Explain frame of reference.

(N/A) In order to specify the position of an object,we need a reference point and a set of axes. It is convenient to choose a rectangular coordinate system consisting of three mutually perpendicular axes,labelled $X$,$Y$,and $Z$-axes.
The point of intersection of these three axes is called the origin $(O)$ and serves as the reference point.
The coordinates $(x, y, z)$ of an object describe its position with respect to this coordinate system.
To measure time,we position a clock in this system.
If one or more coordinates of an object change with time,we say that the object is in motion. Otherwise,the object is said to be at rest with respect to this frame of reference.
The choice of a set of axes in a frame of reference depends upon the situation. For example,for describing motion in one dimension,we need only one axis. To describe motion in two or three dimensions,we need a set of two or three axes.
To describe motion along a straight line,we can choose an axis,say the $X$-axis,so that it coincides with the path of the object.
We then measure the position of the object with reference to a conveniently chosen origin,say $O$,as shown in the figure. Positions to the right of $O$ are taken as positive and to the left of $O$ as negative.
The position coordinates of points $P$ and $Q$ in the figure are $+360 \ m$ and $+240 \ m$. Similarly,the position coordinate of point $R$ is $-120 \ m$.