1. Unit F, Chapter 1, Section 1 pF6-11
Gravity
Unit F, Chapter 1, Section 1
pF6-11

2. Forces Make Things Move
Force: push or pull
Act of objects
Often affect motion of objects
Cause objects to:
Speed up
Slow down
Stop
Change direction
Gravity – force always acting on us

3. The Force of Gravity
Gravitation: every piece of matter in the universe, no matter how large or small, pulls on every other piece of matter
Gravity: force that pulls us and everything around us toward the center of the Earth
“Gravitation” on Earth

4. The Force of Gravity Why are planets and stars “round?”
Gravity: force that pulls everything to Earth’s center
Pulls on solids:
Us and all objects around us
Pulls on gases
Keeps our atmosphere in place
Moon – weaker gravitational pull
Couldn’t hold an atmosphere if it had one
Pulls on liquids
Holds oceans tight to Earth’s surface
Keeps ocean levels fairly level

5. The Force of Gravity What causes the tides on Earth?
Attraction between moon and Earth’s oceans
Oceans bulge toward moon on one side of Earth
Oceans bulge away from moon on other side of Earth
Remember:
Gravitation: every piece of matter in the universe, no matter how large or small, pulls on every other piece of matter
The moon is a piece of matter in universe
The water on Earth is a piece of matter in the universe

6. Mass & Gravity Mass: amount of matter in an object
Objects with large masses affect the motion of other objects
Two objects have same gravitational pull on each other
Gravity affects movement of smaller objects more
Relationship: Increase mass= stronger force

7. Mass & Gravity
Objects with large masses affect the motion of other objects
Earth-sun system:
Gravitational force between Earth and sun
Sun’s force on Earth keeps Earth in orbit
Earth’s force on sun isn’t noticeable
Solar systems with star and very massive planet
Gravitational force between massive planet and star
Star’s force on planet keeps planet in orbit
Massive planet’s force on star makes star wobble

8. Mass & Gravity
Q: Why don’t we see all the objects around us moving toward each other?
Remember: Gravitation: every piece of matter in the universe, no matter how large or small, pulls on every other piece of matter
A: Masses are too small
Objects move toward Earth because of large mass

9. Distance & Gravity Gravitational force is affected by: Mass of objects
Distance between objects
More distance between two objects the weaker the gravitational force between them

10. (It is 330,000 times as massive as the Earth)
Distance & Gravity
Q: Why don’t we fly off the Earth toward the sun?
(It is 330,000 times as massive as the Earth)
A: We are much closer to the center of the Earth
We are much farther from the center of the sun
We feel Earth’s gravitational force pulling on us more than the sun’s gravitational force

11. Distance & Gravity Distance between objects affect gravitational force
Compare force of gravity on two people:
Person standing at sea-level
Force of gravity is slightly more
Closer to Earth’s center
Person standing on tall mountain
Force of gravity is slightly less
Farther from Earth’s center
Relationship: Increase distance = weaker force

12. Distance & Gravity Gravitational pull is affected by:
Distance between objects
Spacecraft problem
Strong gravitational force when rocket is close to Earth
Need incredible force to escape gravitational pull
Gravitational pull weakens as spacecraft moves farther

13. How Gravity is Measured
Scale: instrument used to measure force of gravity
Weight measure of the force of gravity on an object
Newton: SI (metric) unit for weight
All forces can be measured in Newtons
Weight is just one type of force
Mass: amount of matter in an object
Stays the same no matter where you go in universe
Weight depends on gravitational force where object is found

14. Summary Forces act on objects and often change motion
Gravitational attraction pulls all objects toward each other
Gravitation strength depends on:
Masses of objects
As masses increase, gravitational attraction increases
Distance between objects
As distance increases, gravitational attraction decreases
Forces are measured in Newtons
Including force of gravity

15. Homework
Read pF6-11
Answer ?s 1-10 on Goggle Form (or get a printed copy of the questions)