Final answer:
Equilibrium involves the balance of forces where the net force is zero, and it is true that equilibrium does not involve stresses. Forces in equilibrium have equal magnitudes but opposite directions, and Newton's third law applies to both internal and external forces.
Step-by-step explanation:
The statement that 'Equilibrium involves forces, not stresses' is true. Equilibrium refers to the state in which all the acting forces on a body are balanced, meaning that the net force is zero. This does not directly relate to stresses, which are the internal forces per unit area within materials arising from externally applied forces. For example, an object at rest on a flat surface is in equilibrium because the gravitational force downwards is counteracted by the normal reaction force, which is an equal and opposite force that the surface exerts on the object.
According to Newton's third law, whenever two objects interact, they exert equal and opposite forces on each other, which is applicable to both internal and external forces. In the case of the snapped tug-of-war rope, Newton's third law indicates that both teams were exerting equal and opposite forces on the rope until it failed. Two forces in equilibrium have equal magnitudes but opposite directions, demonstrating how the forces are balanced.
An external force is indeed required to set motion or change the motion of a body, even in outer space devoid of gravitational influences and atmospheric friction. However, in space, this force would primarily serve to overcome inertia rather than balance out any existing forces as there would typically be no opposing forces to start with in such an environment.