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I was trying to think of a situation in which an observer would be able to determine whether he is moving or not. Since velocity is a relative quantity I was unable to do so.

However, consider a situation in which an observer is sitting on a cycle moving with constant velocity (he is not pedaling the bike so, as such he cannot say whether he is moving or not).

In this situation, there is a way he can tell whether the cycle and he are moving or not. This is because if a cycle is moving then it will not fall. However, if it is not moving then it will fall with the observer like it would normally do. Thus the observer would be able to say whether he is moving or not.

Can someone clarify as to what point I am missing and how velocity is relative in this scenario.

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    $\begingroup$ Hint: put that bicycle on a treadmill. Does it fall? $\endgroup$
    – Emilio Pisanty
    Commented Nov 29, 2017 at 13:55
  • $\begingroup$ yes it does i guess. so? $\endgroup$
    – spatialdelusion
    Commented Nov 29, 2017 at 14:12
  • $\begingroup$ No, a bike wouldn't fall on a treadmill. Essentially you are talking about a gyroscope keeping the direction of its axis of rotation. This proves that rotation is not relative. Check this out: en.m.wikipedia.org/wiki/Mach%27s_principle $\endgroup$
    – safesphere
    Commented Nov 29, 2017 at 14:48
  • $\begingroup$ i didnt understand cld u explain in detail.Pl keep it simple.I dont understand GR $\endgroup$
    – spatialdelusion
    Commented Nov 29, 2017 at 14:51
  • $\begingroup$ can someone answer pl $\endgroup$
    – spatialdelusion
    Commented Nov 29, 2017 at 16:37

2 Answers 2

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Imagine you are on a planet the size of Earth, but which has a rotational period of 1 month 21 days. To avoid complexity let's assume this planet has no Sun: it's just sitting in space far from any star. You are on the equator, and you are cycling west at 20 miles an hour (someone has built a road around the equator).

Do you fall off your bike?

Because if you do the maths (and if I've done the maths right) you will discover that you are stationary (relative to the centre of the planet): you are cycling west at just the right speed to overcome the rotation of the planet towards the east.

Well, the answer is, of course, no, you don't. Because motion is relative, and the bike can't somehow magically know that it is going just the right speed that it is stationary.

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As long as the bike is moving forward unwaveringly in a straight line, there is no experiment you can do while on the bike to prove that it is you who is moving forward rather than the surrounding scenery moving backwards. If the bike stops and starts to tilt, you are no longer in an inertial frame of reference. So, yes, you have identified a way of determining whether you are in an inertial frame of reference or not (i.e., by experiencing an acceleration with no force apparently acting on you). But this has nothing to do with whether you can tell while in an inertial frame of reference whether it is you who is moving rather than another inertial frame of reference or whether it is moving. All you can tell is that they are moving relative to one another.

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