If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.

When an object does not change its position with respect to a reference point with elapse of time, the object is said to be at rest.

Example – When a vehicle changes its position with respect to an electric pole (a reference point) with elapse of time, then the vehicle is called in motion. And if the same vehicle does not change its position with respect to that electric pole, the vehicle is called in the state of rest.

Thus to observe the motion of an object; two things are necessary – a reference point and time.

In the above example; the vehicle is the object and electric pole is the reference point. A building, tree, or any other static thing can be taken as the reference point to observe the motion of an object.

When an object moves along a straight line, the motion of the object is called rectilinear motion. For example; the motion of a vehicle along a straight road.

Distance is the length of path covered by a moving object in the given time irrespective of direction. Distance has only magnitude and no direction.

**SI unit of distance is meter (m).**

Kilometer is used to measure bigger distance and it is abbreviated as ‘km’.

Displacement is the shortest possible distance covered by a moving object from the initial point in a particular direction. In other words, the shortest distance between the initial point and final point is called the displacement.

Displacement has both magnitude and direction while distance has only magnitude.

When an object covers the equal distance in equal interval of time, the motion is called uniform motion. For example – if a moving vehicle covers a distance of 10 km every hour, the motion of the vehicle is called uniform motion.

When an object covers the unequal distance in equal interval of time, the motion is called non-uniform motion. For example – If moving vehicle covers a distance of 10 km in the first hour, covers a distance of 20 km in the second hour, covers a distance of 5 km in the third hour, etc. the motion of the vehicle is called non-uniform motion.

Distance covered by a moving object in unit time is called distance.

Speed = Distance covered x Total time taken

⇒v=st

Where, v = speed, s = distance, t = total time.

SI unit of speed is meter per second (m/s).

The average distance covered in unit time by a moving object is called average speed. Average speed is the ratio of total distance covered and total time taken.

Average speed =Total distance covered x Total time taken

⇒v=st

Where, v = Average Speed, s = Total distance covered, t = total time taken.

SI unit of average speed is meter per second (m/s).

The speed of a moving object in particular direction is called velocity. Velocity has both magnitude and direction while speed has only magnitude and no direction.

Velocity of an object is the distance covered in particular direction in unit time.

Velocity=Distance covered x Time taken

⇒v=st

SI unit of velocity is meter per second.

Uniform speed of an object in the same direction is called uniform velocity.

Non-Uniform velocity: Velocity of an object is changed in following two conditions.

(a) When speed is changed

(b) When the direction is changed.

Thus, the non-uniform speed of a moving object in the same direction, or non-uniform speed in different directions or uniform speed in different directions is called non-uniform velocity.

Example – If a vehicle is moving on a circular path with uniform speed, then its velocity is said to be non-uniform because on a circular path the direction of moving body changes along with the direction of curve.

If a vehicle moves with uniform speed on crisscross path, the velocity of the vehicle will be non-uniform because the direction of the vehicle is changed with the change of direction of path.

The arithmetic mean of velocity of an object moving along a straight line is called the average velocity.

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

v_{av}=(u+v)/2

Where, u is the initial velocity and v is the final velocity.

The displacement of a moving object in unit time is also called the average velocity.

The rate of change in velocity is called acceleration. Acceleration is generally denoted by a

Thus, Acceleration =Change in velocity /Time taken

⇒a=(Final velocity - Initial velocity)/Time taken in change in velocity

⇒a=(v−u)/ t

Where, ‘a’ is acceleration, ‘v’ is final velocity, ‘u’ is initial velocity and ‘t’ is time taken for change.

A positive sign of the magnitude of acceleration shows an increase in velocity and a negative sign shows a decrease in velocity. If there is a decrease in acceleration, it is called Retardation. This means the rate of decrease in velocity is called Retardation.

The SI unit of velocity is meter /second

The SI unit of time is second.

Thus, SI unit of acceleration (a) =m/s^{2}

In the case of uniform velocity, the speed or direction of a moving object is not changed and thus there is no change in acceleration. Therefore, in the case of uniform velocity acceleration will be zero.