Same Force, Different Point — The Sign of a Moment
One body, one 4.0 N force, and three different answers: +13.9 N·m about A, −10.4 about B, −6.9 about C. Slide the point you take moments about and the graph comes out as a straight line through zero — crossing exactly where the point meets the force's line of action, which is also where the sign changes over.
How to use this simulation
- Press A, B or C to take moments about each of the three marked points
- Drag the pivot position continuously and watch the moment trace a straight line
- Press On the line of action to find the point where the moment vanishes
- Press Pin it here and let go to see which way the body actually turns
What to look for
- The same force gives a different moment about every different point
- The graph of moment against position is a straight line, not a curve
- The moment passes through zero exactly where the point sits on the line of action
- The sign reverses as the chosen point crosses that line
- A positive and a negative moment of the same size turn the body opposite ways
The physics behind it
A moment is never a property of a force by itself; it is a property of a force and a chosen point together. Moving that point changes the perpendicular distance linearly, so the moment plotted against position along the body is a straight line. It passes through zero where the chosen point lies on the line of action, since the perpendicular distance vanishes there, and the sign flips as the point crosses over to the other side. Positive and negative simply record which way the force would turn the body about the point you picked.