Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Confused about General Theory

  1. Apr 29, 2006 #1
    Hi,

    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.

    Thanks.
     
  2. jcsd
  3. May 1, 2006 #2
    Hi,

    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:

    http://www.polarhome.com:763/~rafimoor/english/GRE.htm

    The article is informal and does not use higher mathematics.

    Regards
    Rafi
     
  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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?