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Gravity.ppt
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- 2. • Would yoube surprised if you let go of a pen you were holding and it did not fall? • You are so used to objects falling that you may not have thought about why they fall. • One person who thought about it was Isaac Newton. • He concluded that a force acts to pull objects straight down toward the center of Earth. • Gravity is a force that pulls objects toward each other.
- 3. Universal Gravitation • Newtonrealized that gravity acts everywhere in the universe, not just on Earth. • It is the force that makes an apple fall to the ground. • It is the force that keeps the moon orbiting around Earth. • It is the force that keeps all the planets in our solar system orbiting around the sun.
- 4. • What Newtonrealized is now called the law of universal gravitation. • The law of universal gravitation states that the force of gravity acts between all objects in the universe. • This means that any two objects in the universe, without exception, attract each other. • You are attracted not only to Earth but also to all the other objects around you.
- 5. • Earth andthe objects around you are attracted to you as well. • However, you do not notice the attraction among objects because these forces are small compared to the force of Earth’s attraction.
- 6. Factors Affecting Gravity •Two factors affect the gravitational attraction between objects: – Mass – Distance • Mass is a measure of the amount of matter in an object. • The SI unit of mass is the kilogram. • One kilogram is the mass of about 400 modern pennies. • Everything that has mass is made up of matter. • The more mass an object has, the greater its gravitational force. • Because the sun’s mass is so great, it exerts a large gravitational force on the planets. • That’s one reason why the planets orbit the sun.
- 7. • In additionto mass, gravitational force depends on the distance between the objects. • The farther apart two objects are, the lesser the gravitational force between them. • For a spacecraft traveling toward Mars, Earth’s gravitational pull decreases as the spacecraft's distance from Earth increases. • Eventually the gravitational pull of Mars becomes greater than Earth’s, and the spacecraft is more attracted toward Mars.
- 9. Weight and Mass •Mass is sometimes confused with weight. • Mass is a measure of the amount of matter in an object; weight is the measure of the gravitational force exerted on an object. • The force of gravity on a person or object at the surface of a planet is known as weight. • So, when you step on a bathroom scale, you are determining the gravitational force Earth is exerting on you.
- 10. • Weight varieswith the strength of the gravitational force but mass does not. • Suppose you weighed yourself on Earth to be 450 newtons. • Then you traveled to the moon and weighed yourself again. • You might be surprised to find out that you weigh only about 75 newtons – the weight of about 8 kilograms on Earth! • You weigh less on the moon because the moon’s mass is only a fraction of Earth’s.
- 12. Gravity and Motion •On Earth, gravity is a downward force that affects all objects. • When you hold a book, you exert a force that balances the force of gravity. • When you let go of the book, gravity becomes an unbalanced force and the book falls.
- 13. Free Fall • Whenthe only force acting on an object is gravity, the object is said to be in free fall. • An object in free fall is accelerating. • Do you know why? • In free fall, the force of gravity is an unbalanced force, which causes an object to accelerate.
- 14. • How muchdo objects accelerate as they fall? • Near the surface of Earth, the acceleration due to gravity is 9.8 m/s2. • This means that for every second an object is falling, its velocity increases by 9.8 m/s.
- 15. • For example, supposean object is dropped from the top of a building. • Its starting velocity is 0 m/s. • After one second, its velocity has increased to 9.8 m/s. • After two seconds, its velocity is 19.6 m/s (9.8 m/s + 9.8 m/s) • The velocity continues to increase as the object falls.
- 16. • While itmay seem hard to believe at first, all objects in free fall accelerate at the same rate regardless of their masses. • The two falling objects in Figure 10 demonstrate this principle.
- 18. Air Resistance • Despitethe fact that all objects are supposed to fall at the same rate, you know that this is not always the case. • 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.
- 19. • Remember thatfriction acts in the direction opposite to motion, so air resistance is an upward force exerted on falling objects. • Air resistance is not the same for all objects. • Falling objects with a greater surface area experience more air resistance. • That is why a 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.
- 20. • You cansee the effect of air resistance if you drop a flat piece of paper and a crumpled piece of paper at the same time. • Since the flat paper has a greater surface area, it experiences greater air resistance and falls more slowly. • In a vacuum, both pieces of paper would fall at the same rate.
- 21. • Air resistanceincreases with velocity. • As a falling object speed up, the force of air resistance becomes greater and greater. • Eventually, a falling object will fall fast enough that the upward force of air resistance becomes equal to the downward force of gravity acting on the object. • At this point the forces on the object are balanced. • Remember that when forces are balanced, there is no acceleration. • The object continues to fall, but its velocity remains constant.
- 22. • The greatestvelocity a falling object reaches is called its terminal velocity. • Terminal velocity is reached when the force of air resistance equals the weight of the object.
- 23. Projectile Motion • Ratherthan dropping a ball straight down, what happens if you throw it horizontally? • An object that is thrown is called a projectile. • Will a projectile that is thrown horizontally land on the ground at the same time as an object that is dropped?
- 24. • The yellowball was given a horizontal push at the same time as the red ball was dropped. • Even though the yellow ball moves horizontally, the force of gravity continues to act on it in the same way it acts on the red ball. • The yellow ball falls at the same rate as the red ball. • Thus both balls will hit the ground at exactly the same time.
- 25. • In asimilar way, an arrow flying toward a target is a projectile. • Because of the force of gravity, the arrow will fall as it flies toward the target. • So if you try to hit the bull’s-eye, you must aim above it to account for gravity’s pull. • When you throw a projectile at an upward angle, the force of gravity reduces its vertical velocity • Eventually the upward motion of the projectile will stop, and gravity will pull it back toward the ground. • From this point, the projectile will fall at the same rate as any dropped object.