Difference Between Kepler And Newton Law

The laws of Kepler and Newton form the backbone of classical mechanics, each laying down fundamental principles that describe the motion of bodies under the influence of forces. Johannes Kepler, through empirical observation, deduced three laws of planetary motion that describe how planets orbit the Sun. On the other hand, Sir Isaac Newton, building on Kepler’s findings and those of his contemporaries, formulated the laws of motion and universal gravitation, providing a comprehensive framework that explains not only planetary motion but also the behavior of objects on Earth.

Kepler’s laws highlight that planets move in elliptical orbits with the Sun at one focus, sweep equal areas in equal times, and the squares of their orbital periods are proportional to the cubes of the semi-major axis lengths of their orbits. Newton’s laws further explain that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force, the relationship between acceleration and applied force, and that for every action, there is an equal and opposite reaction.

These principles not only revolutionized the way we understand the heavens but also laid the groundwork for modern engineering and physics, offering predictive power and explanatory depth that have stood the test of time. These laws, distinct yet complementary, are crucial for the advancement of astrophysics and mechanical engineering.

Kepler’s Laws Overview

Johannes Kepler, through meticulous observation and calculation, formulated three laws that describe planetary motions. These laws not only explain how planets move but also provide insights into the gravitational interactions that govern their orbits.

First Law: Elliptical Orbits

Kepler’s First Law, often called the Law of Ellipses, states that planets move in elliptical orbits with the Sun at one focus. Unlike a perfect circle, an ellipse has two focal points, and the Sun occupies one of these points. This was a groundbreaking revelation because it challenged the long-held belief that orbits were perfectly circular.

  • Key Points:
    • Planets orbit in shapes called ellipses.
    • The Sun is not at the center but at a focal point of the ellipse.
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Second Law: Equal Areas

Kepler’s Second Law or the Law of Equal Areas describes the speed at which any planet travels through space. According to this law, a line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time. This means that a planet moves faster when it is closer to the Sun and slower when it is farther from the Sun.

  • Key Points:
    • A line from a planet to the Sun covers equal areas in equal times.
    • Planets accelerate as they near the Sun and decelerate as they move away.

Third Law: Harmonic Law

The Third Law, known as the Law of Harmonies, compares the orbital period and radius of orbit of a planet to those of other planets. It states that the square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit. This law allows astronomers to predict the position of planets in their orbits.

  • Key Points:
    • The square of the orbital period (T^2) is proportional to the cube of the semi-major axis (a^3) of the orbit.
    • This relationship holds true for all planets.

Newton’s Law of Motion

Isaac Newton built on Kepler’s laws and others’ work to formulate his three laws of motion, which explain the relationship between a body and the forces acting upon it, and its motion in response to those forces.

First Law: Inertia

Newton’s First Law, also known as the Law of Inertia, states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. It introduces the concept of inertia, which is the tendency of an object to resist changes in its state of motion.

  • Key Points:
    • Objects at rest stay at rest unless a force acts upon them.
    • Objects in motion stay in motion with the same speed and in the same direction unless acted upon by an external force.

Second Law: F=ma

The Second Law defines the relationship between an object’s mass, its acceleration, and the applied force. This law is often expressed in the equation F=ma, where F is the force applied, m is the mass of the object, and a is the acceleration caused by the force.

  • Key Points:
    • Force equals mass times acceleration (F=ma).
    • The acceleration of an object depends on the mass of the object and the amount of force applied.

Third Law: Action and Reaction

Newton’s Third Law states that for every action, there is an equal and opposite reaction. This means that for every force there is a reaction force that is equal in size, but opposite in direction.

  • Key Points:
    • Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
    • This law explains the nature of forces between two interacting objects.
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Newton’s Law of Gravitation

Universal Gravitation Concept

Newton’s Universal Law of Gravitation states that every point mass attracts every other point mass by a force acting along the line intersecting both points. This law explains that the gravitational force between two objects depends on their masses and the distance between their centers.

  • Key Points:
    • All objects with mass are attracted to each other.
    • Gravitational force decreases with the square of the distance between the objects.

Formula and Application

The gravitational force between two masses is given by the equation: 𝐹=𝐺𝑚1𝑚2𝑟2F=Gr2m1​m2​​ where 𝐹F is the magnitude of the gravitational force between the two masses, 𝐺G is the gravitational constant, 𝑚1m1​ and 𝑚2m2​ are the masses of the objects, and 𝑟r is the distance between the centers of the two masses.

