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**Chapter 27.2 Models of the solar system**

Std. IE 1k- Students will recognize the cumulative nature of scientific evidence Objective 1: Compare models of the solar systems Objective 2: Summarize Kepler’s three laws of planetary motion Objective 3: Describe how Newton explained Kepler’s laws of motion

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**Models of the Solar System**

Geocentric – Earth centered model (Aristotle) the sun, stars, and planets revolve around the Earth Did not explain retrograde motion - the appearance of some planets moving backward

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**Epicycles – small circle movements (Ptolemy)**

Heliocentric model - the sun the center (Nicolaus Copernicus)

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**Objective 1: Compare models of the solar systems**

What is the geocentric model? Earth centered model What is the heliocentric model? Sun centered model What is the movement of planets in small circles? Epicycles What is the appearance of planets moving backward in the sky? Retrograde motion

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Kepler’s Laws Johannes Kepler discovered patterns which led him to develop 3 laws that explained planetary motion 1) Law of Ellipses 2) Law of Equal Areas 3) Law of Period A Polish clergyman named Nicholas Copernicus figured out by 1543 that those motions made sense if planets moved around the Sun, if the Earth was one of them, and if the more distant ones moved more slowly --so sometimes the Earth overtakes them, and they seem to move backwards for a while. Brilliant mathematician Interested in astronomy early – saw the great comet 1577 and luner (red) eclipse Small pox left him visually impaired and hands were crippled

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1) Law of Ellipses 1st law - States that each planet orbits the sun in a path called an ellipse (not a circle ) Elliptical orbits can vary in shape (elongated or circles) Eccentricity – the degree of elongation of an elliptical orbit explains that planets are orbiting the sun in a path described as an ellipse As the 2 objects are located closer together, the shape is similar to circle

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2) Law of Equal Areas 2nd law - states that equal areas are covered in equal amounts of time as an object orbits the sun It describes the speed at which objects travel at different points in their orbits Ex. Mars moves fastest in its elliptical orbit when it is closest to the sun describes the speed at which any given planet will move while orbiting the sun

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3) Law of Periods 3rd law – states the relationship between the average distance of a planet from the sun & the orbital period of the planet Scientist can find out how far away the planets are from the sun by using this law Orbital Period - the time required for a body to complete a single orbit Unlike Kepler's first and second laws that describe the motion characteristics of a single planet, the third law makes a comparison between the motion characteristics of different planets.

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**Objective 2: Summarize Kepler’s three laws of planetary motion**

Which of Kepler’s laws states that equal areas are covered in equal amounts of time? 2nd law – Law of Equal Area Which of Kepler’s laws states that each planet orbits the sun in a path called an ellipse? Law of Ellipses What is the time required for a body to complete a single orbit? Orbital Period

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**Newton’s Explanation of Kepler’s Law**

Newton used inertia to help explain Kepler’s laws. Inertia - a moving body will remain in motion and resist a change in speed or direction until an outside force acts on it Kepler’s laws explained how the planets moved, but not why they moved Plus 3rd law explained how planets moved around the sun, but did not explain the movement of moons Newton's Laws of Motion and Newton's Universal Law of Gravitation. 1st law of motion (inertia) – if there is acceleration then there is a force 2nd law of motion (F = ma) – calculates the force 3rd law of motion ( action reaction) according to Newton's first law of motion, there must be a force acting on the planet that is always directed toward the center of the orbit -- that is toward the Sun! Newton's second law of motion allows us to state what the magnitude of that force

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**Newton’s Models of Orbits**

Planets do not follow a straight path Gravity = an outside force that cause the orbit to curve Gravity exists between any two objects in the universe Formation of curved orbits Gravitational pull sun pulls objects towards it Inertia keeps the object moving in a straight line Universal law of Gravitation - Fgravity = G M1 M2 / r2 G = 6.67 X 10-11 meters3 kilograms-1 seconds-2

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**Objective 3: Describe how Newton explained Kepler’s laws of motion**

What exists between any 2 objects in the universe? Gravity What is a body remaining in motion unless acted on by an outside force called? Inertia What is pulling the planets toward the sun? What gives planets their curved orbits? Inertia and gravitational pull

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