Much of the work of accident analysis and reconstruction involves the application of general physical laws to the particular situation at hand. These physical laws are expressed in terms of Newtonian mechanics, which was developed by the great English scientist Sir Isaac Newton in the seventeenth century. Newtonian mechanics is the base science of physics and engineering.
Velocity is a vector, which means it has both a magnitude and direction.
Note that the more common term, Speed, is a velocity without a direction, so that 30 miles per hour or 20 meters per second are speeds, and not velocities, since no direction is specified. Thus, any time a body changes its rate of travel or its direction of travel, it is said to be accelerating. Acceleration is a vector requiring both magnitude and direction. They will have fallen approximately 16.1 feet in this one-second interval. This relationship is not so much a natural law as a rule for assigning a magnitude to forces.