A Classical Approach of Newtonian Mechanics

1 INTRODUCTION 1.3 mks units

1.3 Mks units

The first principle of any exact science is measurement. In mechanics there are

three fundamental quantities which are subject to measurement:

1. Intervals in space: i.e., lengths.


2. Quantities of inertia, or mass, possessed by various bodies.


3. Intervals in time.

Any other type of measurement in mechanics can be reduced to some combina-

tion of measurements of these three quantities.

Each of the three fundamental quantities—length, mass, and time—is mea-

sured with respect to some convenient standard. The system of units currently

used by all scientists, and most engineers, is called the mks system—after the first

initials of the names of the units of length, mass, and time, respectively, in this

system: i.e., the meter, the kilogram, and the second.

The mks unit of length is the meter (symbol m), which was formerly the dis-

tance between two scratches on a platinum-iridium alloy bar kept at the Inter-

national Bureau of Metric Standard in S`evres, France, but is now defined as the

distance occupied by 1, 650, 763.73 wavelengths of light of the orange-red spectral

line of the isotope Krypton 86 in vacuum.

The mks unit of mass is the kilogram (symbol kg), which is defined as the mass

of a platinum-iridium alloy cylinder kept at the International Bureau of Metric

Standard in S`evres, France.

The mks unit of time is the second (symbol s), which was formerly defined in

terms of the Earth’s rotation, but is now defined as the time for 9, 192, 631, 770
oscillations associated with the transition between the two hyperfine levels of the
ground state of the isotope Cesium 133.

In addition to the three fundamental quantities, classical mechanics also deals

with derived quantities, such as velocity, acceleration, momentum, angular mo-