Gravitation

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Gravitation
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Cavendish Experiment

• What do you need to calculate the force of
  attraction between two bodies?
• the masses of the two objects
• the distance between the two objects
• the gravitational constant.
• Therefore, to be able to prove the law of
  gravitation you need to be able to calculate the
  gravitational constant (G).

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The Problem

• The strength of attraction between two small
  masses will be extremely small! Therefore, hard
  to measure in a laboratory.
• Despite the weakness of the attraction, Henry
  Cavendish was able to perform an experiment to
  measure the force between two small objects which
  led to the calculation of the gravitational
  constant (G) .

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• He performed the experiment inside a closed shed
  and observed the result from outside through a
  telescope.

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Torsion Balance

• For his experiment in 1798, Cavendish hung a
  dumbell from a fine string.
• He then placed two large lead weights below the
  dumbell, and was able to see a small twisting in
  the string.
• From this small twist in the string he was able
  to measure the force between the objects.
• After measuring the force, masses, and distance,
  the gravitational constant could be calculated

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Did Cavendish determine G?

• In actuality, Cavendish's only goal was to
  measure the density of the Earth he called it
  'weighing the world'.
• The method Cavendish used to calculate the
  Earth's density consists in measuring the force
  on a small ball caused by a large ball of known
  mass, and comparing it with the force on the
  small ball caused by the Earth, so the Earth can
  be calculated to be N times more massive than the
  large ball without the need to obtain a numeric
  value for G.
• The gravitational constant does not appear in
  Cavendish's paper, and there is no indication
  that he regarded it as a goal of his experiment.
• One of the first references to G is in 1873, 75
  years after Cavendish's work.

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What is the difference between mass and weight?

• Mass
• The amount of matter in an object
• Mass is measured in kg or g
• Measured using a balance

• Weight
• The force of gravity on an object
• Weight is measured in Newtons (N)
• Measured using a scale
• What does weight depend on?
• mass
• gravity
• What does gravity depend on?
• mass of planet
• distance to planet
• gravitational constant

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Gravitational Force

• Weight is another name for the gravitational
  force from the Earth.
• What can we use to measure the gravitational
  force acting on an object?
• A spring balance/ Newton meter
• People often use the word weight when they
  really mean mass

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Summary
Compare your weight

• 1) Mass is a measurement of the amount of matter
  something contains, while Weight is the
  measurement of the pull of gravity on an
  object.2) Mass is measured by using a balance
  comparing a known amount of matter to an unknown
  amount of matter. Weight is measured on a
  scale.3) The Mass of an object doesn't change
  when an object's location changes. Weight, on the
  other hand does change with location

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Interplanetary Can Experiment

• You have nine cans from the nine planets and each
  has the same mass.
• Find the weight of each can using a Newton meter
  and work out which can is from which planet.
• Here is one to get you started - is the
  symbol for Earth
• The mass of each can 581g 0.581kg
• Good Luck!

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What is gravity?

• We don't really know.
• We can define what it is as a field of influence,
  because we know how it operates in the Universe.
• Some scientists think that it is made up of
  particles called gravitons which travel at the
  speed of light.
• If we are to be honest, we do not know what
  gravity "is" in any fundamental way - we only
  know how it behaves.

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Scientific Revolution

• Modern work on gravitational theory began with
  the work of Galileo Galilei in the late 16th
  century
• Galileo showed that gravitation accelerates all
  objects at the same rate.
• This was a major departure from Aristotle's
  belief that heavier objects are accelerated
  faster.
• Galileo's work set the stage for the formulation
  of Newton's theory of gravity.

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Is there a link?

• On Earth, the acceleration of free fall is 10m/s2
• On the Earth, there is a gravitational force of
  10 newtons on every kilogram
• These two facts are connected

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• Here is what we do know...
• Gravity is a force of attraction that exists
  between any two masses. Sir Isaac Newton (1642 --
  1727) realized that the force called "gravity"
  must make an apple fall from a tree.
• Newton's "law" of gravity is a mathematical
  description of the way bodies are observed to
  attract one another, based on many scientific
  experiments and observations.
• The effect of gravity extends from each object
  out into space in all directions, and for an
  infinite distance. However, the strength of the
  gravitational force reduces quickly with
  distance.

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Actually.

• Einstein later came along and redefined gravity,
  so there are now two models -- Newtonian and
  Einsteinian.
• Einsteinian gravitational theory has features
  that allow it to predict the motion of light
  around very massive objects and several other
  interesting phenomena
• Stay tuned!!!

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• Unfortunately, Newtonian gravity falls apart when
  we try to combine it with what we've learned
  about Special Relativity.

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Space time continuum

• In the early twentieth century, Albert Einstein
  developed his theory of general relativity in
  which he described gravity as a deformation in
  space, caused by the presence of massive objects.
• This deformation 'told' smaller things how to
  move through space, so they either went into
  orbit or fell onto the larger celestial object.

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Einstein's idea that space was like a fabric
stretched across the Universe was revolutionary.
He called it the 'space-time continuum'.
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• The presence of mass or concentrated energy
  causes a local curvature in the space-time
  continuum.
• This curvature is such that the inertial paths of
  bodies are no longer straight lines but some form
  of curved (orbital) path, and this acceleration
  is what is called gravitation

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Balling ball and Lycra Demonstration

• Matter tells space how to bend.
• Space tells matter how to move