1. Classroom Reminders
Always follow directions- this will keep you out of trouble
Raise your hand before speaking or leaving your seat- this will also keep your name off the board
Keep hands, feet and objects to yourself- any violation of this will get your name on the board
If your name ends up on the board and does not come off by the end of the period then you will be written up in Review 360.

2. How does gravity affect our solar system?
Bellringer
How does gravity affect our solar system?

3. Tuesday March 18, 2014
Objective: SW understand that gravity is the force that governs the motion of our solar system.

4. Historical Models of our Solar System and Kepler’s Laws of Planetary Motion

5. Geocentric Model Earth is center of our Solar System
Aristotle- Over 2000 years ago,
Unexplained on how planets appear to move backwards
Ptolemy- Planets move in small circles

6. Heliocentric Model Sun is center of our Solar System
Copernicus- Over 450 years ago (1543 ad)
First to suggest heliocentric theory and vaguely mapped out the planets orbiting the sun in a circular orbit.
Theory published on his deathbed. Initially not supported and took over 50 years for it to be accepted. Now backwards motion of planets (retrograde) was explained.

7. Galileo Supporter of Copernicus’s Heliocentric theory House arrest
Observed moons orbiting Jupiter and theorized objects can revolve around other planets not just Earth
placed under house arrest and was forced to denounce theory or be executed

8. Solar System precisely measured
Tycho Brahe- studied Solar System and made very accurate recordings of his observations
Tycho’s assistant, Kepler, used information for the details of orbits
Tycho’s published recording of the solar system not used by Kepler until after Tycho’s death

9. Kepler’s Laws of Planetary Motion
1st Law - Law of Ellipses
Each planet orbits the sun in a path called an ellipse or elongated circle
Ellipse is a closed curve whose path is determined by 2 points or foci within the ellipse
Focus 1 is the Sun and Focus 2 is an imaginary point
1st Law) Law of Ellipses
Each planet orbits the sun in a path called an ellipse or elongated circle
Ellipse is a closed curve whose path is determined by 2 points or foci within the ellipse
(circle use 1 point or focus )
One focus or point is located within the sun
The other focus or point is not within an object
The major axis runs through both foci points and the semi major axis is the planet’s average distance from the sun

10. TW demonstrate how ellipses are formed using string and markers.
Ellipse Demo
TW demonstrate how ellipses are formed using string and markers.

11. Universal gravitation
Remember that gravity is a universal force that attracts all objects that have a mass to each other.
You don’t notice the attractive force between ordinary objects because the masses are small which makes the gravity between them weak.
It takes a huge mass to create gravity that is strong enough to feel.

12. Gravity Strength
Remember the strength depends on two factors: mass and distance.
The force of gravity increases as the mass of an object increases.
The force of gravity increases as objects get closer to each other; it decreases as objects get farther apart.

13. Earth’s Gravity The strength of Earth’s gravity is 9.8 N.
In comparison, the strength of the Moon’s gravity is 1.6 N.
The Moon’s mass is much less than Earth’s, so it creates less gravity.
This means that objects on the Moon weigh less than on Earth.

14. Satellites
Remember that an orbit is the path that an object in space follows around another object.
An object in orbit is called a satellite. It can be natural like the Moon, or artificial like television satellites.
Gravity is what keeps satellites in orbit around a particular object.

15. Circular Motion and Force
Imagine a ball tied to a string. If I were to swing it around, it would move in a circle. The tension in the string is the force (gravity) that changes the ball’s direction.
What would happen if I released the string? The ball would fly off in a straight line (inertia).

16. TW demonstrate circular motion using string and ball.
Circular Motion Demo
TW demonstrate circular motion using string and ball.

17. The Sun The Sun is enormous, hot ball of gas held together by gravity.
Because of its size, the Sun contains about 99.8 percent of the mass in our solar system.
Because of its mass, the Sun’s gravity is enough to hold the entire solar system in orbit.

18. Revolving and Rotating
Planets both revolve around the Sun and rotate on their axes. An axis is the imaginary line that an object spins around.
Earth, like most of the other planets, spins from west to east. One complete rotation is called a day. One Earth day is 24 hours long. As Earth rotates, the Sun appears to move across the sky from East to West.

19. One Year
All of the planets orbit, or revolve, around the Sun in the same direction (counterclockwise).
A year is the time it takes a planet to complete one revolution around the Sun. A year on Earth takes approximately 365 days.
The farther a planet is from the Sun, the longer it takes it to complete one revolution.

20. Check Your Understanding
1. Gravity exists between all objects with mass. Why don’t you
notice the force of gravity between you and all of the objects
around you?
2. Gravitational force gets weaker as ________________ increases and gets
stronger as the _______________ of the objects increases.
3. A bag of flour weighs 22.2 newtons (5 pounds) on Earth. Will it
weigh more, less, or the same on the Moon? Why?
4. Is a satellite orbiting Earth free from Earth’s gravity? Why or why
not?
5. Do the planets move in perfect circles around the Sun?
6. Does the Moon rotate or revolve around Earth?
7. What are the meanings of the terms day and year in terms of
Earth’s motion?

21. Exit Slip
1. The force of (gravity/mass) causes the planets to orbit around the Sun.
2. Gravitational force between the Sun and the planets causes the planets to (order/orbit).
3. Gravity between Earth and the Moon keeps the Moon in orbit around the (Sun/Earth).