When forces balance, nothing moves. Add up all the forces and you get zero. Draw them tip to tail and the shape closes — the last arrow lands exactly where the first one started. Here you push forces around until they balance, and then look at the special case of exactly three forces, where the angles between them tell you the sizes.
Two tabs. In Equilibrium, add up to five forces, drag their tips, and turn on Head to tail to chain them. Shuffle order rearranges the chain — the shape changes, the closing does not. Show resultant draws the leftover force. In Lami's Theorem you get exactly three forces; drag two and the third adjusts itself so they always balance.
Each force's size, the resultant, and whether you are in equilibrium. Get the resultant close enough to zero and a green In equilibrium badge appears. In Lami's tab, turn on the angles to see α₁, α₂ and α₃, and turn on the formula to watch the three ratios F/sin α stay equal as you drag.
Two dimensions only. Lami's works with three forces, never more.
You should be able to say why a closed tip-to-tail polygon means equilibrium, and use Lami's theorem when three forces meet at a point.