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What is kepler's 2nd law of planetary motion? Why is kepler's second law important?


Kepler's Second Law, The Law of Equal Areas:

Kepler's second law of planetary motion states that "a line joining a planet and the Sun sweeps out equal areas in equal time intervals". This means that the planet moves faster when it is closer to the Sun and slower when it is farther away. To understand Kepler's second law, it is helpful to think about the concept of angular momentum. Angular momentum is the product of an object's mass, its velocity, and the distance from the object to the center of rotation. In the case of a planet orbiting the Sun, the angular momentum of the planet is constant.

Kepler's second law of planetary motion.


Angular momentum = Mass*Velocity*Distance from Sun to celestial body.

L = MVR

Now, angular momentum is constant (as angular momentum is conserved). Therefore, L and M are constant. R is a variable as the orbit is elliptical in shape (from the first law of Kepler). So, to compensate for the variable R, the velocity V of the planet changes, As R increases V decreases, and vice versa. Hence, keeping the angular momentum constant.

The change in the planet's velocity is what allows it to sweep out equal area in equal time intervals. When the planet is closer to the Sun, it has high velocity and travels a greater orbital distance. When the planet is farther away from the Sun, It has lower velocity and travels a smaller orbital distance in a particular time but the area swept is equal in both cases for that particular time

Kepler's second law has a number of important implications. First. it shows the planets do not move with constant speed throughout their orbits. Second, it shows that the angular momentum of the planet orbiting the sun is constant. Third it shows that the closer the planet is to the sun, the faster it moves.

Kepler's second law is one of the three laws of planetary motion that was first published by Johannes Kepler in 1609. The other two laws are Kepler's first law and Kepler's 3rd law of Planetary motion.

Kepler's law of planetary motion was a major breakthrough in the understanding of the solar system. It provided a mathematical description of the motion of the planet that was consistent with observation. Kepler's law also confirmed the heliocentric model of the solar system, which places the Sun at the center of the solar system and planets orbit around the sun. Kepler's laws of planetary motion are still used today to study the motion of planets, moons, comets, and other objects in the solar system. They are also used to study the motion of stars and galaxies in the other part of the universe.

Geocentric and Heliocentric Model.



Nuances and subtleties to Kepler's second law.

There are a few nuances and subtleties to Kepler's second law that are worth noting.

  •  First, the law only applies to closed orbits and open orbits such as the orbit of a comet does not sweep out equal areas in equal time intervals. 
  • Second. the law only applies to the orbits that are not perturbed by the other objects. The orbits of the planet in the solar system is perturbed by the gravitational pull of other objects or planets. This perturbation causes the planets to move at a slightly varying speed throughout their orbits.
  •  Finally, the law only applies to orbits that are relativistic The effect of special relativity must be taken into account. This causes the planet to move at slightly different speeds throughout its orbits 
Despite these nuances and subtleties, Kepler's second law is a remarkably accurate description of the motion of planets in the solar system. It is a testament to Kepler's genius that his laws of planetary motion are still used today to study the motion of objects in the solar system and beyond.



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