magnitude and direction. Arrows often represent vectors.
The length of the arrow represents the speed of an object. The
arrow points in the direction in which the object is moving.

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 Constant and Changing Velocity Velocity is constant, or
does not change, when an object’s speed and direction of
Scientific Vocabulary movement do not change. Look at arrows in the left box in the
segment figure below. Each segment of the arrow shows the distance and
(noun) a part of the direction the cyclists move in a given unit of time. Each
something segment is the same length. This means the cyclists are moving
the same distance and in the same direction during each unit of
time. Both speed and direction of movement are constant, so the
cyclists are moving at a constant velocity. The arrows in the
middle box get larger each second. The speed is changing, so
the velocity is also changing. Velocity can change even if the
speed of an object remains constant. Recall that velocity
includes an object’s speed and the direction it is traveling.

Constant Velocity Changing Velocity

Changing Speed Changing Direction

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

How can a graph help you understand

an object’s motion?
In the study of motion, the two measurements that are
compared are time and distance.

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Distance-Time Graphs Graphs that show comparisons
between time and distance are called distance-time graphs. The
change in distance is the same each second on the graph below,
which represents an object moving at constant speed. Constant
speed is shown as a straight line on distance-time graphs.
Distance-Time
Graph
60
Distance (m)

50
40
30
20
10
0 1 2 3 4 5 6
Time (s)

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 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?
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