# Gravity Inertia

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```					          Gravity

Why do planets stay in place?
1
What is Gravity?
 Gravity is a force of attraction between objects
– The force of gravity is dependent on the mass of the
objects and the distance between them.
– The greater the mass, the stronger is the gravitational
attraction between the objects
– The further away the objects are the weaker the
gravitational attraction.
– Newton’s law of gravity: F=(G *M1*M2)/r2 the
force of gravity gets reduced by the square of the
distances
the mass of Earth
2
Effects of Gravity
 Oceans rise and fall twice daily – called tides.
 Tides are the result of the gravitational pull of the moon
and sun.
 They pull on the water in the ocean pulling it toward the
moon, causing a high tide.
 The moon has a greater affect on tides because it is
closer than the sun.
 Spring tides, the highest high and lowest low tides,
occur when the moon, Earth & Sun are aligned i.e. a
new or full moon
 Neap tides- the smallest changes in the tides, occurs
when the Earth, Sun Moon form a right angle i.e. 1st or
last quarter moon
3
Orbital Speed & Distance from Sun
 Gravity is the force that holds planets and other
objects of the solar system in their orbits
 The closer a planet is to the sun, the faster it
moves. Therefore if we know a planet’s distance
from the sun we can calculate its velocity.
 Planets with elliptical orbits, move faster when
they are closer to the sun than when they are
further away from the sun.

4
Inertia & gravity combine to fix orbits
 Inertia – the tendency of an object at rest to stay at rest
or to keep moving in the same direction it is moving at
a constant speed
 If a satellite has a circular orbit, inertia and the force of
gravity are constant. With the change in direction the
satellite keeps its orbital path. Gravity causes the
direction to change continuously
 If the orbit is elliptical, gravity also causes the speed to
change when the satellite is closer to its primary (the
sun)

5
Gravity & Planet Orbits

   What is Gravity?
   Effects of Gravity
   Orbital Speed & Distance from Sun
   Inertia & gravity combine to fix orbits

6

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 views: 78 posted: 3/23/2011 language: English pages: 6