Question

define motion in a straight line

Answer 1

The straight line motion can be of two types: uniform linear motion with constant velocity or zero acceleration; non uniform linear motion with variable velocity or non-zero acceleration. The motion of a particle (a point-like object) along a line can be described by its position , which varies with (time).

Answer 2

Continuously increasing displacement from a point of origin on a line parallel to one axis and perpendicular to other is motion in a straight line.