You know that, speed is related to distance and is a scalar
quantity. Similarly, velocity is a vector quantity which is related to
displacement of the particle.

You can easily find the velocity of a particle if you know
the displacement. From previous section you have studied how to calculate
displacement in a 3 dimensional space.

**Average velocity**of a particle in a time interval t1 to t2 is defined as its displacement divided by the time interval.

Please note in mind that, we need just initial and final
position of the particle to calculate average velocity.ie, the position in
between t1 and t2 need not to bother.

**Instantaneous velocity**is the velocity at a given location in a time.

For a very small interval of time the displacement ∆r, will be along the line of motion of the particle. In this small interval of time the distance and displacement will be equal. Which means, magnitude of instantaneous speed and instantaneous velocity are same.

At the school time, I had a doubt that when will be the velocities
become zero and negative? Do you have the same? No need to worry. Here I would
like to explain the concept, with figure 3 shown below.

In the figure 3, at initial stage displacement is
increasing. This means, slope of the curve is positive. ie, velocity is
positive at this stage. But at peak, the change of displacement with respect to
time is zero. Velocity is zero at that position. At final stage of journey, the
displacement is decreasing with respect to time which leads to a negative
velocity.

Figure
3

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