Gravity and Motion

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Gravity and Motion
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Gravity and Motion

• Gravity is what gives the universe its _________
• A universal force that acts on _________ the
  objects in the universe
• Every particle in the universe is drawn ________
  every other particle in the universe
• Gravity is what also controls the motion of
  astronomical bodies, holding planets in orbit
• Gravity and motion are tightly connected in the
  universe

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Solving the Problem of Astronomical Motion

• Ancient astronomers didnt make the connection
  between gravity and motion that we understand
  today. How come we dont fly off the face of the
  Earth? What keeps the planets moving?
• ___________________ was the first to make the
  connection in the 1600s with a series of
  carefully constructed experiments using
  projectiles and balls rolling down planks
• These experiments demonstrated the power of the
  _________________________

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Inertia

• _________________ the tendency of a body at rest
  to remain at rest and a body in motion to keep
  moving.
• Both Aristotle and Kepler recognized inertia, but
  it was Galileo who demonstrated its importance
  through experimentation
• _________________ was the first to use the term
  inertia
• Newton recognized the special importance of
  inertia

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Inertia

• Galileo was the first to demonstrate it by real
  experimentation
• Rolled balls down an incline speed
  _____________ until it reached the bottom
• Rolled balls up an incline speed
  ________________ as it approached the top
• Hypothesized that a ball rolled along a flat
  surface with no other forces acting on it would
  neither increase or decrease
• In the absence of outside forces, inertia keeps
  an object already in motion moving at a fixed
  speed

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Newtons Laws of Motion

• Newtons first law of motion (The Law of
  Inertia) a body continues in a state of
  _________ or uniform motion in a straight line
  unless made to change that state by forces acting
  on it.
• ______________ something that pushes or pulls
• Net force the total of ___________ forces acting
  on a body
• _______________ forces lead to _______
  acceleration

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Newtons Laws of Motion

• If a body in not moving in a ________________
  line at constant speed, some net force must be
  acting on it
• Newton actually wasnt the first to state this
  law (Christiaan Huygens was), but Newton showed
  how to ___________________ it to the universe
• Because the planets and other heavenly bodies
  follow a curved path, some net force must be
  acting upon them.

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Orbital Motion and Gravity

• Kepler recognized that something must be holding
  the planets in orbit around the sun and proposed
  that it was something similar to
  ____________________ (this was 100 years before
  Newton)
• ______________________ suggested it was
  _____________ that held the planets in motion
  (suggested several years before Newton published
  the laws of gravity in 1687)
• Newton was the one to publish his findings and
  demonstrate the properties that gravity must have
  to fulfill its role as the controller of
  planetary motion
• Newton also came up with the mathematical
  ____________ that describe gravity and its effect
  on motion
• Newton correctly deduced that Earths gravity, if
  weakened by distance, could explain the motion of
  the moon.

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Orbital Motion and Gravity

• If a projectile moves fast enough, the Earths
  surface will curve away below the projectile such
  that it will __________________ hit the ground.
• This is the nature of orbital motion the
  __________________ between inertia and the force
  of gravity maintain the orbit
• The question becomes how fast does the object
  need to move to maintain its orbit?

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Newtons Second Law of Motion

• Motion of an object is a change in its position
  which can be characterized by the direction the
  object is moving and its speed
• ________________ nonuniform motion a change in
  speed either increasing or decreasing
• We can produce an acceleration by changing the
  _______________ a body is moving.

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Newtons Second Law of Motion

• A body moving in a circular orbit is
  ________________ accelerating even though its
  speed is not changing (the direction is changing)
• For a body to accelerate a force must act upon it
• Acceleration is proportional to the force that is
  applied
• It also depends on the amount of _____________
  being accelerated

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Newtons Second Law of Motion

• ________________ the amount of matter an object
  contains
• It is also technically is a measure of
  ________________
• The more mass the more inertia
• What is the difference between mass and weight?
• Weight measures the force of gravity acting upon
  a given mass

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Newtons Second Law of Motion

• The law states the amount of acceleration (a)
  that a force (f) can produce depends directly on
  the mass (m) of the object being accelerated
• __________________________
• This is what allows us to launch satellites and
  send them to a pinpoint destination

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The Law of Gravity

• Every mass exerts a force of attraction on every
  other mass. The strength of the force is directly
  proportional to the product of the masses divided
  by the square of their separation
• F GMm
• r2
• G 6.67 X 10-11 meters3/(kilogram-sec.2)
• G Universal Gravitational Constant

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The Law of Gravity

• What does this law tell us?
• If M or m increases and the other factors remain
  the same, the force increases
• If r increases the force gets weaker by the
  square of the distance (if we double the distance
  the forces gets weaker by a factor of 4)
• The gravitational force _______________ really
  disappears
• What does that mean?

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Newtons Third Law (The Law of Action and
Reaction)

• When two bodies interact, they create equal and
  opposite forces on each other.
• The forces acting on the Sun and Earth are
  exactly equal
• The Earth orbits the sun because its mass is much
  smaller (300,000 times) than the sun because
  acceleration is inversely proportional to the
  mass of an object
• The acceleration a body feels is inversely
  proportional to its mass
• The more massive the body is the greater the
  force that is needed to accelerate that mass
• The Sun accelerated 300,000 times less than the
  Earth because it is 300,000 times more massive
• Therefore the Earth does the moving and not the
  Sun

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Surface Gravity

• This is a measure of the gravitational
  ________________ at a planets or stars surface
• A bodys weight depends on its mass and the
  acceleration of gravity
• Determines what an object _____________ on its
  surface
• Influences a planets _______________ and whether
  it has an _______________________
• Asteroids are not spherical because the surface
  gravity is too weak to press them into round
  shapes
• The strength of surface gravity is different from
  body to body.

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Escape Velocity

• The force need to overcome a planets
  gravitational force and __________________________
  ____
• This will also influence whether a planet has an
  atmosphere
• It also influences the nature of
  ____________________.
• Vesc v2GM/R
• V Escape Velocity
• G universal gravitational constant 6.67 X 10-11
  m3/kilogram sec2
• M Mass of the Body to be escaped from
• R Radius of the body to be escaped from