1. Gravity

2. SPI 0807.12.4 Distinguish between mass and weight using appropriate measuring instruments and units.
SPI Determine the relationship among the mass of objects, the distance between these objects, and the amount of gravitational attraction.
SPI Illustrate how gravity controls the motion of objects in the solar system.

4. Who discovered gravity?
Edwin Hubble
Isaac Newton
Thomas Jefferson
Albert Einstein

5. ______ is the measure of the force of gravity on an object
mass
weight
density
equation

6. A push or pull on an object is a/an .
inertia
gravity
force
momentum

7. The rate of change in speed or direction of movement.
acceleration
mass
gravity
friction

8. Key Words
gravity law of universal gravitation Newton’s First Law of Motion Inertia Weight mass

9. The Idea
During the 1600’s, Sir Isaac Newton thought about why objects fall to Earth.
Newton realized that a force acts on objects and pulls them toward Earth’s center.
He soon theorized that this force was everywhere in the universe, not just on Earth.
Using Newton’s theories, astronomers were able to explain the motions of the planets and other processes in the universe.

10. The Law of Universal Gravitation
GRAVITY- is an attractive force that pulls objects toward each other.
The LAW OF UNIVERSAL GRAVITATION states that the force of gravity acts between all objects in the universe.
A gravitational attraction exists between any two objects, from the smallest particle of an atom to the largest stars.

11. Gravity
Gravity is the force that keeps you grounded on Earth and prevents you from floating off into space.
The law of universal gravitation explains that, just as you are attracted to Earth, Earth is attracted to you as well.
You also attract all the other objects around you.
But, our gravity is much smaller than Earth’s, so we don’t notice it.

12. Gravitational Force
The gravitational force between two objects depends on two factors: MASS & DISTANCE
Mass- is the amount of matter in an object.
The more mass an object has, the more gravitational attraction it exerts on other objects.
For small objects, such as two apples, this force is barely measurable.
However, for larger objects, such as Earth and a weather satellite, the force is much greater.
Earth’s mass is so great it exerts a strong gravitational force on a satellite.
This is why they don’t fly off into space as it moves rapidly around the Earth.

13. Distance
Distance also affects the force of gravity between two objects.
The force of gravity is inversely related to the distance between the objects.
As the distance between objects increases, the force of gravity between them decreases.
Likewise, as the distance decreases, the force of gravity increases.

14. As objects move closer together, their pull on each other grows.
As objects move farther apart, their pull on each other weakens.
A good example is to consider a traveling spacecraft.
As it moves away from Earth, the pull of Earth’s gravity becomes weaker.
At the same time, as the spacecraft moves closer to another planet, the pull of that planet’s gravity increases.

15. Distance, Mass & Gravity
Gravity attract all objects toward one another.
If mass increases, gravity increases.
If distance increases, gravity decreases. m m 2m 2m m m

16. Gravity & Orbits
Since gravity pulls objects toward each other, what keeps the moon from crashing into Earth?
Newton’s first Law of Motion- explains that- An object at rest will stay at rest, and an object in motion will stay in motion, unless an unbalanced force acts on the object.
INERTIA- is an objects tendency to resist a change in its motion.
Inertia keeps a heavy box sitting in the same place on a ramp, where a light object would slide down.
Inertia also keeps moving objects in motion along a straight path.
The more mass the object has, the greater its inertia.

17. This diagram shows how inertia and gravity work together to keep the moon orbiting Earth.
The moons inertia tends to cause the moon to travel continuously in a straight line.
At the same time, Earth’s
`Gravity is pulling the moon toward Earth.
These combined
`Forces cause the moon to move in a
curved path around Earth.
`Without Gravity, the moon would
fly off into space.
`Without Inertia the moon would fall
Into Earth.
Earth
Gravity
Actual Orbit
moon
Inertia

18. Weight versus Mass
As we discuss the effects of mass on gravity and inertia, you may wonder whether mass and weight are the same.
NO- they are not.
Mass- is the amount of matter in an object.
Weight-is a measure of the pull of gravity on an object.
Your mass is the same on the moon or in outer space as it is on Earth. You are made up of the same amount of matter no matter where in the universe you are.
The moon is much smaller than Earth, so it’s gravity is less.
If you were measuring your weight on the moon you would weigh LESS.

19. Let’s recap with . . .