# Concise Selina Solutions for Class 9 Physics Chapter 2 Motion in One Dimension

A body is said to be at rest if the position for its surrounding does not change. In contrast, a body is said to be in motion if its position changes for its surroundings. When a body moves along a straight-line path, its motion is said to be one-dimensional motion. One dimensional motion describes objects moving in straight lines. If an object's velocity changes with time, the object is said to be accelerating. When restricted to one dimension, there are only two possible directions for the velocity and acceleration vectors to point in. By one dimension, one means motion along a line, or in one particular direction. Think of a car going down a straight road, or a person running on a straight track. You could also think of an object being thrown up vertically in the air and watching it fall. Each position on the straight line represents the position of the object at a different time, after having studied about the various aspects of measurements and experimentation in the previous chapter which sheds light on physical quantities, their corresponding units, symbols, instruments used to measure various dimensions. The chapter discusses the different examples of scalar and vector quantities, distance, displacement and velocity, acceleration, retardation, different graphs, etc.