Natural motion
- What is natural motion?
- If we want to study motion this is the first question we need to answer.
- If we want to study motion this is the first question we need to answer.
- By natural motion it is meant motion that continues on its own unless disturbed
- Galileo believed that circular motion was natural motion
- I need to find the exact place where he says this probably in his dialogues
- In this case motion or orbits are not dynamical
- orbits are not forceful
- you don’t need Newton’s force to keep a satellite in orbit
- I need to find the exact place where he says this probably in his dialogues
- Newton believed that you need a force to keep satellites in orbit and it was linear motion that was natural
- This is the crucial difference that I need to study because I side with Galileo
- Newton intentionally mixed up or confused or conflated Huygens’ rotational motion, the motion of a whirling stone tied on a string and orbital motion
- these two motions are governed by different rules
Natural motion or motion?
- I have files for both motion and natural motion. We’ll see how this develops
My own development of the concept of motion
- I want to develop my own development of the story of motion.
- It looks like we have 3 reference points:
- Aristotle
- Newton
- Einstein
- Aristotle
- But we must include Kepler too.
- Kepler did not exactly believe in Galileo’s circular motion as natural motion.
- He has a notion of elliptical orbits
- Obviously, it becomes clear that when we say “motion” that we are talking about orbits
- So in our world all motion is curved motion
- Kepler’s elliptical motion is curved motion too
- Even a hyporbolic orbit is curved, if there is such an orbit.
- So Newton’ assuption of rectilinear motion on a straight line makes no sense
- No one has obsereved any straight line motion in this world.
- It looks like Einstein took Newton’s concept of rectilinear inertial motion (natural motion) without any criticism and accepted it as true
- So Einstein’s reference frames all move with Newtonian rectilinear inertial motion or uniform motion on a straight line. This is a fiction. Uniform motion on a straight line does not exist in this world. All motion is orbital motion and obey Kepler’s rule.
Kepler’s concept of natural motion
Johannes Kepler’s views on natural motion, particularly in relation to celestial bodies, were somewhat distinct from those of Galileo and Newton. Kepler’s ideas about motion were deeply influenced by his belief in the connection between geometry and the harmonies of the heavens, but he also made significant strides toward a more physical understanding of planetary motion.
Key Aspects of Kepler’s Views on Motion:
- Departure from Circular Motion:
- Unlike Galileo, who maintained the idea that circular motion could be natural for celestial bodies, Kepler rejected the notion of perfect circular orbits. He is famous for his discovery that planetary orbits are elliptical, not circular, as outlined in his first law of planetary motion.
- Kepler’s work marked a break from the Aristotelian and Ptolemaic tradition that circular motion was the natural, ideal state of celestial bodies.
- Unlike Galileo, who maintained the idea that circular motion could be natural for celestial bodies, Kepler rejected the notion of perfect circular orbits. He is famous for his discovery that planetary orbits are elliptical, not circular, as outlined in his first law of planetary motion.
- Kepler’s Force-like Concept:
- Kepler didn’t yet have the precise laws of motion that Newton would later develop, but he came very close. He speculated that there was a force exerted by the Sun, which he described as “like a magnet,” that influenced the motion of the planets.
- In his book “Astronomia Nova” (1609), Kepler discussed the idea that the Sun imparts a driving force that keeps planets in motion, which is akin to a form of natural motion but understood as driven by a physical influence (i.e., the Sun).
- This force idea was a precursor to Newton’s concept of gravity, although Kepler didn’t frame it in terms of mass and attraction.
- Kepler didn’t yet have the precise laws of motion that Newton would later develop, but he came very close. He speculated that there was a force exerted by the Sun, which he described as “like a magnet,” that influenced the motion of the planets.
- Natural Motion and Elliptical Orbits:
- Kepler believed that the natural motion of planets around the Sun was elliptical. While earlier thinkers, like Galileo and Aristotle, thought that either rest or uniform circular motion was the natural state, Kepler showed that celestial objects move in non-uniform elliptical paths, with varying speeds.
- He understood this variation through his second law of planetary motion (the law of equal areas), which describes how planets sweep out equal areas in equal times, moving faster when they are closer to the Sun and slower when they are farther away.
- Kepler believed that the natural motion of planets around the Sun was elliptical. While earlier thinkers, like Galileo and Aristotle, thought that either rest or uniform circular motion was the natural state, Kepler showed that celestial objects move in non-uniform elliptical paths, with varying speeds.
