For rectilinear motion the relationship between displacement, s, velocity, v, acceleration, a, and time, t, s = s0 + v0t + (1/2)a t2 is valid:

The rectilinear motion is given as:

s = s0 + v0 t + (1/2) a t^2

We can see that the variables stand for:

s = final distance

s0 = initial distance

v0 = initial velocity

t = time

a = acceleration

We can see that all these variable are always constant EXCEPT acceleration. The rectilinear motion can only be applied if and only if acceleration is CONSTANT.