Équilibre statique - I

Static equilibrium requires two simultaneous conditions: first, translational velocity must be zero, meaning no net external force acts on the body; second, rotational velocity must be zero, meaning net torque from all external forces equals zero. When both conditions are satisfied, an object remains at rest.

Static equilibrium occurs when an object is at rest with zero linear and angular velocities. Dynamic equilibrium occurs when an object moves with constant velocity and zero angular acceleration. Both satisfy Newton's second law with zero net force and torque, but differ in motion state relative to the reference frame.

The distinction is artificial because equilibrium depends on the reference frame. An object at rest in one frame may appear to move at constant velocity in another frame. Since physics laws are identical for all inertial frames, an object can be in static equilibrium to one observer and dynamic equilibrium to another simultaneously.

According to Newton's second law, net force causes linear acceleration. For static equilibrium, the sum of all external forces must equal zero, preventing acceleration. This zero-force condition, combined with zero net torque, ensures the body maintains its state of rest or constant motion.

A weighing balance with equal masses on each side achieves static equilibrium when net force and net torque are balanced. Equal masses create equal downward forces that cancel, and symmetric placement ensures torques cancel. This balanced state exemplifies both force and torque equilibrium conditions.

Torque causes angular acceleration in rigid bodies. For rotational equilibrium, net torque from all external forces must equal zero, resulting in zero angular acceleration. This condition, paired with zero net force, ensures the body neither translates nor rotates, achieving complete static equilibrium.

Static equilibrium principles establish that net force and net torque must both equal zero. Applying these conditions systematically allows you to solve for unknown forces and torques in rigid body systems. These fundamentals form the foundation for rigid body equilibrium problems.