The motion along a straight line is known as rectilinear motion. The important parameters required to study the motion along a straight line are position, displacement, velocity, and acceleration.

Position, displacement and distance travelled by the particle
The motion of a particle can be described if its position is known continuously with respect to time. The total length of the path is the distance travelled by the particle and the shortest distance between the initial and final position of the particle is the displacement.
The distance travelled by a particle, however, is different from its displacement from the origin. For example, if the particle moves from a point O to position P1 and then to position P2, its displacement at the position P2 is – x2 from the origin but, the distance travelled by the particle is x1+x1+x2 = (2×1+x2) (Fig below).

The distance travelled is a scalar quantity and the displacement is a vector quantity.

Speed and velocity
Speed
It is the distance travelled in unit time. It is a scalar quantity.
Velocity
The velocity of a particle is defined as the rate of change of displacement of the particle. It is also defined as the speed of the particle in a given direction. The velocity is a vector quantity. It has both magnitude and direction.
Velocity = displacement / time taken
Its unit is m s−1 and its dimensional formula is LT−1.

A particle is said to move with uniform velocity if it moves along a fixed direction and covers equal displacements in equal intervals of time, however small these intervals of time may be. In a displacement – time graph, (Fig. below) the slope is constant at all the points, when the particle moves with uniform velocity.

The velocity is variable (non-uniform), if it covers unequal displacements in equal intervals of time or if the direction of motion changes or if both the rate of motion and the direction change.

Average velocity
Let s1 be the displacement of a body in time t1 and s2 be its displacement in time t2 (Fig.below). The average velocity during the time interval (t2 – t1) is defined as
Vaverage = change in displacement / change in time
= S2 – S1 / t2 – t1 = △s/△t
From the graph, it is found that the slope of the curve varies.

Instantaneous velocity
It is the velocity at any given instant of time or at any given point of its path. The instantaneous velocity v is given by
V = Lt△t->0 △s/△t = ds/dt

Acceleration
If the magnitude or the direction or both of the velocity changes with respect to time, the particle is said to be under acceleration. Acceleration of a particle is defined as the rate of change of velocity. Acceleration is a vector quantity.
Acceleration = change in velocity/time taken
If u is the initial velocity and v, the final velocity of the particle after a time t, then the acceleration,
a = v – u /t
Its unit is m s−2 and its dimensional formula is LT−2. The instantaneous acceleration is, a= dv/dt = d/dt =(ds/dt)=d2s/dt2

Uniform acceleration
If the velocity changes by an equal amount in equal intervals of time, however small these intervals of time may be, the acceleration is said to be uniform.

Retardation or deceleration
If the velocity decreases with time, the acceleration is negative. The negative acceleration is called retardation or deceleration.

Uniform motion
A particle is in uniform motion when it moves with constant velocity (i.e) zero acceleration.

Graphical representations
The graphs provide a convenient method to present pictorially, the basic information’s about a variety of events. Line graphs are used to show the relation of one quantity say displacement or velocity with another quantity such as time. If the displacement, velocity and acceleration of a particle are plotted with respect to time, they are known as,
● displacement – time graph (s – t graph)
● velocity – time graph (v – t graph)
● acceleration – time graph (a – t graph)

Displacement – time graph
When the displacement of the particle is plotted as a function of time, it is displacement – time graph. As v = ds/dt, the slope of the s – t graph at any instant gives the velocity of the particle at that instant. In Fig. below the particle at time t1, has a positive velocity, at time t2, has zero velocity and at time t3, has negative velocity.

Velocity – time graph
When the velocity of the particle is plotted as a function of time, it is velocity-time graph.
As a = dv/dt, the slope of the v – t curve at any instant gives the acceleration of the particle (Fig. below).

If the displacements are s1 and s2 in times t1 and t2, then

The area under the v – t curve, between the given intervals of time, gives the change in displacement or the distance travelled by the particle during the same interval.

Acceleration – time graph
When the acceleration is plotted as a function of time, it is acceleration – time graph (Fig. below).

If the velocities are v1 and v2 at times t1 and t2 respectively, then

The area under the a – t curve, between the given intervals of time, gives the change in velocity of the particle during the same interval. If the graph is parallel to the time axis, the body moves with constant acceleration.

Equations of motion
For uniformly accelerated motion, some simple equations that relate displacement s, time t, initial velocity u, final velocity v and acceleration a are obtained. As acceleration of the body at any instant is given by the first derivative of the velocity with respect to time,
a = dv/dt (or) dv = a.dt
If the velocity of the body changes from u to v in time t then from the above equation,

Expression for the distance travelled in nth second
Let a body move with an initial velocity u and travel along a straight line with uniform acceleration a.
Distance travelled in the nth second of motion is,
sn = distance travelled during first n seconds – distance travelled during (n –1) seconds
Distance travelled during n seconds
Dn = un + 1/2an2
Distance travelled during (n -1) seconds

Special Cases
Case (i) : For downward motion
For a particle moving downwards, a = g, since the particle moves in the direction of gravity.
Case (ii) : For a freely falling body
For a freely falling body, a = g and u = 0, since it starts from rest.
Case (iii) : For upward motion
For a particle moving upwards, a = − g, since the particle moves against the gravity.