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The equivalence principle?

  1. Oct 24, 2015 #1
    Here is what I understand of the equivalence principle. However, given that the concept is still a bit cloudy to me, please tell me if I am wrong:
    The equivalence principle is a phenomena where all inertial objects experience the same laws of physics. In other words, if you were going at constant speed, you would not be able to tell that you were moving unless there is a window because the laws of physics(ex. throwing an object in the air and expecting it to not fall forward) is the same. The equivalence principle states that if we were to close the window, we could not tell if we were accelerating upwards in a rocket or towards the Earth if the acceleration is 9.25. Only in cases where we accelerate to more than such, such as in a free-falling elevator, would we be able to tell that we are moving. Astronauts in space likewise experience weightlessness because all objects are free-falling together, and hence, there is nothing to push against them so they can " feel" gravity.
    This can all be used to explain why the apple " knows" to fall downwards. Gravity causes the apple to fall towards the Earth at 9.25 so it would meet my head. If we were all experiencing free-fall and are experiencing acceleration at the same rate, the apple would not " fall" downwards.
    If all that I said above was correct(which very likely it is not), does that mean our rates of accleration/gravity on Earth is different?
  2. jcsd
  3. Oct 24, 2015 #2

    Buzz Bloom

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    Gold Member

    Hi Hihello:

    I think for the most part, what you wrote is correct. However, there are a few places where I have made suggestions for changes.

    I think this needs a bit of editing.
    (1) "going at constant speed" fails to eliminate moving at constant speed in a train along a track on the Earth's surface.
    (2) "unless there is a window" fails to require that there is something useful to see outside the window.

    (3) "upwards in a rocket" creates an ambiguity. I suggest "in a rocket upward".
    (4) I think "9.25" is intended to refer to the Earth's acceleration of gravity. I suggest you substitute "9.81 m/s2".

    (5) "in a free-falling elevator, would we be able to tell that we are moving" is confusing. If you are in a free falling elevator, you would not necessarily experience this as moving, since the elevator could just as well be out in deep space.

    (6) I think you are confused about acceleration and free fall. You experience the Earth's gravitational acceleration when you are standing on the earth's surface and feel the upward pressure against your feet. When you are in free fall, you do not experience this pressure, and you might be stationary in outer space, or moving at a constant speed in outer space, of accelerating towards the Earth in a falling elevator. In all these cases, you would fell the same experience.

    You may also find it useful to look at https://en.wikipedia.org/wiki/Equivalence_principle.

  4. Oct 25, 2015 #3


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    Staff: Mentor

    That's not the equivalence principle, it's the principle of relativity. It's many centuries older than the equivalence principle - we generally credit it to Galileo. The equivalence principle is what you say next, with one correction that I've made inline:
    It is the same, 1 G or 9.8 meters per second per second, for everything at or near the surface of the earth. Imagine an observer so far away from earth that he can completely the effect of earth's gravity on him. If he's watching through a powerful telescope, he will observe that everything in the vicinity of the earth's surface is either accelerating towards the center of the earth at 1 G or is being prevented from doing so by some opposing force (for example, it's resting on the surface of the earth so it can't move towards the center).
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