Question

Why can a pace or a footstep not be used as a standard unit of length?

Answer 1

Length is the term used to measure the size of an object or the distance from one point to another. There is a standard unit of length that does not change with time, place, or person. It is a fixed length all over the world. The pace or a foot step of different people are different.

Complete answer:
A pace or a foot step cannot be used as a standard unit of length because every person has a different size of pace or foot step. So while measuring a particular length different people will have a different count of pace or foot step. So if we use footstep as a standard unit of length then the two measured quantities will not be the same.

Therefore we have a standard unit of length based on a metric system which is meter.Denoted as $'m'$ . So no matter who or how many people measure a particular length, with the standard unit meter a particular length will be of particular meters every time and everywhere.

Hence, we can say that a pace or a foot step cannot be used as a standard unit of length.

Note: apart from meter to measure length there are other units also: kilometer denoted as $(km)$ , millimeter denoted as $(mm)$ , etc. Millimeter is used to measure a smaller length and a kilometer is used to measure a larger length. by knowing the value in meters we can get the value in kilometer or millimeter or vice versa by the following conversion.
$1\,m = 0.001\,km$
$\Rightarrow 1\,m = 1000\,mm$