This shows a triangular rigid body with forces applied at the points A, B and C. The equivalent force and couple at the point 'AboutPoint' is also shown. You can move the points to see how the equivalent system changes. The equivalent system is made up of the sum of the force vectors plus the sum of the moments of the vectors about the point 'AboutPoint'. For a rigid body all applied forces and moments can be replaced by a single force and moment at a point and give the same result.

[list][*]Move the 'AboutPoint' around and note how the resultant and couple change[/*][/list][br][list][*]Move the tip of F_A to (1,0) and move the 'AboutPoint' around[/*][/list]How does the resultant force and couple change with position?[br][br][list][*]Move the tip of F_A to (0,0), F_B to (1,3) and F_C to (7,1)[/*][/list]Find where the couple is zero[br][br][list][*]Move the tip of F_A to (-1,1)[/*][/list]Find where the couple is zero[br]What is different about the last two results?[br][br][list][*]Adjust the forces so that the resultant is zero[/*][/list]How does the couple vary as you move 'AboutPoint' around?[br][br][list][*]Adjust the forces so the triangle is in equilibrium. The resultant is zero and [br]the line of action of the three forces pass through the same point.[/*][/list]What happens to the resultant force and couple and why?[br]