2.5 Speed and Velocity

2.5 Speed and Velocity

We are often interested to know how fast a body is moving. For this purpose, we have to find the distance covered in unit time, which is known as speed. If a body covers a distance S in time t, its speed v will be written as:

Speed = Distance / Time
v = S / t
S = vt

Speed is a scalar quantity. The SI unit of speed is m s⁻¹ or km h⁻¹.

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Brain Teaser! The car while moving on a circular road may have constant speed, but its velocity is changing at every instant. Why?

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Do You Know? The fastest land mammal (cheetah) and the fastest fish (sailfish) have the same highest recorded speed of 110 km h⁻¹.

It is obvious that the speed of a vehicle does not remain constant throughout the journey. If the reading of the speedometer of the vehicle is observed, it is always changing. The speed of a vehicle shown by its speedometer at any instant is called instantaneous speed. Practically, we make use of average speed:

Average speed = Total distance covered / Total time taken
v_av = S / t