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CHAPTER 2

Motion Along a Straight Line

2-1POSITION, DISPLACEMENT, AND AVERAGE VELOCITY

After reading this module, you should be able to ...

2.01Identify that if all parts of an object move in the same di-
rection and at the same rate, we can treat the object as if it
were a (point-like) particle. (This chapter is about the mo-
tion of such objects.)
2.02Identify that the position of a particle is its location as
read on a scaled axis, such as an xaxis.
2.03Apply the relationship between a particle’s
displacement and its initial and final positions.

2.04Apply the relationship between a particle’s average

velocity, its displacement, and the time interval for that

displacement.

2.05Apply the relationship between a particle’s average

speed, the total distance it moves, and the time interval for

the motion.

2.06Given a graph of a particle’s position versus time,

determine the average velocity between any two particular

times.

●The position xof a particle on an xaxis locates the particle
with respect to the origin, or zero point, of the axis.

●The position is either positive or negative, according
to which side of the origin the particle is on, or zero if
the particle is at the origin. The positive direction on
an axis is the direction of increasing positive numbers;
the opposite direction is the negative direction on
the axis.

●The displacement xof a particle is the change in its
position:

●Displacement is a vector quantity. It is positive if the
particle has moved in the positive direction of the xaxis
and negative if the particle has moved in the negative
direction.

xx 2 x 1.

●When a particle has moved from position x 1 to position x 2

during a time interval tt 2 t 1 , its average velocity during

that interval is

●The algebraic sign of vavgindicates the direction of motion

(vavgis a vector quantity). Average velocity does not depend

on the actual distance a particle moves, but instead depends

on its original and final positions.

●On a graph of xversust, the average velocity for a time in-

tervaltis the slope of the straight line connecting the points

on the curve that represent the two ends of the interval.

●The average speed savgof a particle during a time interval t

depends on the total distance the particle moves in that time

interval:

savg

total distance

t

vavg

x

t



x 2 x 1

t 2 t 1

What Is Physics?

One purpose of physics is to study the motion of objects—how fast they move, for
example, and how far they move in a given amount of time. NASCAR engineers
are fanatical about this aspect of physics as they determine the performance of
their cars before and during a race. Geologists use this physics to measure
tectonic-plate motion as they attempt to predict earthquakes. Medical
researchers need this physics to map the blood flow through a patient when
diagnosing a partially closed artery, and motorists use it to determine how they
might slow sufficiently when their radar detector sounds a warning. There are
countless other examples. In this chapter, we study the basic physics of motion
where the object (race car, tectonic plate, blood cell, or any other object) moves
along a single axis. Such motion is called one-dimensional motion.

Key Ideas

Learning Objectives

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