Gravity and Motion

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Gravity and Motion
Chapter Two
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• Gravity gives the Universe its structure
• It is a universal force that causes all objects
  to pull on all other objects everywhere
• It holds objects together
• It is responsible for holding the Earth in its
  orbit around the Sun, the Sun in its orbit around
  the Milky Way, and the Milky Way in its path
  within the Local Group

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2.1

• Astronomers of antiquity did not make the
  connection between gravity and astronomical
  motion
• Galileo initiated the investigation of this
  connection with his famous experiments
  (projectiles and balls rolling down planks)
• His efforts led to laws of motion

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2.2

• Galileo established the idea of inertia
• A body at rest tends to remain at rest
• A body in motion tends to remain in motion

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• Through experiments with inclined planes, Galileo
  demonstrated the idea of inertia and the
  importance forces (friction)

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This concept was incorporated in Newtons First
Law of Motion
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• http//www.physicsclassroom.com/mmedia/newtlaws/cc
  i.html

• http//www.physicsclassroom.com/mmedia/newtlaws/mb
  .html

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• Important ideas of the Newtons first Law
• The concept of a force is introduced
• -anything that pushes or pulls-
• The force referred to is a net force
• -the total of all forces acting on a body-
• The law implies that if a body is not moving in a
  straight line at a constant speed, some net force
  must be acting on it
• The fact that planets travel in curved paths
  suggests that some force must be present

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2.3

• Although not the first to propose gravity as
  being responsible for celestial motion, Newton
  was the first to
• Spell out the properties of gravity
• Write the equations of gravity-induced motion

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• According to the legend, Newton realized
  gravitys role when he saw an apple falling from
  a tree
• This made him wonder if Earths gravity extended
  to the moon
• Newton correctly deduced that Earths gravity, if
  weakened by distance, could explain the moons
  motion

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• Newton theorized that
• The Moons motion could be explained by the
  existence of a force (to deviate the Moon from a
  straight inertial trajectory) and that such a
  force decreased with distance
• Orbital motion could be understood as a
  projectile moving parallel to the Earths
  surface at such a speed that its gravitational
  deflection towards the surface is offset by the
  surfaces curvature away from the projectile

AHHHHH!
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• Swanson says
• In an absence of forces, a projectile (moon) will
  travel in a straight line at a constant speed
• BUT, because a force, gravity, is acting on the
  moon, the path is not straight but curved
  parallel to Earths surface
• Note the projectile will not stop because its
  inertia will carry it forward
• Also Note the strength of gravity decreases
  with distance

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• From your everyday experiences, you know that
  when you throw an object horizontally, gravity
  pulls it downward.
• The faster you throw it, the further it travels
  before striking the ground

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2.4

• Newtons Second Law of Motion
• An object is said to be in uniform motion if its
  speed and direction remain unchanged
• An object in uniform motion is said to have a
  constant velocity
• A force will cause an object to have non-uniform
  motion, a changing velocity
• Acceleration is defined as a change in velocity

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• Acceleration
• A object increasing or decreasing in speed along
  a straight line is accelerating
• An object with constant speed moving in a circle
  is accelerating
• Acceleration is produced by a force and
  experiments show the two are proportional

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• Mass
• Mass is the amount of matter an object contains
• Technically, mass is a measure of an objects
  inertia
• Mass is generally measured in g or kg
• Mass should not be confused with weight, which is
  a force related to gravity weight may change
  from place to place, but mass does not

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• Newtons Second Law of Motion
• Force equals mass times acceleration
• This equation applies for any force,
  gravitational or otherwise

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2.5

• The Law of Gravity is
• Where G is the gravitational constant (dependent
  on units), M and m are two masses that are
  undergoing mutual gravitational attraction, and r
  is the distance (center-to-center) between them
• All masses exert a gravitational force of
  attraction on all other masses

AHHH!
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2.6

• Newtons Third Law of Motion
• This law is also known as the law of
  action-reaction
• Consequence the gravitational force of the Earth
  on the Sun is exactly equal to the gravitational
  force of the Sun on the Earth

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2.7

• The masses of the orbiting bodies determines the
  gravitational force between them
• Note you can determine physical properties
  of orbiting objects, such as their mass
  by understanding their orbital periods.

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2.8

• Surface gravity is the acceleration a mass
  undergoes at the surface of a celestial object
  (e.g., an asteroid, planet, or star)
• Surface gravity
• Determines the weight of a mass at a celestial
  objects surface
• Influences the shape of celestial objects
• Influences whether or not a celestial object has
  an atmosphere

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2.9

• To overcome a celestial objects gravitational
  force and escape into space, a mass must obtain a
  critical speed called the escape velocity
• Escape velocity
• Determines if a spacecraft can move from one
  planet to another
• Influences whether or not a celestial object has
  an atmosphere
• Relates to the nature of black holes
• Note black hole is an object whose escape
  velocity equals the speed of light, thereby,
  light cannot escape from itblack.

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So Whats Important ?!?

• A gravitational force exists between any two
  objects in the universe
• The strength of this force depends on the masses
  of the bodies and their separation
• Gravitational forces hold astronomical bodies
  together and in orbit around one another

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• A bodies inertia makes it remain at rest or move
  in a straight line at a constant speed unless the
  body is acted on by a net force
• For a planet to orbit the Sun, the Suns force of
  gravity must act on it

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• The gravitational force exerted by a planet
  determines its surface gravity and escape
  velocity.
• Surface gravity determines your weight on a
  planet
• Escape velocity is the speed necessary to leave
  the surface and escape without falling back