6. What exactly is Moment of a Force?

What is a moment of a Force?

The Moment of a force is a measure of its tendency to cause a body to rotate about a specific point or axis.

As stated by the definition of moment above we can deduce, as we move a body with mass away from the axis of rotation on the pivot, the moment (or torque) increases.

moment of Force

The picture above demonstrates moment in action. If we increase d (distance of Garfield from the pivot) the moment will increase. Adding to it, if we would increase the weight F the moment will also increase.

This leads us to our formula:


Where, F is the force

and d is the perpendicular distance from the axis at point O to the line of action of the force.

Moment being a vector quantity

  1. Magnitude: The units for Moment are Nm or lbft. Magnitude can be determined using the formula mentioned above.
  2. Direction: The direction of the moment can be determined using the moment axis (RHR). the moment Axis is perpendicular to the pane that contains the force F and its moment arm d.

Moment Axis —–> Out of the page —–> Anti-Clockwise

Moment Axis —–> Into the page ——> Clockwise

Resultant Moment

For 2D problems, the resultant moment about a point can be determined by finding the algebraic sum of the moment caused by all the forces in the system.

Also Read: Couple Moment part 2