Torque

A $100 \mathrm{kg}$ cubical box lies on a floor. A child pushes horizontally at a top edge. What force magnitude puts the box on the verge of tipping over if there is sufficient friction between it and the floor to prevent sliding?

A $60 \mathrm{kg}$ father and $20 \mathrm{kg}$ child sit on opposite ends of a seesaw consisting of a board of length $4.0 \mathrm{m}$ and negligible mass. The pivot can be placed anywhere between the father and the child. At what distance from the child should it be placed so that the seesaw is balanced when the father and child are stationary?

The body in the figure shown below is pivoted at $O$. Three forces act on it, 

$F_A=12 \mathrm{N}$ at the point $A, 8.0 \mathrm{m}$ from $O$,

$F_B=14 \mathrm{N}$ at the point $B, 4.0 \mathrm{m}$ from $O$ and

$F_C=23 \mathrm{N}$ at the point $C, 3.0 \mathrm{m}$ from $O$. 

What is the net torque about $O$?

The body in the given below figure is pivoted at $O$ and two forces act on it as shown. If $r_1=1.30 \mathrm{m}$, $r_2=2.15 \mathrm{m}, F_1=4.20 \mathrm{N}, F_2=4.90 \mathrm{N},$ ${\text{θ}}_1=75.0°$ and ${\text{θ}}_2=60.0°$, what is the net torque about the pivot?