1. Chapter 12
Section 2 Gravity
Objectives
Explain that gravitational force becomes stronger as the masses increase and rapidly becomes weaker as the distance between the masses increases.
Evaluate the concept that free-fall acceleration near Earth’s surface is independent of the mass of the falling object.
Demonstrate mathematically how free-fall acceleration relates to weight.
Describe orbital motion as a combination of two motions.

2. Law of Universal Gravitation
Section 2 Gravity
Law of Universal Gravitation
Sir Isaac Newton (1642–1727) generalized his observations on gravity in a law now known as the law of universal gravitation.
Universal Gravitation Equation
m1 and m2 are the masses of the two objects
d is the distance between the two objects
G is a constant

3. Law of Universal Gravitation, continued
Section 2 Gravity
Law of Universal Gravitation, continued
All matter is affected by gravity.
Two objects, whether large or small, always have a gravitational force between them.
When something is very large, like Earth, the force is easy to detect.
Gravitational force increases as mass increases.
Gravitational force decreases as distance increases.

4. Law of Universal Gravitation
Section 2 Gravity
Law of Universal Gravitation

5. Law of Universal Gravitation
Section 2 Gravity
Law of Universal Gravitation

6. Section 2 Gravity
Free Fall and Weight
Free fall is the motion of a body when only the force of gravity is acting on the body.
Free-fall acceleration near Earth’s surface is constant.
If we disregard air resistance, all objects near Earth accelerate at 9.8 m/s2.
Freefall acceleration is often abbreviated as the letter g, so g = 9.8 m/s2.

7. Free Fall and Weight, continued
Section 2 Gravity
Free Fall and Weight, continued
Weight is equal to mass times free-fall acceleration.
weight = mass  free-fall acceleration
w = mg
Weight is different from mass.
Mass is a measure of the amount of matter in an object.
Weight is the gravitational force an object experiences because of its mass.

8. Comparing Mass and Weight
Section 2 Gravity
Comparing Mass and Weight

9. Free Fall and Weight, continued
Section 2 Gravity
Free Fall and Weight, continued
Weight influences shape.
Gravitational force influences the shape of living things.
Velocity is constant when air resistance balances weight.
The constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity is called the terminal velocity.

10. Section 2 Gravity
Terminal Velocity

11. Free Fall and Motion Orbiting objects are in free fall.
Section 2 Gravity
Free Fall and Motion
Orbiting objects are in free fall.
The moon stays in orbit around Earth because Earth’s gravitational force provides a pull on the moon.
Two motions combine to cause orbiting.

12. Two Motions Cause Orbiting
Section 2 Gravity
Two Motions Cause Orbiting

13. Section 2 Gravity
Free Fall

14. Projectile Motion and Gravity
Section 2 Gravity
Projectile Motion and Gravity
Projectile motion is the curved path an object follows when thrown, launched, or otherwise projected near the surface of Earth.
Projectile motion applies to objects that are moving in two dimensions under the influence of gravity.
Projectile motion has two components—horizontal and vertical. The two components are independent.

15. Section 2 Gravity
Projectile Motion

16. Projectile Motion and Gravity, continued
Section 2 Gravity
Projectile Motion and Gravity, continued
Projectile motion has some horizontal motion.
Horizontal motion is motion that is perpendicular (90º) to Earth’s gravitational field.
The horizontal velocity is constant.
Projectile motion also has some vertical motion.
The vertical motion is the same as downward free-fall motion.

17. Section 2 Gravity
Projectile Motion