An Application of the Principles of Projectile Motion to a Homicide Investigation

Abstract

Biomechanics testimony was sought to assess the feasibility of a claim made by the defendant that a fatal head injury sustained by the victim arose as a result of the defendant throwing a child's bicycle a horizontal distance of 7 m which accidentally struck the victim. The method of projection was claimed to involve pushing the bicycle from the chest equally with both hands from a stationary upright standing position in a manner similar to a chest pass in basketball. A further consideration in the case was that the maximum height of the projectile during flight was constrained by the presence of an overhead ceiling. Using equations for uniform motion and based on measurements made at the crime scene, it was determined that the theoretical release speed of the projectile could not have been less than 10.0-11.0 m/s and was probably closer to 14 m/s. These estimated release speeds are substantially higher than the release speeds associated with a comparable movement pattern, namely, the maximal two-handed bench throw (~2.5 m/s), and instead approximate the release speeds recorded by the best three throwers in the Men's Shot Put at the 2004 Athens Olympic Games (13.60-13.95 m/s). It therefore seems highly unlikely that the defendant could have thrown the projectile a horizontal distance of 7 m as claimed.