This is , I suppose not a good question , but I think I am missing something which confuses me in this question.
So my question is -
Velocity is relative. So suppose a car is moving at a speed 200 km/hr with respect to an inertial frame. In its frame its speed is 0. Now suppose in a time dt its speed increases by dv , i.e it has an acceleration say, a. We can extend the time interval but the problem will be the same. So after some time say, its speed becomes 210 km/hr with respect to the same inertial frame( the acceleration being constant and continuous ) but still its speed in its own frame is 0. So its speed in its own frame has not changed. So in its own frame ( which is not inertial ) it has no acceleration.
Now someone sitting in the car will say that the universe accelerated past him. An inertial frame observer will say that the car accelerated with respect to him. Its true that someone sitting in the car will feel the jerk due to acceleration. But just like the concept of relative velocity and its result which ascertains the fact that no one in an inertial frame can tell whether it is he or the surroundings which are moving. Similarly an accelerated observer ( though he feels the jerk due to acceleration) will say the universe accelerated past him. So who has accelerated ? Is absolute acceleration real. I might have confused the question particularly the last line but that's because I am confused myself.
Actually we say that there isn't any inertial frame. But even to say this we need to compare with some basic reference (inertial) frame because a frame is Inertial when it moves at a constant velocity w.r.t to another inertial frame. So how do we say that there isn't any inertial frame ? With which frame do we compare ? I think that SIMPLY SAYING THAT NEWTON'S LAWS ARE NOT VALID WOULDN'T SUFFICE. BECAUSE IF YOU ARE IN THE SAME ACCELERATING FRAME IN WHICH SOME FORCES ARE ACTING ON A BODY , HOW WOULD YOU KNOW THAT YOU ARE IN A NON-INERTIAL FRAME (forgetting for the time being that there aren't any inertial frame and actually trying to prove this very fact). It might be that the frame with which you compare be the accelerating one and you could be an inertial observer w.r.t to some still unknown inertial frame . SORRY , I MIGHT HAVE CONFUSED THE LAST FEW LINES , BUT THAT'S BECAUSE , I AM CONFUSED MYSELF. AND DON'T KNOW EXACTLY HOW SHOULD I FRAME MY QUESTION ! Can just saying that Newton's law aren't valid in the frame would suffice and wouldn't have to do any comparison ?
Edit In a nutshell I mean that is there any way to know if a frame is Inertial ? With which frame would one compare it to show it's inertial since different observers can measure different accelerations . I mean in the end with reference to the question above who has accelerated. It's ok that observations of all inertial observers are correct but the truth is there can't be any inertial observer.
How can one know that the frame is Inertial? Does just the application of Newton's laws guarantee it since different observers can measure different accelerations ?