There is no universally accepted "Newton's Fourth Law of Motion." Newton himself formulated three laws of motion. However, the term is sometimes used informally or in specific contexts to refer to different concepts.
Interpretations of "Newton's Fourth Law"
Several interpretations exist depending on the context:
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Newton's Law of Universal Gravitation: This is frequently referred to as a "fourth law" by some, although it's more accurately a separate principle. It states that every particle attracts every other particle in the universe with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. This is expressed mathematically as F = G(m₁m₂)/r², where F is the gravitational force, G is the gravitational constant, m₁ and m₂ are the masses of the two particles, and r is the distance between them. Sources like Quora and CK-12 Foundation mention this interpretation. A more precise description of this "fourth law" can be found at Beason Brackin's website.
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Newton's Law of Cooling: This is another law sometimes mistakenly associated with a "fourth law" of Newton's. It describes the rate at which an object cools down to the surrounding temperature (jasmcole.com). It's a distinct concept from Newton's three laws of motion.
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Informal or Humorous References: The phrase "Newton's Fourth Law" is also used informally or humorously online to describe various phenomena, lacking any scientific basis. Examples can be found on platforms like Reddit (r/EngineeringStudents and r/wholesomememes).
It's crucial to understand that Newton only proposed three laws of motion. Any reference to a "fourth law" should be critically examined for its context and accuracy.
