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.