Flash animation: Two spring scales in interaction.
Video: After several examples, such as gravitational interaction, the interaction between two skaters and that of an object and its support, a warning recommends the voluntary forgetting of this 3rd law in favor of the 1st (principle of inertia ) to take stock of the forces exerted on a body whose speed does not vary, or the 2nd (Fundamental principle of dynamics) when the speed varies.
Definition - Statement
Third law of motion : "Any body A exerting a force on a body B undergoes a force of equal intensity, in the same direction but in the opposite direction, exerted by the body B."
FA/B = - FB/A