Kepler’s Laws of Planetary Motion Bormann Honors Science - 8

Lets make some connections… Aristotle – ( BC) Aristotle – ( BC) Nicolaus Copernicus ( ) Nicolaus Copernicus ( ) Tycho Brahe ( ) Tycho Brahe ( ) Johannes Kepler ( ) Johannes Kepler ( ) Galileo Galilei ( ) Galileo Galilei ( ) Isaac Newton ( ) Isaac Newton ( )

Johannes Kepler came before Newton’s time. Between the years (1571 and 1630) he developed Johannes Kepler came before Newton’s time. Between the years (1571 and 1630) he developed Who is Johannes Kepler? a quantitative description of the motions of the planets in our solar system  We classify these descriptions as the laws of planetary motion

Kepler’s 1 st Law Orbits of planets are ellipses with the Sun at one focus. **Aphelion is the point on the orbit furthest to the Sun **Perihelion is the point on the orbit closest to the Sun

What is an ellipse? Circle – has same diameter whether you measure across or up an down. Ellipse – has diameters of different length Major axis – longest length Minor axis – shortest length The ratio of the axis lengths determines the eccentricity of the ellipse. Eccentricity (e) – measure of how elliptical a planet is Circle e=0 Very stretched out ellipse e=1 Mercur y 0.206Saturn0.054 Venus0.007Uranus0.048 Earth0.017Neptune0.007 Mars0.094Pluto0.253 Jupiter0.048

Kepler’s 2 nd Law The Speed of Planets  A line from a planet to the Sun sweeps out equal areas in equal times.  More simple way to say it: planets move faster when closer to the Sun. 2 nd Law Animation

Kepler’s 3 rd Law The square of a planet’s period equals the cube of the semi-major axis (average distance between the planet and its Sun.. Kepler's Laws Animation Period (P) – The time it takes for one objet to make one complete orbit around another object. Distance (d) – distance between planet and the sun measured in Astronomical Units. Astronomical Unit (AU) – convenient way to measure distances in the solar system. 1 AU is the distance from Earth to the Sun (also equals to 150 million km or 93 million miles.

Planet T(yrs) R(au) T 2 R 3 Venus Earth Mars Jupiter When we compare the orbits of the planets… We find that T 2 and R 3 are essentially equal.

Kepler was able to describe the motion of the planets, however he didn’t provide an explanation as to WHY the planets move this way. Kepler was able to describe the motion of the planets, however he didn’t provide an explanation as to WHY the planets move this way. Isaac Newton came along in Isaac Newton came along in Newton built on Kepler’s Laws by using as a tool for understanding Newton built on Kepler’s Laws by using as a tool for understanding He provided the general explanation of the motions of planets through He provided the general explanation of the motions of planets throughand What does all of this have to do with Isaac Newton? 1642 (died in 1727) Newton’s Laws of Motion The Universal Law of Gravitation mathematics physics

Kepler and Newton Kepler’s Laws define the motion of the planets, but Newton’s Laws define motion. Kepler’s Laws define the motion of the planets, but Newton’s Laws define motion. Newton realized that all motion, regardless if it is occurring on a small scale in front of you or a large scale in space, follows the same basic principles. Newton realized that all motion, regardless if it is occurring on a small scale in front of you or a large scale in space, follows the same basic principles.

Discuss with a partner – How do you think Newton used his laws of motion to expand on Kepler’s Laws of Planetary Motion? – Be ready to share! 1 - An object in motion will stay in motion and an object at rest will stay at rest until acted upon by an unbalanced force. 1 - An object in motion will stay in motion and an object at rest will stay at rest until acted upon by an unbalanced force. 2 - Acceleration of an object is dependent on the net force acting upon the object and the mass of the object. 2 - Acceleration of an object is dependent on the net force acting upon the object and the mass of the object. 3 – For every action there is an equal and opposite reaction. 3 – For every action there is an equal and opposite reaction. Universal Law of Gravitation – Any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of distance between them. Universal Law of Gravitation – Any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of distance between them.

Putting it all together… A planet would move in a straight path at a constant speed forever unless an unbalanced force acted on it. So why do planets move in an ellipse around the Sun? A planet would move in a straight path at a constant speed forever unless an unbalanced force acted on it. So why do planets move in an ellipse around the Sun? Because of gravity why don’t planets just get sucked into the Sun? Because of gravity why don’t planets just get sucked into the Sun? Why do planets move faster when closer to the Sun? Why do planets move faster when closer to the Sun? A force constantly tugging at the planet (gravity) which would cause it to curve inward. An orbit is the balance between inertia and gravitational force. The planets in our solar system continually fall toward the Sun, but inertia also wants them to keep moving in a straight line, when these two balance out it results in a stable orbit. The closer the planet is to the Sun the stronger the gravitational force between them, increasing the acceleration of the object. The more distance, the slower they will move which enables the sun to still have a gravitational effect curving the planet toward the sun keeping it in its orbit.