Newton’s third law

Newton’s Third Law, also known as the Principle of action and reaction, tells us that if a body A exerts an action on another body B, it performs on A another action that is equal and in the opposite direction.


In more explicit terms: The third law states that for each force acting on a body, it performs a force of equal intensity and direction but in the opposite direction on the body that produced it.

In other words, the forces always appear in pairs of equal magnitude, opposite sense and are located on the same line. Mathematically Newton’s third law of motion is usually expressed as follows: F1 = F2 ‘So F1 is the force acting on body 1 and F2’ the reactive force acting on body 2.

This is something that we can check daily on numerous occasions. For example, when we want to jump up, we push the ground to propel ourselves. The reaction of the ground is what makes us jump up.

When we push a person, a car, etc., we also move in the opposite direction. This is due to the reaction that the other person or the car makes about us, even if they do not try to push us.

It should be noted that, although the pairs of action and reaction have the same value and opposite directions, they do not cancel each other out, since they act on different bodies.

This principle assumes that the interaction between two particles propagates instantaneously in space (which would require infinite speed), and in its original formulation it is not valid for electromagnetic forces since they do not propagate through space instantaneously but do at finite speed “c”.

It is important to note that this principle of action and reaction relates two forces that are not applied to the same body, producing different accelerations, depending on their masses. Otherwise, each of these forces obeys Newton’s second law separately.


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