Summary of Kepler’s View on Natural Motion:
- Circular motion is not natural: Kepler moved away from the ancient and early-modern idea that circular motion was the natural state for celestial bodies. He discovered that planets follow elliptical orbits, which was a significant departure from the idea of perfect circles.
- Motion driven by the Sun: Kepler believed that the Sun exerts a force-like influence that drives the planets’ motions, an idea that laid the groundwork for Newton’s later law of universal gravitation.
- No straight-line motion: Kepler did not think in terms of Newton’s concept of natural straight-line motion, but he was closer to modern physics than earlier thinkers in considering the role of forces in shaping planetary orbits.
In contrast to both Galileo and Newton, Kepler’s work bridged the gap between the old Aristotelian ideas of natural motion and the Newtonian understanding of motion influenced by forces, especially gravity.
Newton’s inherent force and natural motion
What was Newton’s understanding of “natural motion”? Is “inertial motion” and “natural motion” refer to the same type of motion? If inertial motion is “unforced motion” why does Newton say that a body in inertial motion moves under the power of its “inherent force.” There seems to be a confusion. Explain.
Newton’s understanding of “natural motion” and “inertial motion” are related but not identical, and the confusion arises from the way terms like “inherent force” are interpreted in Newtonian physics.
Aristotle vs. Newton on Natural Motion
- Aristotle’s Natural Motion:
- For Aristotle, “natural motion” is the motion that objects undergo due to their nature, such as a rock falling toward the earth or fire rising toward the heavens. This motion is intrinsic to the object’s essence and occurs without any external force.
- For Aristotle, “natural motion” is the motion that objects undergo due to their nature, such as a rock falling toward the earth or fire rising toward the heavens. This motion is intrinsic to the object’s essence and occurs without any external force.
- Newton’s Natural Motion:
- Newton redefined the concept of natural motion in terms of inertia. For Newton, natural motion is straight-line motion at constant speed (or rest) unless acted upon by an external force, as described in his First Law of Motion. This inertial motion reflects the state of a body under no net force, aligning with the principle of inertia.
- Newton redefined the concept of natural motion in terms of inertia. For Newton, natural motion is straight-line motion at constant speed (or rest) unless acted upon by an external force, as described in his First Law of Motion. This inertial motion reflects the state of a body under no net force, aligning with the principle of inertia.
Inertial Motion as “Unforced Motion”
Newton’s First Law (the Law of Inertia) states:
> Every body persists in its state of rest, or of uniform motion in a straight line, unless compelled to change that state by forces impressed upon it.
This law defines inertial motion as “unforced motion,” meaning the body continues moving uniformly without external interference.
“Inherent Force” in Newton’s Framework
The term “inherent force” in Newton’s writings can be misunderstood without context:
- Inherent Force = Inertia:
- Newton sometimes uses the term “inherent force” (or “vis insita”) to refer to a body’s inertia, which is its resistance to changes in its state of motion or rest. This is not a force in the modern sense but a property of matter.
- Newton sometimes uses the term “inherent force” (or “vis insita”) to refer to a body’s inertia, which is its resistance to changes in its state of motion or rest. This is not a force in the modern sense but a property of matter.
- Inherent Force and Motion:
- Inertial motion under the influence of “inherent force” refers to the natural tendency of a body to maintain its current state (rest or uniform motion). It does not imply an actual force propelling the body but rather the absence of forces causing change.
- Inertial motion under the influence of “inherent force” refers to the natural tendency of a body to maintain its current state (rest or uniform motion). It does not imply an actual force propelling the body but rather the absence of forces causing change.
Resolving the Confusion
- Unforced Motion and Inherent Force:
- “Unforced motion” means no external forces are acting to change the body’s motion.
- The “inherent force” is not an active force but a passive property that ensures the body maintains its current state until a force acts.
- “Unforced motion” means no external forces are acting to change the body’s motion.
Key Takeaways
- In Newtonian mechanics, inertial motion is “natural motion” in the sense of being the default state of an unforced body.
- “Inherent force” refers to inertia, which is a property, not a force in the active sense.
- The confusion often stems from interpreting “inherent force” as something actively driving motion, which it is not.
Newton’s framework revolutionized the Aristotelian view by shifting the focus from intrinsic teleology (purpose-driven motion) to a precise, measurable understanding of forces and motion.