Chapter 12 - Equilibrium and Elasticity - Problems - Page 346: 19

$F = 8.7~N$

The torque about the rotation axis from the applied force $F$ must be equal in magnitude to the opposing torque from the nut. We can find $F$: $\tau_{net} = 0$ $L~F - d~(40~N) = 0$ $L~F = d~(40~N)$ $F = \frac{d~(40~N)}{L}$ $F = \frac{(2.6~cm)(40~N)}{12~cm}$ $F = 8.7~N$