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

In the figure,the tension in the diagonal string is $60\,N$. Find the magnitude of the horizontal forces $\overline{F}_1$ and $\overline{F}_2$ that must be applied to hold the system in the position shown. What is the weight of the suspended block?

$A$ stone of mass $0.05 \,kg$ is thrown vertically upwards. What is the direction and magnitude of the net force on the stone during its upward motion?

$A$ body of mass $2 \ kg$ is suspended as shown in the figure. The tension $T_1$ (in $kg-wt$) in the horizontal string is :-

The following forces start acting on a particle at rest at the origin of the coordinate system simultaneously: ${\vec F_1} = -4\hat i - 5\hat j + 5\hat k$,${\vec F_2} = 5\hat i + 8\hat j + 6\hat k$,${\vec F_3} = -3\hat i + 4\hat j - 7\hat k$,and ${\vec F_4} = 2\hat i - 3\hat j - 2\hat k$. The particle will move:

Three forces $\vec{P}, \vec{Q},$ and $\vec{R}$ act at a point in a plane. The angles between $\vec{P}$ and $\vec{Q}$,and $\vec{Q}$ and $\vec{R}$ are $150^{\circ}$ and $120^{\circ}$ respectively. What is the ratio of the magnitudes of the forces $\vec{P}, \vec{Q},$ and $\vec{R}$ if the system is in equilibrium?