A seesaw consisting of a uniform board of mass M and length \ell \text { supports a father and daughter with masses } m_{f} \text { and } and m_d, respectively, as shown in Figure 12.8. The support (called the fulcrum) is under the center of gravity of the board, the father is a distance d from the center, and the daughter is a distance \ell / 2 from the center.
(A) Determine the magnitude of the upward force n exerted by the support on the board.
(B) Determine where the father should sit to balance the system.