Moon falls toward Earth center by Earth gravity {free fall}|. Gravity is centripetal force. Orbital speed moves Moon tangentially in orbit. Tangential movement accelerates Moon away from Earth along radius. This acceleration is centrifugal force. Centrifugal force and centripetal force are equal, and motion rates away from and toward center are equal, so Moon maintains approximately same distance from Earth.
weightless
Moon in orbit has no weight, because centrifugal force equals centripetal force, just as astronauts in orbit are weightless. If jumping from height, in free fall, one feels weightless, because no force is opposing fall.
Kepler formulated three planetary-motion laws {Kepler's laws} {Kepler laws}.
first law
Radius from orbiting body to Sun sweeps out equal areas in equal times, because velocity is slow at large radius and fast at small radius.
second law
If object in orbit moves closer, speed increases as potential energy changes to kinetic energy and moves object back outward. If object in orbit moves farther away, speed decreases as kinetic energy changes to potential energy and moves object inward.
If spinning object becomes more compact, radius decreases and speed increases as potential energy changes to kinetic energy. If spinning object expands, radius increases and speed decreases as kinetic energy changes to potential energy.
third law
Acceleration cubed is directly proportional to time squared, because acceleration is highest at greatest curvature point, where velocity is highest.
Two objects in different orbits interact by gravity or electromagnetism to make torque that changes object spin axis {nutation}|.
Earth spins on an axis that is at an angle to axis of Earth orbit around Sun. Sun gravity causes torque on Earth axis and causes it to rotate {precession}| {precession of the equinoxes, Earth}, as angular velocity around axis interacts with angular velocity around orbit. Object spin and orbital motion interact to cause spin-axis precession.
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Date Modified: 2022.0225