Suppose, a body is moving in a particular direction, i.e., it is moving along a straight line. Although the direction of velocity is fixed, its magnitude is varying. Let the magnitude of velocity v is dependent on time t. That means, v is the function of t. is indicated by v (t).

Let initial time be t_{A} and final time be t_{B}

We will find the distance travelled by the body during this time interval. Let us divide the time interval t_{B} – t_{A} into exceedingly small N number of equal section ∆t. Suppose the time interval from the initial time t_{A} to first small section t_{A} + ∆t is ∆t. This time interval is so small that we can consider that the value of v(t) is almost constant. Let the constant value of v(t) be v. If the distance travelled in time ∆t is ∆S_{1} then we get,

**∆S _{1} = v_{1}∆t**

Similarly, for the second, third sections time interval is ∆t and the distance travelled will be respectively ∆S_{2} = v_{2}∆t and ∆S_{3} = v_{3}∆t. For the Nth section, distance travelled is ∆S_{N} = v_{N}∆t. Now, distance travelled between the time intervals t_{A} and t_{B} will be the summation ∆S_{1}, ∆S_{2}, ∆S_{3}……., ∆S_{N}.

So, S = ∆S_{1} + ∆S_{2} + ∆S_{3}…….+ ∆S_{N}.

or, S = v_{1}∆t + v_{2}∆t + v_{3}∆t + ……… + v_{N}∆t.

or, **S = ^{N}∑_{n = 1 }= V_{n} ∆t.** … … … (1)

It is to be noted that here we have assumed that the value of velocity is constant for small-time Interval ∆t. if the value of v would have been exactly constant, then we could get the more accurate value of the distance from the equation (1). Now as the time interval becomes more and smaller, the value of v will be more and closer to the constant value. We can get the accurate value of distance if ∆t tends to zero and number of sections N becomes infinity. Then we can write;

**S = lim _{∆t→0} ^{N}∑_{n = 1 }= V_{n} ∆t**

In calculus, the quantity **lim _{∆t→0} ^{N}∑_{n = 1}**; is written as,

^{tB}∫_{tA}v(t)dtThe quantity ** ^{tB}∫_{tA} v(t)dt** indicates the integration of v(t) from t

_{A}to t

_{B}with respect to t.

Integration is a type of summation, and the symbol ∑ or ∫ indicates that summation or integration.

So,** s = ^{tB}∫_{tA} v(t)dt **… … … (2)

v(t) is called integrand. dt after v(t) indicates that integration is to be done with respect to t. Differentiation and integration are opposite to each other. For example,

**d/dt (sin x) = cos x**

again,** ∫cos x dx = sin x.**

That means, if sin x as differentiated we get cos x, on the other hand, if cos x is integrated we get sin x.