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.

Bellringer

•How does gravity affect our solar system?

Tuesday March 18, 2014

•Objective: SW understand that gravity is the force that governs the motion of our solar system.

Historical Models of our Solar Systemand

Kepler’s Laws of Planetary Motion

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

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.

Galileo• Supporter of

Copernicus’s Heliocentric theory

• House arrest • Observed moons

orbiting Jupiter and theorized objects can revolve around other planets not just Earth

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

Kepler’s Laws of Planetary Motion1st 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

Ellipse Demo

•TW demonstrate how ellipses are formed using string and markers.

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.

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.

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.

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.

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).

Circular Motion Demo

•TW demonstrate circular motion using string and ball.

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.

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.

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.

Check Your Understanding1. Gravity exists between all objects with mass. Why don’t younotice the force of gravity between you and all of the objectsaround you?2. Gravitational force gets weaker as ________________ increases and getsstronger as the _______________ of the objects increases.3. A bag of flour weighs 22.2 newtons (5 pounds) on Earth. Will itweigh more, less, or the same on the Moon? Why?4. Is a satellite orbiting Earth free from Earth’s gravity? Why or whynot?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 ofEarth’s motion?

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).