Advanced mechanics of materials/Motion and displacement

Definition
Displacement, also known as distance, is the amount of space that exists between an object and an observer. As the object moves away from the observer, it increases the amount of space between it. When the object does this, it increases displacement, or displaces more of the space between it and the observer. As the object moves towards the observer, it decreases the amount of space between it. When the object does this, it decreases displacement, or displaces less of the space between it and the observer.

Number line displacement
In math, this concept of displacement is sometimes represented by a number line, with the number zero often thought of as the starting point or initial observer. This is because the zero is the most widely used system by which displacement is measured on the number line, due to the fact that it is most easily calculated by determining the number upon which the object is located, because zero subtracted from any number is always that number.

For example: