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Confused about General Theory

  1. Apr 29, 2006 #1

    I was wondering if someone could explain things simply to me (sorry, Im a high school science teacher and not really into the high level maths side of things).

    I would like to ask if the General Theory of Relativity says anything about objects obeying Newtons Laws in non-inertial reference frames. A lot of the sources Ive come across say that it confirms that Newtons Laws are obeyed in ALL reference frames - including non-inertial ones -hence its name "General" theory of relativity.

    But I dont get how the laws are obeyed in a car accelerating from a traffic light- where objects appear to change their motion without an applied force.

  2. jcsd
  3. May 1, 2006 #2

    The general theory of relativity is based on the equivalence principle that says that acceleration is equivalent to a gravity field. If, for this discussion, we ignore earth gravity, an accelerating car can be considered stationary in a constant uniform gravity field. Everything outside the car is free-falling in this gravity field and constantly accelerating backwards. Things inside the car are either free-falling in it, or held by the matter of the car that applies a force equals to m*a on them.

    You can read about general relativity here:


    The article is informal and does not use higher mathematics.

    Last edited by a moderator: Apr 22, 2017
  4. May 1, 2006 #3
    They do have a force - its the frictional force - If you put your briefcase on the car seat and accelerate your car - the briefcase also gets accelerated because it is pushed by the friction of the seat or becasue it contacts the backrest - try putting some wheels or bearings on your lose items in the car and accelerate - they want to stay at rest so they slide backward when you accelerate forward - when you are stopping, they slide forward. Things at rest stay at rest unless acted upon by some force.
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