The idea that gravity extends throughout the universe is credited to Sir Isaac Newton

The apple is falling toward Earth

Reason: pulled by Earth’s gravity

Newton thought that the moon must be falling around Earth

The moon falls beneath the straight line it would follow if no force acted on it› See figure 12.1

He hypothesized that the moon was simply a projectile circling Earth under the attraction of gravity

The moon has a component of velocity parallel to Earth’s surface

The sideways or tangential velocity is sufficient to ensure nearly circular motion around Earth rather than into it

If there’s no resistance to reduce its speed, the moon will continue falling around and around Earth indefinitely

Is the sideways velocity-the component of velocity perpendicular to the pull of gravity

Newton’s theory confirms the Copernican theory of the solar system

The Earth and the planets continually “fall” around the sun in closed paths

Why don’t the planets crash into the sun?

They don’t because the planets have tangential velocities

What would happen if the tangential velocities of the planets were reduced to zero?

Their motion would be straight toward the sun and they would indeed crash into it

Newton discovered that gravity is universal

States that every object attracts every other object with a force that is directly proportional to the mass of each object

F ~ m1 * m2

d2

m1 = the mass of one object

m2 = the mass of the other object

d = the distance between their center of mass

The greater the masses, the greater the force of attraction between them

The greater the distance between the objects, the weaker the force of attraction

G measures the strength of gravity G = 6.67x10-11 N*m2/kg2

F = G * m1 * m2

d2

The value of G tells us that the force of gravity is a very weak force

We sense gravitation only when masses like that of Earth are involved

Between you and a classmate, it is too weak to notice (but it’s there!)› Weakest of the 4 fundamental forces

Between you and Earth it’s easy to notice, it’s your weight

Mass of Earth = 6x1024 kg

An apple weighs 1N at Earth’s surface, weighs only 0.25N when located twice as far because the pull of gravity is only ¼ as strong

3x as far, weighs 1/9

A physical quantity varies inversely as another quantity is squared

The Earth is round, but why? Because of gravitation Since everything attracts everything

else, Earth had attracted itself together before it became solid

Any “corners” of Earth have been pulled in so that Earth is a giant sphere

How is the gravitational force between a pair of planets altered when the mass of one of the planets is doubled?

The force is doubled When both are doubled? The force is four times as great

When they are twice as far apart? The force is decreased to ¼ as much

When they are three times as far apart?

The force is decreased to 1/9 as much

Ten times as far apart? The force is decreased to 1/100 as

much