I am a high-school student. Recently we learned the concepts of relative motion and velocity. The idea that anything in motion can subsequently be at rest depending on the frame of reference fascinates me. I am however unsure about a thought experiment I conceived . If an observer was trapped in a closed box with no way to interact with the external surroundings how will he know if he is moving or at rest ( by moving i mean with a constant velocity )? Is there any way to know as motion is relative? What noticeable (noticed by the observer) effect will a force applied on the box have on the observer?
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1$\begingroup$ Does this answer your question? physics.stackexchange.com/q/423597 $\endgroup$– AllureCommented May 27 at 4:48
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1$\begingroup$ Does this answer your question? Can we really not tell if we are moving? $\endgroup$– naturallyInconsistentCommented May 27 at 5:02
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1$\begingroup$ And the fact that you can't tell how fast you're moving is an essential insight which lead to the special theory of relatively :) $\endgroup$– user34722Commented May 27 at 5:32
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1$\begingroup$ You are not alone. ‘Galilean invariance or Galilean relativity states that the laws of motion are the same in all inertial frames of reference. Galileo Galilei first described this principle in 1632 in his Dialogue Concerning the Two Chief World Systems using the example of a ship travelling at constant velocity, without rocking, on a smooth sea; any observer below the deck would not be able to tell whether the ship was moving or stationary.’ en.m.wikipedia.org/wiki/Galilean_invariance $\endgroup$– my2ctsCommented May 27 at 7:05
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1$\begingroup$ It's great that you're thinking about this. Thinking this through is exactly the point of relativity of motion. Not only that you can't tell, but neither is "true" in absolute terms - motion is only meaningful with respect to some reference frame. Imagine an astronaut floating next to a spaceship doing repairs, somewhere in outer space. It makes no difference if they are stationary with respect to some distant star, or if they are moving in a straight line at an enormous but constant speed with respect to the same star. Note that due to Newton's 1st law, no engine thrust is required for this $\endgroup$– Filip MilovanovićCommented May 27 at 15:32
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3 Answers
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how will he know if he is moving or at rest
He can't. The two situations are indistinguishable.
If the box is in uniform (constant velocity) motion, there is no way the person inside can sense how fast he is going, or if he is at all.
The human body can only sense accelerations.
Is there any way to know as motion is relative?
If, as your question states, the person in the box has no way to communicate with the outside of the box, then no. The fact that motion is relative is not really the point here. The point is that provided no (net) forces operate the box, there is no way to tell.
What noticeable (noticed by the observer) effect will a force applied on the box have on the observer?
Because of inertia, any force will cause a feeling of "motion" that is usually in the opposite direction of the applied force. For example, if a force is applied accelerating him forward, he will feel a backward jerk. If the force is applied in the opposite direction, he will feel a forward push.
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This is the essence of Einstein's conception of special relativity: the laws of physics do not make any distinction between being at rest (relative to what?) and moving at constant velocity (again, relative to what?). This means there is no physics experiment you could perform from inside that box that could possibly tell you if you were sitting still or moving at constant velocity.
answered May 27 at 5:48
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Your inner ear will tell you, it will pick up the motion, be it ever so slight, but one symptom that you may feel is nausea or get a sore head. I get a form of morning sickness from a genetic condition I have and if I cannot see where I am going, or I am pointing in another direction from the way I am moving I feel nauseous.
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1$\begingroup$ As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center. $\endgroup$– Community BotCommented May 27 at 5:09
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1$\begingroup$ Your inner ear can only detect acceleration, not constant velocity. $\endgroup$– KDPCommented May 27 at 6:34
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$\begingroup$ Therefore, the question is flawed, your inner ear can detect movement, that is what it is for, it could only be deceived in space, as our bodies are designed only to work in gravity. $\endgroup$ Commented May 28 at 6:24