1. Gravity and Inertia

2. What is gravity?
Gravity is a force of attraction between objects that is due to their masses and the distances between them.
Objects with greater masses have a greater force of attraction than objects with lesser masses have. 
Objects that are close together have a greater force of attraction than objects that are far apart have. 

3. Why is gravity important?
Gravity accounts for the formation of planets, stars, and galaxies. 
It also keeps smaller bodies in orbit around larger bodies. 

4. Johannes Kepler
During the 1600s, a German astronomer named Johannes Kepler made careful observations of planets and their movements around the Sun. 
Kepler discovered that planets travel around the Sun in a path called an elliptical orbit.  
aphelion - where the object is farthest from the sun
perihelion where the object is closest to the sun.
One complete orbital path around the Sun is a revolution. 

5. Sir Isaac Newton
Sir Isaac Newton made several discoveries that helped explain Kepler’s observation about the orbits of planets.  
Newton’s law of universal gravitation states that all objects in the universe attract each other through gravitational force.

6. Gravity and Planetary Motion
The inward force that causes an object to move in a circular path is called centripetal force

7. Newton's 1st Law of Motion - Inertia
Inertia is the tendency of an object in motion to continue moving in a straight line unless acted upon by another force. 
Planets are held in orbit around the Sun due to the balanced forces of gravity and inertia.
If gravity of the sun disappeared, the planets would fly off into space in a straight line

9. Lesson Check
What two forces are responsible for keeping the planets in orbit of the sun?
What two factors affect gravitational pull?
Label the model below with the words gravity and inertia along with arrows showing which way the force is moving.
GRAVITY AND INERTIA
MASS AND DISTANCE
INERTIA
GRAVITY