1. Stars, Galaxies, and the Universe- Course 3
3-3 Gravity and Motion

2. Vocabulary
Orbit-The path of an object as it revolves around another object in space. Gravity-The attractive force between objects, the force that moves objects downhill. Law of universal gravitation- The scientific law that states that every object in the universe attracts every other object. Mass- a measure of how much matter is in an object. Weight- a measure of the force of gravity acting on an object. Inertia- The tendency of an object to resist a change in motion. Newton’s first law of motion-The scientific law that states that an object at rest will stay at rest and an object in motion will stay in motion with at constant speed and direction unless acted on by a force. Accretion-The process of building something up gradually by the gathering together of smaller pieces.
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3. My Planet Diary pg. 104
Gravity Assists You might think that gravity only brings objects down. But gravity can also speed things up and send them flying! If a space probe comes close to a planet, the planet’s gravity changes the probe’s path. Engineers plan space missions to take advantage of these “gravity assists.” A gravity assist can shorten the probe’s interplanetary trip by many years. The diagram shows how the probe Voyager 2 used gravity assists to visit all four outer planets! How does a planet’s gravity change the path of a space probe? ______________________________________________________________________________________________________

4. Gravity and Motion Gravity Assists
How does a planet's gravity change the path of a space probe?

5. Gravity and Motion
Gravity, Mass, and Distance
Compare and contrast the force of gravity in the second and third pictures. Draw arrows showing the force of gravity in the second and third pictures.

6. What Determines Gravity? Pg. 104
Objects in space are affected by different forces. A force is a push or a pull. A force called gravity attracts all objects toward each other. Sir Isaac Newton’s law of universal gravitation states that every object in the universe attracts every other object. The strength of the force of gravity between two objects depends on two factors: 1. the masses of the objects 2. the distance between them.

7. Gravity, Mass, and Weight pg. 105
Mass is the amount of matter in an object.
Because Earth is so massive, it exerts a much greater force on you than this book does.
Weight is the measure of the force of gravity on an object.
An object’s weight can change depending on its location.
On the moon, you would weigh about one sixth as much as on Earth. That’s because the moon has less mass than Earth, so the pull of the moon’s gravity on you would also be less.
The force of gravity between two objects decreases rapidly as distance between the objects increases. If the distance doubles, the force of gravity decreases to one fourth of its original value.

8. Assess Your Understanding pg. 105
I get it! Now I know that the force of gravity depends on ______________________________________________________________________________________________________

9. What Keeps Objects in Orbit? Pg. 106
Earth and the moon remain in orbit because of inertia.
Inertia is the tendency of an object to resist a change in motion.
You feel the effects of inertia when you are riding in a car and it stops suddenly. You keep moving forward until your seat belt catches you.
The more mass an object
has, the greater its inertia.
An object with greater
inertia is harder to start or
stop.

10. What Keeps Objects in Orbit? Pg. 106
Newton stated his ideas about inertia as a scientific law.
Newton’s first law of motion says that an object at rest will stay at rest and an object in motion will stay in motion with a constant speed and direction unless acted on by a force.
Newton concluded that inertia and gravity combine to keep Earth in orbit around the sun and the moon in orbit around Earth.

11. Orbital Motion
Why would the motion of the moon change if the mass of Earth Increased?

12. Gravity vs. Distance pg. 107 do the math! Gravity Versus Distance
As a rocket leaves a planet’s surface, the force of gravity between the rocket and the planet changes. Use the graph to answer the questions below.
The variables being graphed are
______________ and ____________
2. What is the force of gravity on the rocket at the planet’s surface?
______________________________
3. What is the force of gravity on the rocket at two units (twice the planet’s radius from its center)?
4. In general, how does the force of gravity on the rocket change as its distance from the planet increases?

13. Assess Your Understanding pg. 107
1a. What two factors keep a planet in orbit around the sun? ______________________________________________________ b. What keeps earth from falling into the sun? c. How would a planet move if the sun suddenly disappeared? Explain. ______________________________________________________________________________________________________________ I get it! Now I know that the objects are kept in orbit by __________ _______________________________________________________

14. How Does Gravity Help Form Planets, Stars, and Solar Systems? Pg. 108
Scientists think that gravity helps form solar systems by bringing material together in a gradual buildup. The process of building something up gradually by the gathering together of smaller pieces is called accretion. When chunks of material get mashed together, their increased mass increases their gravitational pull, attracting more material. Think of marbles on a rubber sheet, The group of marbles will cause the sheet to curve at the center from the weight of the marbles. This makes the sheet bend more as more marbles are added making it more likely that new marbles will roll into the center

15. Gravity Forms Solar Systems pg. 108
Scientists think that all solar systems begin as clouds of materials including hydrogen, helium, rock, ice, and other materials. For any solar system to form, gravity must pull the cloud’s material together. As the cloud collapses and starts to rotate, it forms a spinning disk. Most of the material in the disk is pulled to the center and packed tightly as it undergoes accretion.

16. Gravity Forms Stars and Planets pg. 109
Gravity Forms Stars: When the pressure on the material gets high enough, nuclear fusion occurs, causing the star to form and release energy. Gravity Forms Planets: Some material remains outside the star and begins to form into planets. Gravity pulls rock and ice together, forming planets and other bodies. Gravity Keeps Solar Systems Together: The star at the center of a solar system exerts a large gravitational force on all other objects in the solar system, so they stay in orbits around the star.

17. Assess Your Understanding pg. 109
2a. What is accretion?_________________________________ ___________________________________________________ b. What would happen to a solar system if gravity suddenly stopped working? _____________________________________ I get it! Now I know that gravity helps form planets, stars, and solar systems by _____________________________________