1. Forces in Space

2. What are the Relationships Among Objects in Space?

3. How do forces explain the structure and function of objects in space ?
Common characteristics:
shape
orbit
Differences:
size
composition

4. Why are the planets round?
Gravity!
In a newly born sun -- or the Earth -- or in any object, anywhere -- gravity pulls from the center on all the outer surfaces. Material the same distance from the center of an object is pulled with the same force -- so you end up with an object the shape of a ball.

5. Why are the asteroids irregular shaped?
The asteroids, mostly orbiting in the asteroid belt between Mars and Jupiter, are small rocky bodies tumbling through space. Because they don't have much mass, their gravity is weak -- so most asteroids have irregular shapes.

6. Why are the planets closest to the sun made of heavier materials?
Gravity and circular forces also explain why denser materials end up closer to the sun. The lightest materials were flung farthest out into space. The gravity of the sun holds the denser material closest to it.

7. Why do the planets stay in an orbit around the sun?
The answer is a balance in forces. The forward motion of the planets is balanced by the pull of gravity exerted by the sun. So, the planets stay at approximately the same distance from the sun.
Remember Sir Isaac Newton?

8. What was Newton’s big idea?
The idea that forces exist throughout the universe, and not just on Earth, made Newton’s ideas revolutionary. His laws of motion explained why the observations of other scientists were correct.

9. What did he say about forces in the universe?
Newton’s First Law of Motion
An object in motion tends to remain in motion in a straight line with a constant speed unless acted upon by an outside force. An object at rest tends to remain at rest unless acted upon by an outside force.
How does this relate to objects in space?
The planets and other bodies in space are traveling in a straight line until acted on by gravity from another object.

10. What did he say about forces in the universe?
Newton’s Third Law of Motion
Every action has an equal and opposite reaction.
How does this relate to objects in space?
The sun pulls on the planets with the same force at which the planets pull away from the sun.