TEXAS WOMAN'S UNIVERSITY KINS 3591 Reaction Board Method This lab was developed by Gary Christopher, MS, ATC, of Texas Woman's University. Reaction Board Method - This method uses the technique of summing torques about an axis of rotation. This method calls for a person to stand or lie on a board that is supported on each end as seen in the below figure. The sum of downward forces is equal to the weight of the body plus the weight of the board. The sum of upward forces is equal to the sum of the reaction forces acting upward at each end of the board.  When added together the reaction forces will equal the weight of the body plus the weight of the board. This can be easily confirmed if a scale is placed under each end of the board. Regardless of where a person stands or lies on the board, the sum of the two scale readings will equal the total body weight plus the weight of the board. If more of the person's weight is placed near one end of the board, the scale at that end of the board will bear more of the person's weight and will have a higher reading than the other scale. In order to determine the location of the center of gravity, we need to sum the torques acting on the system. In a static situation like this, the sum of torques will be equal to zero (ST = 0). To sum the torques, an axis of rotation must be selected. In order to compute the location of the center of gravity (CoG), we must select the axis at the end away from the scale (regardless of the orientation of the person on the board. Torque is defined as the product of a force and the moment arm (perpendicular distance from an axis of rotation) (T = F^d). If we choose the axis at the location of the reaction force away from the scale, the moment arm of that force is zero, so only two torque producing forces will be acting -  the reaction force at the scale end of the board, and the weight of the person on the board. The body weight line of force extends from the CoG toward the center of the Earth. The reaction force at the scale acts upward on the board. Because the board is not rotating, these two torques must be equal in magnitude and opposite in direction. The torque produced by the scale is equal to the product of the scale reading (reaction force) and the perpendicular distance to the axis of rotation. The perpendicular distance will equal the distance between the board supports. The opposing torque produced by the body is equal to the product of body weight and the perpendicular distance from the line of gravity to the axis of rotation. It is this distance that will be determined in this lab.     Assignment Compute your own center of gravity. Complete the data sheets and turn in to your instructor.