I was listening to The Infinite Monkey Cage on the BBC and they were talking about general relativity and gravity.

They were saying that gravity is not a force as Newtonian laws describe and is at odds with your real world experience. They said that if you are sitting on a chair, you don't feel a force pulling you down onto the chair. You feel the chair pushing up onto your bottom - at least that's what it feels like when you sit down and try it. They seem to be saying that gravity is not a force pulling from the center of the Earth, it's me that's still and the chair pushing up against me due to acceleration.

My question is if this description is accurate or have I misunderstood this?

If you are sitting on a chair, is what we call gravity actually the chair accelerating up onto me due to the geometry of Space Time?

Is there a better analogy for this?


Yes, that is a correct analogy if you want to be "relativistic" about it. Spacetime around a gravitating mass is like a river moving downstream. If you sit still on its surface, then you are moving relative to the shoreline. If you want to be still relative to the shores, then you have to exert a force against the water by paddling hard. The more mass there is (and the closer you are to it), the faster the river flows in the analogy.

The cool thing about this picture is that it also has an illustration for a black hole. A black hole is such a large mass that the difference in elevation between the source of the river (at infinity) and its mouth (at the center of the black hole) is so large, that a waterfall forms. No matter how hard you are trying to swim upstream, once you go over the falls (the event horizon), you can only fall deeper with the water, but there is no swimming back.

  • $\begingroup$ Good one about the waterfall and the black hole, have not head that one before. $\endgroup$ Jan 4 '16 at 12:38
  • $\begingroup$ @MikaelFremling: It's not mine, of course, I heard it first from Kip S. Thorne himself in a lecture about general relativity. $\endgroup$
    – CuriousOne
    Jan 4 '16 at 12:41
  • $\begingroup$ @CuriousOne that makes a lot more sense and is a great analogy. Thanks $\endgroup$
    – Phil_12d3
    Jan 4 '16 at 20:02

Gravity is pulling down on you from the centre of the earth but the force you are feeling is normal reaction.

The thing is, gravity is acting on you and everything around you so if you want to deal with the forces felt by objects around you, you can ignore gravity altogether.

Some examples:

1) If you are running behind a truck, at the same speed it is moving, you don't feel it. But if you move slower than it, you will bang into it and feel pain.

2) Take a weighing machine with you into an elevator. Weigh yourself when it accelerates upwards and downwards. You weigh heavier upwards and lighter downwards.

3) drop a heavy object on top of a light object. The heavy object does not damage the light object till they touch the ground.

The assumptions you can make in these cases:

1) both you and truck are still, and then truck accelerated towards you.

2) both you and lift are still then sudden upward or downward force acted on you.

3) both objects are still and suddenly normal reaction pushed them upwards.


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