There are two cases in the simulation:
The left one is a simpler one: An object hanged with a string. You can drag the blue arrow to change the external force.  The equilibrium state will be shown when you release the mouse.
If you click the [b]always[/b] checkbox. The program will try to show the equilibrium state while you are dragging. But you will find it jump from one state to another. It is not a continuous one. Because in real case, we can not determine the force externally. Because force is an interaction between two object. It is not determined by one object alone. (The tension of the string will also change, when the interaction force between object and external world).
You can check the [b]spring[/b] mode, to view the interaction in a more realistic way.
(Use spring to simulate the interaction : force).

The gray line shows the original length of the spring. (It will be the same as the arrow when you switch to spring mode)
Click [b]play[/b] to start the simulation.
You can then drag the spring to change the interaction (force).

The second case is ?
two objects connected by strings. The default setting is Fx1=Fx2 (horizontal forces for object1 and object2 cancel each other).
Use mouse to drag the arrows to change the external forces.
You can un-check the check box to disable it and play with more complicate situation.

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Enjoy the fun of Physics! You can access this simulation online from
static equilibrium
Designed by Fu-Kwun Hwang (http://www.phy.ntnu.edu.tw/ntnujava/)
Deptartment of Physics, National Taiwan Normal University, Taiwan