A rigid body diagram is a simplified representation of an object that assumes it is perfectly rigid and does not deform under the action of forces.
Free-body diagram: A free-body diagram shows all the forces acting on an object, represented by arrows pointing in different directions.
Torque is a measure of how effectively a force can cause an object to rotate. It depends on both the magnitude and direction of the force.
Equilibrium occurs when an object has zero net force and zero net torque acting on it, resulting in no acceleration or rotation.