This model opens with two applied forces already in place. You may click on the head of either force vector and drag it to a new location. The model calculates the reaction force vector required to maintain static equilibrium with the two applied forces. The radius vector for the reaction force points to the place where the reaction force vector must be attached for equilibrium. If the attachment point for the reaction force vector moves off the disk, no equilibrium is possible, given the applied forces. The last line written on the drawing area lists the torque about the center of mass for each of the force vectors, applied force 1, applied force 2 and reaction force, respectively. These values are calculated independently, rather than forcing TR = -(T1+T2). If you check closely, you may see minor departures from zero for T1 + T2 + TR, due to calculation precision.
Clicking on the Clear button wipes out all the vectors for a new start. Pick any point on the disk and click and drag the mouse to set the first force. If you release the mouse button before dragging, the magnitude of the first applied force is set to zero. You may come back to it later to adjust it. Next choose a location for the application of the second force and lay it out the same way.
For details on the operation of controls, see the Model Controls help page.