Use of couples of forces to express torque in situations of static equilibrium

Abstract

A rigid body is in static equilibrium if the following two conditions are met: the sum of external forces is zero, and the sum of external torsion moments (or torques) is zero. The condition for torques turns out to be difficult for some students, because it involves a vector product. In this work we discuss an easier alternative to apply that condition, by using just scalar elements. We show some examples of a bar in static equilibrium, where we use the equilibrium of forces in order to find out the couples of forces producing torques.