Table of Contents
What is speed?
The rate of change of position of an object with time in any direction is called its speed. It is equal to the distance travelled by the object per unit time.
Speed = Distance / Time
Speed has only magnitude and no direction, so it is a scalar quantity. Also the distance travelled by an object is either positive or zero, so the speed may be positive or zero but never negative.
Units of speed
The SI unit of speed is ms-1.
The CGS unit of speed is cms-1.
The dimensional formula of speed is [MoL1T-1].
Different Types of Speed:
(i) Uniform speed
An object is said to be moving with uniform speed, if it covers equal distances in equal intervals of time, however small these time intervals may be.
(ii) Variable speed
An object is said to be moving with variable speed if it covers unequal distances in equal intervals of time.
(iii) Average speed
For an object moving with variable speed, the average speed is the total distance travelled by the object divided by the total time taken to cover that distance.
Average speed = Total distance travelled / Total time taken
(iv) Instantaneous speed
The speed of an object at any particular instant of time or at a particular point of its path is called the instantaneous speed of the object. Suppose an object covers distance ∆x in a small time interval ∆t around the instant f, then its average speed is ∆x / ∆t. The limiting value of this average speed when the time interval ∆t approaches zero, gives the instantaneous speed at the instant t. Thus
Instantaneous speed,
v = dx/dt
Here dx/dt is the first order derivative of distance x with respect to time t.
The speedometer of an automobile indicates its instantaneous speed at any instant.