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

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

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

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

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

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

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

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

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

(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

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

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

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

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

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