kilogram as the unit of force, we have thereby also settled what the unit of mass must be.
A stone weighing one kilogram, and in fact any stone, when starting to fall from rest, acquires in the first second a velocity of 9.81 metres per second. Its acceleration or gain of speed is therefore 9.81 metres per second per second. Since the force which pulls the stone towards the earth is one kilogram, and since in any system of units the product of mass and acceleration represents force, the mass of our stone is the 9.81th part of unit mass. Therefore in the system of units chosen in this example, a stone weighing 9.81 kilograms has unit mass.
Expressing now the known masses of earth and moon in this system, and remembering that the average radius of the moon's orbit is 385,080 km., and the length of the month 27 days 7 hours 43 minutes, it is easy to calculate the centrifugal force from the well-known relation between mass, radius and time of revolution. The result is in round numbers 20,000 million million tons. It is difficult to grasp the meaning of so prodigious a force, but we may get an idea of its magnitude by calculating the diameter of a cylindrical bar of the strongest steel able to just support the
