Transcript 19.2 Gravity and the Moon

19.2 Gravity and the Moon
p. 666-669
Gravity
The force that holds the moon in orbit is a
force that acts over long distances and
without direct contact.
 Gravity is a force of attraction. Newton’s
Law of universal gravitation states that
gravity exists between all objects in the
universe.
 G = 6.67 x 10-11 N m2 / kg2
 Gravity is measured in Newtons.

Newtons
Named after Isaac Newton, it is a kg per
m/s2, or a mass times an acceleration.
 Also a delicious cookie.

Measuring Gravity

2 things tell us how strong the force of
gravity is between 2 objects:
1. The masses of the objects
2. The distance between the 2 objects.
Mass is the amount of matter in an object.
 Weight is the amount of gravity pulling on
an objects and is determined by location.

Oh, so you want to know the
formula, do ya?

The force of gravitational attraction is given by:
F = G M m / r2
where
F is the gravitational force (in newtons)
M, m are the masses of the attracting bodies (in
kilograms)
r is the distance between the (centers) of the bodies
G is a proportionality constant that depends on units
Distance and gravity
The farther away the object the less
gravity’s effect.
 If distances increases, force decreases
exponentially

Inertia and orbital motion
Inertia: the tendency of any object to resist
a change in motion.
 Planets, moons, are not going to stop their
motion unless something (huge) gets in
their path.
 Newton’s first law explains inertia: an
object at rest will remain at rest and an
object in motion will remain in motion
unless acted on by an outside force.

How do Earth and moon remain in
their orbits?
1. Inertia- moon keeps traveling in its path
2. Gravity- Earth keeps pulling on the moon
Moon’s
velocity
Moon
Gravitational
Force
Earth
Path followed by the Moon
Orbits
The earth, moon, and the other planets
are continuously falling AROUND
whatever object they are orbiting.
 The same is true of satellites and space
shuttles around earth.
 If you fired a rocket at a fast enough
speed, it would continue to circle the earth
indefinitely (8,000m/s)
