The second law is of great practical use.
One can use experiments to determine the manner in which the
force depends on the position and velocity of the bodies and then use
calculus (which was also invented by Newton)
to determine the motion of the bodies by obtaining
the position as a function of time using the known
form of *F* and the equation *F* = *m a*.
Note that in this equation *m* measures how
strongly a body responds to a given force (the larger *m* is the less
it will be accelerated); *m* measures the inertia of the body.

Suppose we choose a test body of mass, say, 1gm. By measuring its
motion one can obtain its acceleration and, using *F* = *m a*, determine
the force. Once *F* is known the motion of *any* body is
predicted: by measuring the falling an apple you can predict the motion
of the Moon.