4.4 : Moment of a Force: Vector Formulation

The moment of force refers to the measure of the rotational tendency of a force. It occurs when a force is applied in such a way that it produces a twisting or rotational motion rather than linear motion. The moment arm of a force is the perpendicular distance from the line of action of the force to the axis of rotation. The moment of force is not a scalar but a vector quantity.

The vector formulation of the moment of force is the cross-product of the position and force vectors. The cross-product of two vectors results in a vector perpendicular to both vectors. The moment about a given point, O, can be calculated using the following formula:

where M_O is the moment of force, r is the position vector, and F is the force vector.

The magnitude of the moment of force is expressed as:

where the moment arm is rsinθ and the magnitude of the moment is maximum when the position vector and force vector are perpendicular to each other.

The right-hand rule determines its direction, stating that if the fingers curl in the direction of the object's rotation, the thumb shows the direction of the moment. This means that its direction is perpendicular to the plane containing both the position and force vectors. Additionally, if the force is applied at two different points but along the same line of action, it creates the same moment of force. This property is commonly known as the principle of transmissibility of force.

Overall, understanding the moment of force and its vector formulation is essential for a variety of fields, including engineering, physics, and mechanics. It enables us to predict and understand the rotational motion of objects under the influence of a force.