Laws of Motion and Energy

Chapter Seven: Gravity and Space

• 7.1 Gravity

• 7.2 The Solar System

• 7.3 The Sun and the Stars

7.1 Gravity

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

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.

7.1 Conserving Energy

• The gravitational force between you and Earth is much stronger than between you and your book.

7.1 Law of gravitation• Newton’s law of universal gravitation says 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).

7.1 Law of gravitation

• If one object doubles in mass, then the gravitational force doubles.

7.1 Law of gravitation

• If both objects double in mass, then the force doubles twice, becoming four times as strong.

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.

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.

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.

7.1 Orbital motion

• Newton’s second law explains the motion of planets, moons, and satellites.

• An orbit is the circular (or elliptical) path an object takes around a gravity source, such as a planet or star.

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.

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.

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.

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.

7.1 The Earth and Moon

• The Moon’s own shadow creates the phases that we see.

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.

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?