  • Applications:
    • Calculation of the gravitational force between Earth and other objects.
    • Understanding the trajectories of satellites and spacecraft.

Key Differences

Understanding the distinctions between Kepler’s laws and Newton’s contributions illuminates their respective impacts on the field of physics and beyond. While both sets of laws describe planetary motions, their approaches and implications differ significantly.

Conceptual Foundations

Kepler’s laws are derived from empirical observations and focus on the motions specific to celestial bodies in the solar system. These laws describe how planets move but do not explain why they move as they do. In contrast, Newton’s laws provide a theoretical framework that explains the forces behind these motions. Newton introduces concepts of mass, gravity, and force to explain not only the motion of celestial bodies but also of objects on Earth.

Mathematical Formulations

  • Kepler’s Laws:
    • Elliptical paths with the Sun at one focus.
    • Planets sweep equal areas in equal time spans.
    • A proportional relationship between the square of the orbital period and the cube of the orbit’s semi-major axis.
  • Newton’s Laws:
    • Objects in motion remain so unless acted upon by a force (Inertia).
    • Force equals mass times acceleration (F=ma).
    • Every action has an equal and opposite reaction.

Newton’s formulas involve calculus, a mathematical method he helped develop, which provides the tools needed to derive Kepler’s laws from his theory of gravity and motion.

Applications in Astronomy

Kepler’s laws are fundamental in the field of astronomy for calculating the positions of planets and other celestial bodies in their orbits. Newton’s laws, however, extend these applications further by enabling calculations involving the motion of any object under any force, not just celestial bodies under gravitational pull.

Practical Examples

To illustrate how these laws apply in practical scenarios, consider their implications in understanding planetary motions and designing satellite trajectories.

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Planetary Motions

Kepler’s laws allow astronomers to predict the position of planets and their travel paths around the sun, essential for both basic astronomy and complex missions involving planetary exploration. For example, knowing that planets sweep equal areas in equal times helps scientists predict the speed of a planet at any point in its orbit.

Satellite Trajectories

Newton’s laws are critical in the field of satellite deployment and trajectory planning. The understanding of gravitational forces and motion dynamics is crucial for placing satellites into orbit and ensuring they maintain their paths without colliding with other objects or re-entering the Earth’s atmosphere prematurely.

Impacts on Modern Science

The implications of Kepler’s and Newton’s laws extend far beyond their initial applications in celestial mechanics.

Advances in Physics

Newton’s laws laid the groundwork for classical mechanics, which has evolved into more advanced theories like relativity and quantum mechanics. These developments have transformed our understanding of not only motion and force but also energy, space, and time.

Technological Applications

The principles derived from both Kepler’s and Newton’s laws have numerous technological applications. These range from the calculation of orbital mechanics for satellites to the design of bridges and buildings that must withstand various forces. Technological advances in GPS systems, telecommunications, and space exploration all rely on principles outlined by Kepler and Newton.


How do Kepler’s laws differ from Newton’s laws?

Kepler’s laws specifically describe the motion of planets in their orbits around the Sun, focusing on the paths and speeds at which they travel. Newton’s laws, however, provide a broader framework applicable to any objects in motion or at rest, explaining the fundamental principles of dynamics and the forces that govern motion.

Why are Kepler’s laws important?

Kepler’s laws are crucial for understanding celestial mechanics, particularly the orbital behavior of planets and other bodies in space. These laws help astronomers predict planetary positions, understand the orbital dynamics within solar systems, and navigate space missions.

Can Newton’s laws explain all motions?

While Newton’s laws form the basis of classical mechanics, they do not fully explain motions at very high speeds or on the quantum scale. For such scenarios, relativistic and quantum mechanics theories are needed, which were developed after Newton’s time.

What practical applications do these laws have?

Both Kepler’s and Newton’s laws have numerous practical applications, from predicting the movement of celestial bodies to designing spacecraft trajectories and satellites. They are also fundamental in technologies like GPS navigation and other systems that rely on understanding the physics of motion.


The foundational principles set forth by Kepler and Newton continue to impact our understanding of the universe and the development of modern technology. Their laws have not only helped us navigate the vast expanse of space but have also facilitated significant advancements in various scientific fields. The continued relevance of these laws in both theoretical and applied physics underscores their importance in our ongoing quest to decipher the cosmos and harness the laws of nature for technological advancement. Their legacy is a testament to the enduring power of keen observation and rigorous scientific inquiry.

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