Speed: The distance travelled by a particle per unit time is called speed. Speed is a scalar quantity. Speed = 
.
If s distance travelled by a particle in time t then speed of the particle is 
.
SI unit of speed is ms-1 and the dimension is [LT -1].
Uniform speed: A particle or a body is said to be moving with uniform speed, if it covers equal distance in equal interval of time.
Variable speed: A particle or a body is said to be moving with variable speed, if it covers unequal distance in equal interval of time.
Average speed: The average speed of a body is defined as the ratio of the total distance travelled by it to the total time taken.
If S1, S2, S3 are the distance travelled by a particle for time t1, t2, t3 respectively, then the average speed of it is 
.
Velocity: The displacement of a particle per unit time is called velocity. Velocity is a vector quantity. Velocity = 
.
If 
is the displacement of a particle in time t then velocity of the particle is 
.
SI unit of velocity is ms-1 and the dimension is [LT-1].
Uniform velocity: A particle or a body is said to be moving with uniform velocity, if it covers equal displacement in equal interval of time.
Variable velocity: A particle or a body is said to be moving with variable velocity, if it covers unequal displacement in equal interval of time.
Average velocity: The average velocity of a body is defined as the ratio of the total displacement travelled by it to the total time taken.
If 
are the displacement travelled by a particle for time t1, t2 respectively, then the average velocity of it is 
.
CONCEPT: 1. The half of the distance travelled by a car with speed 
and the other half with speed 
. Calculate the average speed of the car.
Let us consider the total distance is 2x. Time for travelling the distance x with speed  is 
.
Time for travelling the distance x with speed 
 is 
.
The average speed of the car is 
.
2. A train travels with uniform acceleration. When the front face of the train passes the electric post its speed is u and when the end face passes that post its speed is v. Calculate the speed of the train when the middle part of it passes that post.
Let the length of the train is L and acceleration is a. Then, 
or, 
.
The speed of the train when the half part of it passes that post is w. So, 
Or, 
Or, 
Or, 
.
Acceleration: The rate of increase of velocity of an object with time is called acceleration.
Acceleration = 
.
SI unit of acceleration ms-2and the dimension is [LT -2].
Retardation: The rate of decrease of velocity of an object with time is called retardation.