Conceptual Physics

2.1 - Position

Position: The location of an object; in physics,

typically specified with graph coordinates.

Position tells you location.

There are many ways to describe location: Beverly Hills 90210; “...a galaxy far, far
away”; “as far away from you as possible.” Each works in its own context.

Physicists often use numbers and graphs instead of words and phrases. Numbers and
graphs enable them (and you) to analyze motion with precision and consistency. In this
chapter, we will analyze objects that move in one dimension along a line, like a train
moving along a flat, straight section of track.

To begin, we measure position along a number line. Two toy figures and a number line
are shown in the illustrations to the right. As you can see, the zero point is called the
origin. Positive numbers are on the right and negative numbers are on the left.

By convention, we draw number lines from left to right. The number line could reflect an
object's position in east and west directions, or north and south, or up and down; the
important idea is that we can specify positions by referring to points on a line.

When an object moves in one dimension, you can specify its position by its location on
the number line. The variable x specifies that position. For example, as shown in the
illustration for Equation 1, the hiker stands at position x = 3.0 meters and the toddler is
at position x = í2.0 meters.

Later, you will study objects that move in multiple dimensions. For example, a
basketball free throw will initially travel both up and forward. For now, though, we will
consider objects that move in one dimension.

Position

Location of an object Relative to origin

x represents position

Units: meters (m)

What are the positions of the

figures?

Hiker:x = 2.0 m

Toddler:x = í3.0 m

2.2 - Displacement

Displacement: The direction and distance of the

shortest path between an initial and final

position.

You use the concept of distance every day. For example, you are told a home run
travels 400 ft (122 meters) or you run the metric mile (1.5 km) in track (or happily watch
others run a metric mile).

Displacement adds the concept of direction to distance. For example, you go
approximately 954 mi (1540 km) south when you travel from Seattle to Los Angeles;
the summit of Mount Everest is 29,035 ft (8849.9 meters) above sea level. (You may
have noticed we are using both metric and English units. We will do this only for the first

Displacement