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Recently I came across the following thought experiment :

Take a string and whirl it in a horizontal circle. Imagine that an ant is sitting on the string . From our point of view , the ant is in accelerated motion. But from the point of view of the ant , it is us who’s in accelerated motion.

Now this probably follows from the fact that rest and motion are only relative terms. We can only be at rest or in motion relative to a frame of reference. However , this holds only when the motion is non accelerated. I believe that since the ant can feel that it’s undergoing acceleration this does not hold true right ? Please advise me about your opinion. Thank you !

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There is a simple way to find out it you are in an accelerating frame. Just drop something and see what happens to it. If the object remains motionless next to you then you are in an inertial frame. If the object accelerates away from you then you are in an accelerating frame. And if the object does accelerate away from you the the magnitude of that acceleration is equal to the proper acceleration of your frame.

Suppose your ant physicist was floating in space, then when it dropped something that object will just float weightless beside it. So this is an inertial frame. However if the ant clinging to the string drops something then that object is going to fly outwards away from the ant. So you ant on a string will know it's in an accelerated frame not an inertial frame.

So unlike velocity an observer can always determine their proper acceleration, but this does not mean acceleration is absolute. Suppose we stand together at the top of a cliff, and I jump while you remain standing at the top. Which of us is accelerating? This experiment is described in Do I have to know the General Relativity theory to understand the concept of inertial frame? Remarkably the answer is that you are one accelerating. This is precisely the thought experiment Einstein used to help him formulate general relativity.

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  • $\begingroup$ Thank you very much . I would love to read more of your answers on GR $\endgroup$ – Aditi May 24 at 5:09

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