Absolute, true, and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration. Relative, apparent, and common time is any sensible and external measure (precise or imprecise) of duration by means of motion; such as a measure-for example, an hour, a day, a month, a year-is commonly used instead of true time.
Isaac NewtonCentripetal force is the force by which bodies are drawn from all sides, are impelled, or in any way tend, toward some point as to a center.
Isaac NewtonEvery particle of matter is attracted by or gravitates to every other particle of matter with a force inversely proportional to the squares of their distances.
Isaac Newton