Mass

Mass, measured in kilograms is also called inertia (resistance to acceleration). Newton's second rule in its modern form...

f = ma

.... defines the mass of an isolated object, when force is in Newtons and acceleration is in m/s/s.

The kilogram is defined as the mass of a particular (unique) platinum iridium cylinder kept in a vault at Servrés near Paris for the last three hundred years.  

The four quantities defined in the MKSA system are the meter, kilogram, second and Ampere.

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