Problem $3$: Motor B opens and closes the door below by drawing in the cable $\backslash ($ A B $\backslash ).$ The two sections of the door are pin connected at $\backslash ($ C $\backslash )$ and $\backslash ($ D $\backslash )$ and the bottom A is attached to a roller that travels along the vertical track. Each section of the door has a uniform weight of $300$ lb and height of $\backslash ($ L$=10\backslash$mathrm$\{$ft$\}\backslash ),$ determine the force on the cable when $\backslash (\backslash$theta$=90^\{\backslash$circ$\}\backslash ).$