Laws of Physics

Applied to Bloodspatter

The flight path of a blood droplet through still air is completely determined by the action of only two forces.

(1)  The force of gravity, a familiar force which acts on all objects with mass, including our bodies. This force, while acting on a particular droplet, is constant in magnitude and direction. Its magnitude is equal to the product of the mass of the object and the acceleration of gravity (mg). The direction of this force is always downward. The action of this force creates the downward acceleration of gravity "g". The gravity force depends only on the mass of an object.

(2)  The force of air resistance or viscous drag:   all objects moving through the air experience this resistance to the motion. This force is different from the gravity force in that it does not depend on the mass of the object. It depends instead, on the shape of the object and the speed at which it moves through the air. Automobiles and aircraft are designed with streamlined shapes in order to reduce the force of air resistance.

The direction of this force is exactly opposite to the direction of motion at all times. The latter property means that the action of this force is to decelerate or reduce the speed of the droplets.

For blood droplets that have a mass greater than about 5 milligrams the air resistance has a small effect on the flight paths. This flight path, in still air, can be shown to lie in a vertical plane and has the familiar shape of a parabola concave downward.

Of major importance to the directional analysis method is the fact that neither of these two forces push the droplets sideways during its flight. The droplets progress through the air moving up and down but not left or right. Therefore under normal conditions, the droplet motion is confined to a vertical plane which passes through the source point and the impact point. The virtual string is also confined to this flight plane and passes directly over the source point.

All the strings belonging to stains from a common source will BackTrack or converge onto that source position when viewed from above, the bird's eye view.