Rectilinear motion involves motion along straight line and thus is described usually in one dimension. Further, rectilinear motion involves only one way of changing direction i.e. the particle under motion can only reverse motion. The particle can move either in positive xdirection or in negative xdirection. There is no other possible direction valid in rectilinear motion.
This attribute of rectilinear motion allows us to do away with the need to use vector notation and vector algebra for quantities with directional attributes like position vector, displacement and velocity. Instead, the vectors are treated simply as scalars with one qualification that vectors in the direction of chosen reference is considered positive and vectors in the opposite direction to the chosen reference is considered negative.
The most important aspect of the sign convention is that a vector like velocity can be expressed by a scalar value say, 5 m/s. Though stated without any aid for specifying direction like using unit vector, the direction of the velocity is indicated, which is in the positive xdirection. If the velocity of motion is 5 m/s, then the velocity is in the direction opposite to the direction of reference.
To illustrate the construct, let us consider a motion of a ball which transverses from O to A to B to C to O along xaxis as shown in the figure.
The velocities at various points of motion in m/s (vector form) are :
v
O
=
2
i
;
v
A
=
3
i
;
v
B
=

4
i
;
v
C
=
3
i
v
O
=
2
i
;
v
A
=
3
i
;
v
B
=

4
i
;
v
C
=
3
i
Going by the scalar construct, we can altogether drop use of unit vector like "i" in describing all vector quantities used to describe motion in one dimension. The velocities at various points of motion in m/s (in equivalent scalar form) can be simply stated in scalar values for rectilinear motion as :
v
O
=
2
;
v
A
=
3
;
v
B
=

4
;
v
C
=
3
v
O
=
2
;
v
A
=
3
;
v
B
=

4
;
v
C
=
3
Similarly, we can represent position vector simply by the component in one direction, say x, in meters, as :
x
O
=
0
;
x
A
=
5
;
x
B
=
10
;
x
C
=

5
x
O
=
0
;
x
A
=
5
;
x
B
=
10
;
x
C
=

5
Also, the displacement vector (in meters) is represented with scalar symbol and value as :
OA
=
5
;
OB
=
10
;
OC
=
5
OA
=
5
;
OB
=
10
;
OC
=
5
Following the same convention, we can proceed to write defining equations of speed and velocity in rectilinear motion as :

v

=

đ
x
đ
t


v

=

đ
x
đ
t

(2)
and
v
=
đ
x
đ
t
v
=
đ
x
đ
t
(3)