Motion along Straight Line

Motion along Straight Line:

 If an object changes its position with respect to its surroundings with time, then it is called in motion. It is a change in the position of an object over time. Motion in a straight line is nothing but linear motion.


According to figure 1 given above, if an object moves from point O to point A then the total distance travelled by the object is given as 60 km.


Uniform Motion:

Definition: The object covers equal distances in equal intervals of time, it is said to be in uniform motion.


Consider an object moving along a straight line. Let it travel 5 m in the first second, 5 m more in the next second, 5 m in the third second and 5 m in the fourth second. In this case, the object covers 5 m in each second.

Non uniform Motion:

 Definition: The object cover unequal distances in equal intervals of time called non uniform motion

Example: when a car is moving on a crowded street or a person is jogging in a park.


Measuring the Rate of Motion

Measuring the rate of motion of an object is to find out the distance travelled by the object in unit time. This quantity is referred to as speed.

The SI unit of speed is metre per second. This is represented by the symbol m s–1 or m/s. The other units of speed include centimetre per second (cm s–1) and kilometre per hour (km h–1).

 Average Speed:

 The average speed of an object is obtained by dividing the total distance travelled by the total time taken. That is,

Average speed = Total distance travelled/Total time taken

An example- A car travels a distance of 100 km in 2 h. Its average speed is 50 km h–1. The car might not have travelled at 50 km h–1 all the time.

Speed with direction:


Velocity is the speed of an object moving in a definite direction. The velocity of an object can be uniform or variable.

It can be changed by changing the object’s speed, direction of motion or both.

Average Velocity:

 When an object is moving along a straight line at a variable speed, we can express the magnitude of its rate of motion in terms of average velocity

«  Average Velocity (in case of uniform motion)-

Average Velocity = (Initial Velocity + Final Velocity)/2

«  Average Velocity (in case of non-uniform motion)-

Average Velocity = Total Displacement / Total Time taken

«  Speed and velocity have the same units, that is, m s–1 or m/s.

Rate of Change of Velocity:


            This rate of change of velocity per unit time is called Acceleration.

«  Acceleration = Change in velocity/ Time taken

«  SI Unit: m/s2

«  Uniform Acceleration – An object is said to have a uniform acceleration if It travels along a straight path Its velocity changes (increases or decreases) by equal amounts in equal time intervals

«  Non - Uniform Acceleration – An object is said to have a non-uniform acceleration if Its velocity changes (increases or decreases) by unequal amounts in unequal time interval

«  Acceleration is also a vector quantity.

«  The direction of acceleration is the same if the velocity is increasing in the same direction. Such acceleration is called Positive Acceleration.

«  The direction of acceleration becomes opposite to that of velocity if velocity is decreasing in a direction. Such acceleration is called Negative Acceleration.

«  De-acceleration or Retardation – Negative acceleration is also called De-acceleration or Retardation.