It's a common experience when we are in some vehicle like a car being accelerated we can feel the "invisible push/pull" or acceleration caused due to the change in velocity I assume we can always trust this ability of ours to decide if we are being accelerated whether in car/bus/train/rocket whenever our velocity changes we can feel it. In physics lectures you often hear professors saying that when a body say a closed box is freely falling (ff) say towards the earth, the man (or whatever) inside can never tell if he is in ff or somewhere in quiet space away from all matter with no gravity around.

But now you can see the problem in former the man's velocity is increasing by $9.8~\mathrm{m/s}$ every second now if I give a super-car and you put on the accelerator definitely your friend sitting besides, even if blind and deaf can tell if he is indeed accelerating or not. So now here is my question why the man inside the ff box can't feel the acceleration and (his velocity is changing wildly) and how is his state identical to that of a man standing still in deep space with no gravitational pull around.

I was seeking a intuitive answer which doesn't involve relativity theory as I don't understand it yet and which could be explained by classical mechanics.


marked as duplicate by Emilio Pisanty, Diracology, Qmechanic Jul 28 '16 at 17:58

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ imagine you're inside a lift. You jump from its floor, and when you reach the maximum height(i.e when you come to relative rest again), the lift is let go. Now, both you and the lift fall identically(accelerate identically). In other words, you will find that you are hanging in mid air, and are not able to reach the floor of the lift. This is essentially similar to that of a gravity free space. You(the observer) is clueless about the field of gravity, as it appears to affect you in no way $\endgroup$ – Lelouch Jul 28 '16 at 15:41
  • $\begingroup$ Hmm i see now after your help and reading other Q's I think accelerating under gravity can't be compared with accelerating in a rocket as former only is illusionic acceleration while latter provides real acceleration,ir may be something exists in gravity that messes up the things so accelerating isn't really accelerating anymore $\endgroup$ – Viney Hill Jul 28 '16 at 15:53
  • $\begingroup$ I think this is a great question, not a duplicate of the referenced post at all. I taught AP Physics for years and never got asked this question. Good thisng too! If I may, restated in brief: In a car, we feel the press of the seat when accelerating. When falling, we do not feel the pull of gravity. I suspect Pixel would like a Newtonian answer. $\endgroup$ – bpedit Jul 28 '16 at 16:42
  • $\begingroup$ It's because gravitational mass is the same as inertial mass. $\endgroup$ – Peter R Jul 29 '16 at 0:37
  • $\begingroup$ I think this is a great question, not a duplicate at all. The acceleration is real not illusory. $\endgroup$ – Rahul J A Jul 29 '16 at 11:42

CASE -I:Consider the acceleration of a body with mass M when a force of 100 N is applied on a body.
CASE-II : Consider the acceleration of the same body when a force of 1000 N and 900 N respectively are applied on it simultaneously in opposite directions.

The acceleration in both cases will be the same as the net force is 100 N.

Now consider the internal changes in the body. The body in case II is experiencing a much severe crushing power than in case I.

Similarly in a free fall only a single force is acting on the body. In all other cases cited in the question two or more forces are acting on the body in opposing directions.


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