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Inertia - Why does it exist?

  1. Jan 20, 2014 #1
    Inertia -- Why does it exist?

    Can someone explain to me in terms of a thought experiement (not equations, please) why it is that inertia exists given that motion is only defined as it relates to another point of reference and given that there is no fixed point of reference but everything is relative? Who's to say something is moving or even accelerating unless defining it relative to another object and then who's to say it's not the other object accelerating and you are standing still...so I don't see how inertia even exists given all these facts.
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  3. Jan 20, 2014 #2


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    Acceleration is not relative. One can always measure whether or not a frame of reference is accelerating.
  4. Jan 20, 2014 #3
    But that is what I don't get, how does one measure whether or not the frame of reference is accelerating if you don't have a seperate frame of reference to measure against? For instance, if there was only one object that existed in the universe, how would you measure that it is accelerating, sitting still, or moving, or anything...when there is nothing to compare it to except itself?
  5. Jan 20, 2014 #4


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    Use an accelerometer. For example, stand on a spring scale. It'll show the same reading regardless of your choice of reference frames.
  6. Jan 20, 2014 #5
    Yes, that makes sense. I mean, I get that it exists, I just don't see how to reconcile it with the theory of relativity. It would reconcile in my mind if we still believed in Newton's concept of ether because then you have a fixed frame of reference.
  7. Jan 20, 2014 #6


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    But the Special Relativity says only inertial(non-accelerating) reference frames are equivalent. You're trying to reconcile a non-existent issue.
  8. Jan 20, 2014 #7
    so I might be getting my terms wrong, but I know that inertia exists, I also think that modern day physists believe that there is no fixed frame of reference...so I am still puzzled how these two facts can both be true. Because to say you are accelerating, you would have to compare motion over time as compared to some other point, but if there is no fixed point, than what other point are you comparing against. Then, that other point you are comparing against, how do you say it is not the one accelerating? Maybe there is just a simple concept I am missing here, but it doesn't seem possible to define motion, acceleration, or anything related to position without a frame of reference to define it against. BUT I know that inertia is a true thing, so doesn't that mean that there is a fixed point of rest in the universe, or some way to determine something is moving other than just how it is moving relative to another object? Am I making any sense?
  9. Jan 20, 2014 #8


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    There's an easy way to tell if you are accelerating or not. Hold a ball in your hand and gently release it. If the ball hovers stationary relative to you, you are not accelerating. If the ball starts to move relative to you, you are accelerating.

    Or, as said in post #4, try weighing yourself on some scales. If the scales read zero you are not accelerating.
  10. Jan 20, 2014 #9
    What gives you that weight on the scale, or makes the ball fall? If everything is relative, you cannot say you are accelerating if you say the whole universe is accelerating around you...however, I know this isn't right because the scale WILL tell me I have weight, or the ball WILL fall, so what I am thinking is that there IS a fixed frame of reference in the Universe and Einstein was not right about that point.
  11. Jan 20, 2014 #10


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    Shoosh, jocanon, don't go accusing Einstein of a crime he didn't commit!
    As said earlier, relativity doesn't say all frames are equivalent. It only says all INERTIAL frames are equivalent. Go and check in the wiki article on SR, it's in the second sentence!

    It's quite simple, really. You can never tell if your velocity is 0, but you can tell if your acceleration is.

    You can't find an universal rest frame of reference(this means v=0 only!), but you can find e.g., one that is universaly non-rotating.
  12. Jan 20, 2014 #11
    lol, i guess my simple mind is just not meant to understand this concept :)
  13. Jan 20, 2014 #12
    Apperently I am not the only one that has this question. I just did some poking around on Google and found that some people believe inertia is the result of all the "fixed stars" mass interacting with ours while others believe it is an effect at the atomic level.
  14. Jan 20, 2014 #13
    See: http://en.wikipedia.org/wiki/Mach's_principle
  15. Jan 20, 2014 #14
    The bottom line is "everything is NOT relative". Some things are relative while others are not. Once you accept that experimental fact, all will be much clearer.
  16. Jan 20, 2014 #15
    In that frame of reference the floor is exerting a normal force on the person and not on the ball.

    In the case of free fall where a capsule is falling in a gravitional field and thus continually changing velocity, direction or both: the ball would float beside the floating person (tidal effects being absent or very small).
  17. Jan 21, 2014 #16
    We can not measure weight without acceleration (gravitational or some other due to interaction forces)
    There can be two reasons for that

    1. Acceleration is the cause for mass

    2. We just have not found a way to measure it without acceleration
  18. Jan 21, 2014 #17


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    Jocanon, Have you read this?
    Last edited: Jan 21, 2014
  19. Jan 21, 2014 #18


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    You would really need to define your word "inertia" before getting a reliable answer to this question. Inertia has at least two distinct meanings, depending on context. Are you referring to Mass, Momentum or what?
    (Why are you rejecting the idea of "equations", out of hand - as if you think everything can be explained without them?)
  20. Jan 21, 2014 #19
    Here are some words about inertia from Feynman

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