Laws of Motion and Energy

2. Laws of Motion and Energy

3. Chapter Seven: Gravity and Space • 7.1 Gravity • 7.2 The Solar System • 7.3 The Sun and the Stars

4. 7.1 Gravity • Gravity creates an attractive force between all objects that have mass.

5. 7.1 Gravity • There is even a gravitational force between you and a book. • You don’t notice gravity between ordinary objects because it takes a huge amount of mass to create a force big enough to feel.

6. 7.1 Conserving Energy • The gravitational force between you and Earth is much stronger than between you and your book.

7. 7.1 Law of gravitation • Newton’s law of universal gravitationsays the strength of the force depends on mass and distance. • If two objects are the same mass they have the pull on each other with the same amount of force (Fg).

8. 7.1 Law of gravitation • If one object doubles in mass, then the gravitational force doubles.

9. 7.1 Law of gravitation • If both objects double in mass, then the force doubles twice, becoming four times as strong.

10. 7.1 Weightlessness • The astronauts in the space shuttle are not far enough away to escape Earth’s gravity. • How do they float around with no apparent weight? • The answer is that they are actually falling around the Earth in their orbit.

11. 7.1 Weightlessness • An elevator accelerating upward makes you heavier. • A scale in the elevator would show your weight plus the additional force applied to accelerate you.

12. 7.1 Weightlessness • The scale “falls away” beneath your feet with the same acceleration as you fall. • The scale would show zero because there would be no force between it and your feet.

13. 7.1 Orbital motion • Newton’s second law explains the motion of planets, moons, and satellites. • An orbitis the circular (or elliptical) path an object takes around a gravity source, such as a planet or star.

14. 7.1 The Earth and Sun • Compared to Earth, the Sun is huge. • The Sun has 330,000 times the mass of Earth and is 110 times larger in diameter.

15. 7.1 The Earth and Sun • At a distance of 150 million km, an 80-kg person is attracted to the Sun only with a force of about half a newton. • Earth’s gravity gives the same person a weight of 784 newtons.

16. 7.1 The Earth and Moon • The Moon is much smaller than Earth. • Because the Moon has much less mass, its surface gravity is about 1/6 as strong as the gravity on Earth.

17. 7.1 The Earth and Moon • The orbit of the Moon is tilted about 5o from the orbit of Earth. • This means the Moon is not in Earth’s shadow except for rare eclipses.

18. 7.1 The Earth and Moon • The Moon’s own shadow creates the phases that we see.

19. 7.1 Tides • A daily rise in ocean level is called a tide. • Tides are caused by the gravitational pull of the Moon acting on the water in the oceans. • The moon-side tide is caused by the moon’s gravity pulling on the ocean.

20. 7.1 Tides • As the Earth rotates around the center of mass, the ocean on the opposite side from the Moon is “flung outward” a little by its own inertia. How many tides are there every 24 hours?