Understanding Newton's Laws in Non-Inertial Reference Frames

[pkc111]

Hi,

I was wondering if someone could explain things simply to me (sorry, I am 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 I've 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 don't 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.

 

[Rafi]

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

 

[yogi]

Hi,



But I don't 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.

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.