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Acceleration due to gravity

  1. Nov 5, 2015 #1
    If gravity is not a force, rather the curvature of space time influenced by a body's mass, then why do we perceive an acceleration due to gravity, as though there was a force? In my mind, it would make sense for the bend in space to only cause a massive object to change direction. I suppose if something is to maintain a constant "speed" while it is changing direction, it also is accelerating. Does this mean that when someone drops a ball and it accelerates towards earth in what appears to be a straight line, really what is happening that it is maintaining constant speed but accelerating in a different direction due to the curvature of space?
  2. jcsd
  3. Nov 5, 2015 #2


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    Truly we do not understand the underlying actions of gravity but we reduce the action to an explanation that best fits the application of gravity that we observe within our environment, similar to the fact that we assign colors the wavelengths of electromagnet waves that are detectable by our optic nerves, although those electromagnetic waves have no intrinsic color at all.
  4. Nov 5, 2015 #3
    You are the one accelerating, not the falling object. The Earth surface is pushing *you* up, it's not gravity pulling the falling object down.
    Yes but that's not what's happening during freefall.
    No, again, a freefalling object is not accelerating. It looks like it is, because your coordinate system is accelerating the other way.
    The curvature of spacetime allows you to maintain constant distance from the Earth center, while accelerating away.

    Unless I'm mistaken :nb)
  5. Nov 5, 2015 #4


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    Imagine standing on a spinning merry-go-round. You feel a force that is pushing you outwards, away from the center and towards the perimeter. If you drop an object, your sense that there is such a centrifugal force is vindicated; the object accelerates outwards as it falls and ends up landing a spot that is not directly underneath your hand.
    But there's not really an outwards force (and this is why you'll hear people calling centrifugal force a "fictitious" force). Instead, you and the dropped object are trying to move along a straight line at a constant speed, just as inertia says you should. The surface of the merry-go-round is accelerating radially inwards as it follows its circular path. Because you are connected to that surface, it pushes you inwards (centripetal force) and that's what you're feeling when you experience the sensation of being forced outwards relative to the merry-go-round. The dropped object, not connected to the merry-go-round, experiences no centripetal force and falls straight down while you, your hand, and the point on the merry-go-round underneath your hand all accelerate away from it.

    Something similar is happening when you see a dropped object accelerating downwards. The object is actually travelling in a straight line at a constant speed, just the way that Newton's law says an object experiencing no force should. However, the surface of the earth is accelerating towards the dropped object at 1g and because you are standing on the surface of the earth, it is pushing you along as well. You can even measure the force that the surface of the earth is exerting on you to accelerate you upwards at 1g: stand on a spring scale and the spring will be compressed as the earth exerts a force on the spring and the spring exerts a force on you; and F=ma will tell you that your upwards acceleration is 1g. The ony reason you don't think you're accelerating is that the surface of the earth is accelerating with you/

    Search this forum for a video done by member @A.T. for a clear visual of what's going on and how an object's natural tendency to follow a straight line through spacetime produces accelerations.
    Last edited: Nov 6, 2015
  6. Nov 6, 2015 #5


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    See the video and links in the its description on youtube:

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