1. Gravity Newton realized that a force acts to pull objects straight down toward the center of Earth. He called this force gravity. Gravity is the force that pulls objects toward each other. Newton realized that a force acts to pull objects straight down toward the center of Earth. He called this force gravity. Gravity is the force that pulls objects toward each other.

2. Free Fall When the only force acting on a falling object is gravity, the object is said to be in free fall. An object in free fall accelerates as it falls. Do you know why? In free fall the force of gravity is an unbalanced force, and unbalanced forces cause an object to accelerate. When the only force acting on a falling object is gravity, the object is said to be in free fall. An object in free fall accelerates as it falls. Do you know why? In free fall the force of gravity is an unbalanced force, and unbalanced forces cause an object to accelerate.

3. How much? How much do objects accelerate as they fall? Near the surface of Earth, the acceleration due to the force of gravity is 9.8 meters per second squared. This means that for every second an object is falling, its velocity is increases by 9.8 meters per second.

4. Think about it… Suppose an object is dropped from the top of a building. Its starting velocity is 0 meters per second. At the end of the first second, its velocity is 9.8 m/s. After two seconds, its velocity is 19.6 m/s or 9.8 m/s + 9.8 m/s. What is the velocity of the falling object after 3 seconds? Suppose an object is dropped from the top of a building. Its starting velocity is 0 meters per second. At the end of the first second, its velocity is 9.8 m/s. After two seconds, its velocity is 19.6 m/s or 9.8 m/s + 9.8 m/s. What is the velocity of the falling object after 3 seconds?

5. Will a bowling ball and a baseball fall at the same rate? While it may seam hard to believe at first, all objects in free fall accelerate at the same rate regardless of mass. Turn to page 58 and look at figure 11. What does figure 11 show us? While it may seam hard to believe at first, all objects in free fall accelerate at the same rate regardless of mass. Turn to page 58 and look at figure 11. What does figure 11 show us?

6. Projectile Motion Rather than dropping a ball straight down, what happens if you throw it horizontally? An object thrown is called a projectile. An object that is simply dropped and one that is thrown horizontally are both in free fall. The horizontal motion of the thrown object does not interfere with its free fall. Both objects will hit the ground at exactly the same time. Rather than dropping a ball straight down, what happens if you throw it horizontally? An object thrown is called a projectile. An object that is simply dropped and one that is thrown horizontally are both in free fall. The horizontal motion of the thrown object does not interfere with its free fall. Both objects will hit the ground at exactly the same time.

7. Air Resistance Despite the fact that all objects are supposed to fall at the same rate, you know that this is not always true. For example, an oak leaf flutters slowly to the ground while an acorn drops straight down. Objects falling through air experience a type of fluid friction called air resistance Despite the fact that all objects are supposed to fall at the same rate, you know that this is not always true. For example, an oak leaf flutters slowly to the ground while an acorn drops straight down. Objects falling through air experience a type of fluid friction called air resistance

8. Remember… Friction acts in the direction opposite the motion so air resistance is an upward force. Air resistance is not the same for all objects. The greater the surface area of an object, the greater the air resistance. That is why the leaf falls more slowly than an acorn. In a vacuum, where there is no air, all objects fall with exactly the same rate of acceleration. Friction acts in the direction opposite the motion so air resistance is an upward force. Air resistance is not the same for all objects. The greater the surface area of an object, the greater the air resistance. That is why the leaf falls more slowly than an acorn. In a vacuum, where there is no air, all objects fall with exactly the same rate of acceleration.

9. Terminal Velocity Air resistance increases with velocity. As a falling object speeds up, the air resistance against it increases. Eventually, the air resistance equals the force of gravity. Remember that when forces are balanced there is no acceleration. Although the object continues to fall, its velocity no longer increases. This velocity, the greatest velocity the object reaches is called terminal velocity. Air resistance increases with velocity. As a falling object speeds up, the air resistance against it increases. Eventually, the air resistance equals the force of gravity. Remember that when forces are balanced there is no acceleration. Although the object continues to fall, its velocity no longer increases. This velocity, the greatest velocity the object reaches is called terminal velocity.

10. Weight Weight is known as the force of gravity on a person or object at the surface of a planet. When you step on a bathroom scale, you are determining the force with which Earth is pulling you. Do not confuse weight with mass. Weight is a measure of the force of gravity on an object and mass is a measure of the amount of matter in that object. Weight is known as the force of gravity on a person or object at the surface of a planet. When you step on a bathroom scale, you are determining the force with which Earth is pulling you. Do not confuse weight with mass. Weight is a measure of the force of gravity on an object and mass is a measure of the amount of matter in that object.

11. Find your weight in Newtons Weight = Mass x Acceleration due to gravity Weight is usually measured in newtons, mass in kilograms, and acceleration due to gravity in meters per second squared. To find the weight of a 50 kilogram person we would multiply 50 kg. x 9.8 m/s = 490 newtons on the Earth’s surface. Weight = Mass x Acceleration due to gravity Weight is usually measured in newtons, mass in kilograms, and acceleration due to gravity in meters per second squared. To find the weight of a 50 kilogram person we would multiply 50 kg. x 9.8 m/s = 490 newtons on the Earth’s surface.

12. Universal Gravitation Newton discovered that gravity acts everywhere in the universe. Gravity is the force that makes an apple fall to the ground, the force that keeps the moon orbiting around the Earth, and the planets orbiting around the sun. The law of universal gravitation states that the force of gravity acts between all objects in the universe. Any two objects in the universe attract each other. You are attracted to the Earth, the Earth is attracted to you and other objects are attracted to you and you to them. Newton discovered that gravity acts everywhere in the universe. Gravity is the force that makes an apple fall to the ground, the force that keeps the moon orbiting around the Earth, and the planets orbiting around the sun. The law of universal gravitation states that the force of gravity acts between all objects in the universe. Any two objects in the universe attract each other. You are attracted to the Earth, the Earth is attracted to you and other objects are attracted to you and you to them.

13. Think about this… Why don’t you notice that the objects around you are pulling on you? The reason is that the strength of the force depends on the masses of the objects involved. The force of gravity is much greater between you and the Earth that between you and your binder. Although your mass would remain the same on another planet or the moon, your weight would be different. Why don’t you notice that the objects around you are pulling on you? The reason is that the strength of the force depends on the masses of the objects involved. The force of gravity is much greater between you and the Earth that between you and your binder. Although your mass would remain the same on another planet or the moon, your weight would be different.

14. Distance between objects. If the gravitational force depends on mass, why don’t you notice a force of attraction from a massive object, such as the moon or the sun? Gravitational force also depends on the distance between the objects. The farther apart the objects are, the weaker the force. Astronauts travel great distances from Earth. As they travel from Earth toward the moon, Earth’s gravitational pull becomes stronger and the astronauts feel the moon’s gravity and no longer notice the pull of Earth’s gravity. If the gravitational force depends on mass, why don’t you notice a force of attraction from a massive object, such as the moon or the sun? Gravitational force also depends on the distance between the objects. The farther apart the objects are, the weaker the force. Astronauts travel great distances from Earth. As they travel from Earth toward the moon, Earth’s gravitational pull becomes stronger and the astronauts feel the moon’s gravity and no longer notice the pull of Earth’s gravity.