Why are the states of a body at rest and a body moving with constant velocity considered equivalent in terms of net force?

$A$ body of mass $m$ is suspended by two strings making angles $\theta_1$ and $\theta_2$ with the horizontal ceiling with tensions $T_1$ and $T_2$ respectively. If $T_1 = \sqrt{3} T_2$,then the angles $\theta_1$ and $\theta_2$ are:

$A$ box of mass $m$ is in equilibrium under the application of three forces as shown below. If the magnitude of $F_1$ is $10 \ N$, what is the magnitude of $F_3$ (in $N$)?

$A$ weight $Mg$ is suspended from the middle of a rope whose ends are at the same level. The rope is no longer horizontal. The minimum tension required to completely straighten the rope is ......

$A$ block is placed between two surfaces as shown in the figure. Find the normal reaction at both surfaces. [Assume $g = 10 \ m/s^2$]

$A$ flexible chain of mass $m$ hangs between two fixed points at the same level. The inclination of the chain with the horizontal at the two points of support is $30^{\circ}$. Considering the equilibrium of each half of the chain, the tension of the chain at the lowest point is . . . . . . .