Newton's third law - Action and reaction are equal and opposite.
For example, a building has a force applied to the corner.
Select 'Play' to see the effect of the force.
Diagram 1: The building is not stable enough to offer any resistance to the resultant force. Structural failure results since the building is no longer in a state of equilibrium.
Diagram 2: The building is now fitted with a diagonal brace or strut. The diagonal brace acts as a reaction force against the resultant force. The building is now in a state of equilibrium.
If a force is applied to move an object, the object produces an equal and opposite force to remain stationary if it can; otherwise it will move.