Center of mass:The center of mass is the point where all the mass of an object can be considered to be concentrated. It is typically located at the geometric center for objects with symmetrical shapes.
Moment arm: The moment arm is the perpendicular distance between the axis of rotation and the line of action of a force. In this case, it would be the distance from one end of the board to where the person is sitting.
Torque:Torque refers to rotational force or twisting force that tends to cause rotation. In this scenario, torque equilibrium needs to be achieved for both chains supporting the board.