07 3-2 (c) The three-point test
Rule
- If the sum of the moments of a system of coplanar forces about three non-collinear points in its plane has the same non-zero value , the system is equivalent to a couple of moment .
Why
- A coplanar system reduces to a single force, a single couple, or equilibrium.
- It is not in equilibrium (the moments are non-zero).
- If it reduced to a single force at distances from the three points, then , so the three points would lie on a line parallel to the force; they do not. So it must be a couple.
✍️ Worked examples
Example 16: A square of side 10 cm is acted on by the forces in Example 14. Use the test to confirm that it is a couple.
Solution:
- Moments about : (computed above).
- Moments about : the forces along and pass through ; has arm 10 and has arm 10, both in the same sense: .
- Moments about : similarly .
- Equal non-zero values at three non-collinear points, so the system is a couple of moment N·cm.