thus at first sight appear that its application to such a bulky object as our earth is not permissible. Neither moon nor earth can be considered infinitely small as compared to their distance. Nevertheless we may use the formula, for a mathematical investigation shows that in the case of spheres the summarised effects of all mass particles is the same as if the total mass were concentrated in the centre. Astronomy gives us all the data required for our calculation; all we need to get a definite numerical result is to agree on a definite system of units in which to express a force.
The definition of a mechanical force is: something which produces acceleration of a ponderable mass. Acceleration is the rate at which speed increases in respect of time. Thus, if an electric tramcar starting from rest attains its full speed of 24 miles per hour in the time of 20 seconds, its average acceleration is 1.2 miles per hour in each succeeding second, or "1.2 miles per hour per second." If instead of giving the speed in miles per hour we give it in metres per second, the acceleration of this car would be 0.535 metres per second per second. Taking the metre as the unit of length, the second as the unit of time, and the
