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SR, Inertial frames, Movement, Testing

  1. Dec 3, 2011 #1
    I wanted to understand something about Inertial frames especially as they are talked about in SR.

    It appears that the laws of physics should hold the same in them.

    Now I understand that inertial frames only involve things moving relative to each other and moving in constant speed so you can never really discern which one is moving.

    Is this a simple test to verify this? If you drop a ball down straight it will hit directly below it, so this should happen in another inertial frame of reference. Both FOR would not be accelerating.

    Another way of thinking here is there is a person stationary on the ground and a car moving at a constant 50mph relative to him. He Throws a baseball in his car at 5 mph. Now he should read that the ball is going 5mph but the guy on the ground should read 55mph. Correct?

    Now how do you test this same principle for light in inertial frames? How do you test the for the person on the ground and the guy in the car measure the same speed for light?
    Last edited: Dec 3, 2011
  2. jcsd
  3. Dec 3, 2011 #2
    The classic experiments are the Michaelson-Morley and Fizeau. I can't think of any simple home experiment.
  4. Dec 3, 2011 #3


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    Things in an inertial frame can be accelerating. What cannot be accelerating is one inertial frame relative to another inertial frame. As PatrickPowers says, a classic test that the speed of light is the same in all inertial frames was the Michelson-Morley experiment. There have been many other tests since then.
  5. Dec 4, 2011 #4
    What if they were both accelerating at the same speed? Would the same laws apply for both of them?
  6. Dec 4, 2011 #5


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    What are they accelerating relative to?
  7. Dec 4, 2011 #6
    Could I say each other?
  8. Dec 4, 2011 #7


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    Any frame accelerating relative to an inertial frame will definitely not be an inertial frame, so if one of them is an inertial frame, the other cannot be, and the laws of physics will not take "standard form" in the accelerated frame.

    I don't find it helpful to think of the primary definition of an inertial frame as "nonaccelerating", since I'd have to say "nonaccelerating relative to an inertial" frame which is consistent, but circular. I define an inertial frame as one in which the laws of physics take "standard form".
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