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How did our understanding of gravity change over
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How did our understanding of gravity change over



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  • 1. How has our understanding of Gravity changed over the years?
  • 2. Greek philosopher
    Looked for the causes of motion
    Classified motion as natural or violent
    An example of a natural motion - rocks falling to the ground because the ground is the natural place for a rock to be
    An example of a violent motion – an arrow shot through space is not natural because the natural motion would be to fall straight to the ground
    Aristotelian explanation of gravity, 1st Law of Motion, was that objects fall to the Earth because they are trying to reach their natural place in contact with the Earth with a constant speed depending on the nature of the object (heavy or light)
    Heavier objects fall faster than lighter objects
    Aristotle (384-322 b.c.)
  • 3. Proposed a whole new way of looking at the problem of motion
    Concentrated on describing mathematically what happened rather than speculate why
    Galileo believed that objects move downward because the gravity disturbs their motion
    Experimented and shared results as evidence for his beliefs
    Galileo believed that in the absence of air resistance all objects will fall to the Earth with a constant acceleration
    Galileo galilei (1564-1642)
  • 4. Born the year thatGalileo died
    Took his work and expounded upon it
    Law of Universal Gravitation - states that every object in the universe attracts every other object with a force which is directly proportional to their masses and inversely proportional to the distance between them
    Three Laws of Motion
    1st Law: Law of inertia – objects at rest stay at rest and objects in motion stay in motion unless acted on by an outside force
    2nd Law: F = ma
    3rd Law: For every action force there is an equal and opposite reaction force
    Newton explained that the motion of objects on Earth and celestial bodies were governed by the same set of laws
    Sir Isaac newton (1642- 1726)
  • 5. This is a great example of this difference between a law and a theory
    There is a major misconception that theories will one day become laws
    This is not true
    Theories and laws are different
    A law is a description of a pattern found in nature. It is often a mathematical equation that is found to fit the pattern.
    A theory is an explanation of a pattern. It is the answer to why there is a pattern.
    this is a greatexample
  • 6. Newton described the relationship between mass, distance, and gravitational attraction. This relationship could be written with the equation F = G(m1m2/R2) where G is the universal gravitational constant, m1 is the mass of one body, m2 is the mass of the second body, and R2 is the distance between them squared
    This is known as Newton’s Law of Universal Gravitation
    What newton knew…
  • 7. There was not in Newton’s time, nor is there now, any well accepted explanation of why there is gravity and why this law holds true. There is no theory of gravity.
    Newton himself understood the difference between law and theory, “…I have not been able to discover the cause of those properties of gravity from phenomena, and I frame no hypothesis … it is enough that gravity does really exist, and act according to the laws which we have explained…” (Newton, 1720/1946, p. 547).
    And what he didn’t know
  • 8. Do Newton’s laws still hold?
    Has anything been discovered since 1727?
    Well actually, there has been
    Newton’s Laws of Motion give us a complete description of the behavior of moving objects at low speeds
    But, Einstein describes the motion of particles moving at close to the speed of light in his special theory of relativity
    How is most of our motion described?
    We move pretty slowly compared to the speed of light, so Newton’s laws describe the motions that we see and deal with every day
    However if you become an atomic physicist, you may work with a particle accelerator that does not follow Newton’s laws
    So is that it?
  • 9. Laws of Motion - examples, body, process, Earth, law, energy, reaction, air, change, surface, History, Newtons three laws. (n.d.). Science Clarified. Retrieved October 24, 2010, from
    Newton, I. (1946). Sir Isaac Newton’s mathematical principles of natural philosophy and his system of the world, A Motte, trans; revised and appendix supplied by F. Cajori, Berkeley, CA, University of California Press, (Original work published 1720).
    Theory: Special Relativity (SLAC VVC). (2009, June 15). SLAC Public Website Server. Retrieved October 24, 2010, from