An object that is moving constantly in a straight line can be described as having rectilinear motion. The direction of travel typically does not change. It is often described in two dimensions but can also be in any direction relative to a reference point. Graphs with an X- and Y-axis configuration are generally used to define the rectilinear motion of an object, as well as motion, velocity, direction, or rates of change in these components of movement. They are typically done so using a straight line, but when changes in motion are compared over time, the straight path of something can be represented by a line that is curved.
There are various rectilinear motion equations, but what most have in common are that motion in the direction of a reference point is typically a positive value. If something is moving away, or in the opposite direction, then the numerical value is usually negative; it is expressed as a number below zero. Motion along the X-axis in a graph, toward the right, is usually positive. When measuring position, the path typically follows this axis, as a function of time or other variable in the Y-axis.
Rectilinear motion with constant acceleration is often how equations are derived, and position, changes in it, and velocity can be determined using pre-defined calculations. Calculus is generally used for determining functions such as velocity by using time and position values in an equation. These are generally dependent on the particle moving in a straight line, but its speed and changes in it over time can be graphed.
Time plots for objects with rectilinear motion are more complex because their speed can change at different rates from one moment to another. While motion is still in a straight line, the variations in velocity can result in a curved line on the chart. The time plots are made using the vertical Y-axis as a reference.
Something with a constant velocity typically has uniform motion. The motion is described for objects not influenced by an external force, which usually includes gravity. It can be factored into objects on Earth as well as planets moving through space. Rectilinear motion is sometimes compared between different objects, especially when one detaches from another with the same velocity. The object that separates often retains the velocity, but the acceleration cannot be defined by a horizontal line, especially if it is dropped from a vehicle being pushed by an engine.