We present a study of the forces, velocities, and trajectories of slender (length/diameter = 10) axisymmetric projectiles using an embedded inertial measurement unit (IMU). Three nose shapes (cone, ogive, and flat) were used. Projectiles were tested at vertical and oblique impact angles with different surface treatments. The trajectory of a half-hydrophobic and half-hydrophilc case impacting vertically was compared to the trajectory of symmetrically coated projectiles impacting the free surface at oblique angles. The oblique impact cases showed significantly more final lateral displacement than the half-and-half case over the same depth. The amount of lateral displacement was also affected by the nose shape, with the cone nose shape achieving the largest lateral displacement for the oblique entry case. Instantaneous lift and drag coefficients were calculated using data from the IMU for the vertical, half-and-half, and oblique entry cases. Impact forces were calculated for each nose shape and the flat nose shape experienced the largest impulsive forces up to 37 N when impacting vertically. The impact force of the flat nose decreased for the oblique entry case. The location of the center of pressure was determined at discrete time steps using a theoretical torque model and values from the IMU. Acoustic spectrograms showed that the sound produced during the water entry event predominately arises from the pinch-off for the cone and ogive nose shapes, with additional sound production from impact for the flat nose shape. Each test run was imaged using two Photron SA3 cameras.

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Hockey helmets represent the best form of head protection available to reduce the occurrence of skull fracture and concussion. Currently, helmet testing protocols focus on the reduction of peak linear acceleration measures. Gaps exist in analyzing how certain impact factors such as angle, neck stiffness, and location influence the energy loaded to the helmet and the risk of injury during head collisions. This study examined the effect of helmet impact angle, neck stiffness-torque levels, and helmet impact locations on energy reduction and risk of head injury grounded on acceleration measures using simulated free fall head collisions. The researchers conducted 540 impacts to collect the data. The results revealed statistical interaction effects between the angle of impact and location on measures of energy and risk of head injury. This study builds on existing literature by introducing an energy measurement technique to assess helmet performance. The outcome also provides an avenue for helmet manufacturers to evaluate the performance of the helmet in reducing concussion risk.

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