Physics of Blood Drops
Blood behaves as a viscous fluid with predictable physical properties. A free-falling blood drop forms a sphere (about 4-5 mm diameter) due to surface tension. When it strikes a surface, the resulting stain's shape depends on the angle of impact: a 90° impact creates a circular stain, while decreasing angles produce increasingly elongated (elliptical) stains with a pointed 'tail' in the direction of travel. The angle of impact can be calculated using the formula: sin(θ) = width/length of the stain. By determining the angle for multiple stains in a pattern and projecting their trajectories backward using strings or software, analysts can locate the area of origin—the three-dimensional point in space where the blood source was located when the spatter was created.