1. Chapter 2: Forces
Gravity

2. Galileo

5. The History
Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th and early 17th centuries. In his famous experiment dropping balls from the Tower of Pisa, and later with careful measurements of balls rolling down inclines, Galileo showed that gravitational acceleration is the same for all objects. This was a major departure from Aristotle's belief that heavier objects have a higher gravitational acceleration. Galileo postulated air resistance as the reason that objects with less mass may fall slower in an atmosphere. Galileo's work set the stage for the formulation of Newton's theory of gravity.

6. Gravity- the force that pulls objects toward each other Law of Universal Gravitation- states that the force of gravity acts between all objects in the universe.

7. Factors Affecting Gravity
Two factors affect gravitational attraction between objects:
Mass
The more mass an object has the greater its gravitational force
Ex. Planets orbit the sun because its mass exerts a large gravitational force
Distance
The farther two objects are the lesser the gravitational force between them
Ex. Spacecraft are eventually pulled away from Earth and pulled toward the planet they are traveling to as they get further from Earth.

8. Weight and Mass Mass - is the amount of matter in an object.
Mass DOES NOT CHANGE with location. Mass stays the same! If you go to the moon your mass stays the same even though you weigh less.
Weight - the measure of the pull of gravity on an object. The more gravity, the more weight. - can change with location. You weigh more on Earth than you would on the moon because of a change in the amount of gravity. - is a force, so it is measured in Newtons (N). - measured using a spring scale.

10. Free Fall
-gravity is pulling it down on an object and no other forces are acting on it
- causes object to accelerate
- Free fall can occur in a vacuum (a place where there is no matter) or in space.

12. Gravity and Acceleration (don’t write)
-The acceleration on Earth due to gravity is the same for all objects. - Acceleration depends on both force and mass. Heavier objects experience more gravity but they have more inertia, so they are more difficult to accelerate.

13. Acceleration Due To Gravity
Acceleration is the rate at which velocity changes.
-All objects accelerate toward Earth at a rate of 9.8 m/s2.
For every second an object falls, its velocity increases by 9.8 m/s2.

15. Let’s look at this a little deeper
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16. Air Resistance
Air resistance- the force that opposes the motion of objects through the air.
Depends on:
- size
- shape
speed of the object
The larger the surface area of an object, the more air resistance acting on it.

20. How do parachutes help sky divers land safely?
They increase air resistance and slow the sky divers to a safe terminal velocity.

21. Acceleration and Terminal Velocity
terminal velocity- the greatest velocity reached by a falling object falling at a constant rate - the upward force of air resistance balances the downward force of gravity, the object stops accelerating. Ex. human without a parachute is about 120 miles per hour.

23. Projectile motion – the curved path an object follows when it is thrown or propelled near the surface of the Earth.