2.14 : Force Vector along a Line

Quite often in three-dimensional statics problems, the direction of a force is specified by two points through which its line of action passes. Consider a three-dimensional static pole with a cable anchored to the ground.

Considering a Cartesian coordinate system with the origin at the pole base, the endpoints of the cable can be denoted as A and B. P_A and P_B represent the position vectors for the two ends of the cable. The triangle law of vector addition is used to obtain the position vector along points A and B. For this purpose, the position vector P_A is subtracted from P_B.

The magnitude of the position vector can be obtained from the square root of the sum of the squares of its components.

The tension force acting on the cable is directed from point A toward point B following the same direction as the position vector P_AB. The unit vector along the cable specifies the direction of the force. It is evaluated by dividing the position vector by its magnitude.

Lastly, the force vector can be expressed in the Cartesian form by multiplying the magnitude of the force vector and the unit vector. This product yields a three-dimensional vector representing the force acting on the pole.