We explain what Newton's Third Law is, which explains the action-reaction principle, its formula and everyday examples.
What is Newton's Third Law?
It is called Newton's Third Law or Principle of Action and Reaction to the third of the theoretical precepts postulated by the British scientist Isaac Newton (1642-1727) in his work Philosohiae naturalis principia mathematica (“Mathematical principles of natural philosophy”) of 1687, influenced by the previous studies of Galileo Galilei and René Descartes.
This work, along with Newton's three laws, is considered a fundamental text of modern physics. Newton's Third Law expresses, in the scientist's words in Latin:
““Actioni contraram semper & æqualem esse reactionem: sive corporum duorum actiones in se mutual semper esse æquales & in parte contrares dirigi”
Which translates as:
“Every action has an equal reaction but in the opposite direction: which means that the mutual actions of two bodies are always equal and directed in opposite directions.”
This law explains that forces in the world always occur in pairs: an action and a reaction, the latter of the same magnitude but opposite direction. This means that when one body exerts a force on another, the latter responds with a force of equal magnitude but opposite direction.
Its mathematical formula is:
F1-2 =F2-1
See also: Dynamics
Examples of Newton's Third Law
Examples of Newton's Third Law in everyday life are easy to find. It is enough to physically imagine a jump, like the one an acrobat makes from his circus springboard, or a swimmer from his diving board at the edge of the pool.
In both cases they rise into the air after applying a certain amount of force to it, pushing it with their feet to jump. So, exert a force F on the trampoline with their legs, which generates a force -F of the same magnitude but opposite direction, lifting it into the air.
The same thing happens in the case of a ball that we throw against a wall with a force F: it will receive a force –F in the opposite direction and of equal magnitude, which will send it rebounding towards us.
Newton's other laws
Apart from Newton's Second Law, the scientist proposed two other fundamental principles:
- Newton's First Law (or Law of Inertia) Which reads: “Every body perseveres in its state of rest or uniform rectilinear motion unless it is forced to change its state by forces imposed on it.” This means that an object moving or at rest will not alter its state unless some type of force is applied to it.
- Newton's Second Law (or Fundamental Law of Dynamics) Which reads: “The change in motion is directly proportional to the impressed driving force and occurs along a straight line along which that force is impressed.” This means that the acceleration experienced by a given body is proportional to the force exerted on it, which may or may not be constant.
Continue with: Laws of thermodynamics
References
- “Newton's Laws” on Wikipedia.
- “What is Newton's Third Law? at Khan Academy.
- “Newton's Third Law” in FisicaLAB.
- “Newton's Laws” in Hyperphysics.
