http://www.ck12.org Chapter 4. One Dimensional Motion Version 2
4.1 The Big Idea
One dimensional motion describes objects moving in straight lines. Speed is a scalar measure of how quickly an
object is moving along this line: units of length per one unit of time. If an object’s speed changes, it is said to be
accelerating (or decelerating). As we will see, understanding an object’s acceleration is the key to understanding its
motion. At this point, we are not worried aboutwherethe acceleration is coming from — we will deal with that
question later.
In general, position, displacement, velocity and acceleration have directions and are therefore vectors. It’s important
to note, however, that in one dimension there are only two possible directions for vectors to point in, and these are
usually labeled+and−. We will therefore typically avoid calling one dimensional quantities vectors, since their
direction can be represented with a sign. The quantities labeled vectors below can and will be treated as scalars with
signs in one dimensional situations throughout the book.