1. May the Force Be With You

2.  Every object in the universe has a mass that exerts a pull (force) on every other mass.  The size of the pull (force) depends on the mass of the object and the distance between objects.

3.  Example: Gravitational pull of the Earth (large mass, strong pull) on molecules of atmospheric gases (small mass, weak pull) keeps the atmosphere close to Earth.  Example: You don’t get pulled from your seat into the Sun because the Earth is exerting a stronger gravitational force on your body than the Sun. This force is stronger because you are closer to the Earth than the Sun.

4.  Everything. You exert a gravitational force on your chair, the floor, and even your classmates sitting beside you.  The force you exert isn’t very strong because people aren’t as massive as Earth.

5. Examples: Mass of Earth = about 6.0 X 10 24 kg (Move the decimal 24 spaces to the right.) 6,000,000,000,000,000,000,000,000 kg Mass of fully loaded plane = about 7.0 X 10 4 kg Average mass of human = 68.5 kg

6.  Isaac Newton was a 17th century English physicist who developed three laws of motion and the law of universal gravitation.  These laws explain motions we observe on Earth and of planets, stars, galaxies, and other celestial bodies.

7.  Myth: An apple fell on his head causing him to define gravity  Fact: He watched an apple fall and wondered if all things were governed by this action. He asked questions, experimented, and used math to find the answers.

8.  Newton’s law of universal gravitation, states that the gravitational force between two objects is proportional to the mass of each and inversely proportional to the distance between them.

9. Large Mass Larger Mass Close Stronger Attraction Large Mass Small Mass Far Apart Weaker Attraction

11. The attractive force of gravity acts between the center of two objects.

12.  In the case of the figures standing on the Earth's surface, the effect of gravity attracts them towards the center of the Earth.  No matter where they stand on the Earth, they don't fall off.

13.  The Sun’s gravitational pull keeps the Earth orbiting the Sun.  The Moon’s gravitational pull on the Earth creates a swell in sea level causing tides.

14.  A gravitational attraction between the Earth and Sun can increase the effect of the Moon on tides.

15.  Planets are larger than their moons and keep the moons from falling away from the planet.  The moons would travel in a straight line without a force acting on it.

16.  Because of the force of the attraction of the planet to the moon, the moon’s straight-line fall is pulled into a curve.  The moon revolves around the planet because of the continuous pull of gravity.

17.  Because of the Earth’s gravity, satellites and the International Space Station (ISS) orbit the Earth.  Astronauts in space appear to be weightless because they are in a state of “free fall.” Due to gravity, the space vehicle and their bodies are falling around (because they are orbiting) the Earth (because they are orbiting) at the same rate so they appear to be floating.

18.  Our solar system has an elliptical shape. The majority of the objects in our solar system stay within this area.

19.  The Sun and most of the planets and their moons stay with in this elliptical area. The dwarf planet Pluto and the comets are the exception.

20.  What is gravity?  How does gravity affect objects?  How do the objects in our solar system stay in their respective orbits?