There is no such thing as inertia of direction, but there is something called inertia.
If any physical body resists a change in its status of either motion or rest, or it tends to object to resist any changes in motion, then this is referred to as inertia.
Inertia is proportional to the mass of an object, and the principles are fundamental to classical physics, which is what is used to describe motion of matter and how it is affected by forces that are applied.
Inertia was the first law that Isaac Newton defined in his Philosophice Naturalis Principia Mathematica', and is often the term used as a shorthand description when talking about the principle of inertia that he outlined in the 'First Law of Motion', which explains how an object will continue to move at its current velocity until something makes it change its speed, its direction, or both.
Inertia on the Earth's surface is often somewhat hidden by the effects of gravity and friction, which are both forces that tend to slow down moving objects, usually to the point of rest. Some classical theorists were completely misled by this because it led them to believe that things would only move if there was some force applied to them.
- Problems with Aristotle's theory
By the 16th century, scientists were finding problems with Aristotle's thinking behind inertia, particularly after Nicolaus Copernicus argued that the earth (and absolutely everything on it) was never still, and in fact was constantly moving around the sun.
Copernicus' thinking was further backed up by Galileo. Galileo also went on to say that it is impossible to ascertain whether something is moving unless there is a point of reference to compare it with.
