2. Kepler's Law of Planetary Motion

Johannes Kepler formulated three empirical laws describing planetary motion.

First Law (Law of Orbits)

Every planet moves around the Sun in an elliptical orbit with the Sun occupying one focus.

Alternatively:

The orbit of each planet about the sun is an ellipse with the sun at one focus. The planet’s closest approach to the sun is called aphelion and its farthest distance from the sun is called perihelion.

Second Law (Law of Equal Areas)

A line joining the planet and the Sun sweeps out equal areas during equal intervals of time. This implies that Planet moves faster when closer to the Sun and Planet moves slower when farther away. This occurs because gravitational attraction becomes stronger when the planet is nearer to the Sun.

Third Law (Law of Periods)

The square of the orbital period is directly proportional to the cube of the semi-major axis.

Where T = orbital period

r = orbital radius

uses of Kepler's Laws

  1. it is used for Satellite design
  2. use for Space exploration
  3. for Prediction of eclipses
  4. useful in Planetary motion
  5. for as Astronomy

 Gravitational Potential Energy

Gravitational Potential Energy (GPE) is the energy possessed by an object because of its position in a gravitational field.

Near Earth's surface:

Example1: A 10 kg object is lifted through 20 m. Find the gravitational potential energy.