# Free fall = standing still

## Main Question or Discussion Point

If you are in free fall you would be standing still, with no acceleration right?
So in a way you are decelerating(or accelerating in the opposite direction) in 9.8m\s^2, when you are standing on the earth, is that true? Or would you have to look at it otherwise?

So free fall cannot be defined as "acceleration" because free fall is acually just standing still, is that correct? (of course you are accelerating compared to a observer watching you fall down from a building, but you could say he was accelerating the other direction )

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Doc Al
Mentor
Hard to tell if you are being serious here. Everything is at rest when viewed from its own frame--so what? When we say that an object in free fall accelerates, we mean with respect to the inertial frame of the earth.

I was being serious...
What i mean, are the acceleration based on graviational or inertial acceleration? They are not the same.

arildno
Homework Helper
Gold Member
Dearly Missed
I think you have misunderstood general relativity a bit here:
In general relativity, gravity is not thought of as a Newtonian-like force, but rather as something warping/molding local spacetime. Any object within the influence of this spatio-temporal warp will still "go along a geodesic", the relativistic analogue to Newton's 1.law that without any external force acting, an object will move uniformly along a "straight line".

I know, that's why i can't image that gravitional "pull" is any pull(force) at all. It's rather the force you must the pull opposite directionthat decides the acceleration. So wouldn't this mean that standing still on earth would be a acceleration of 9.8\m\s

Doc Al
Mentor
So wouldn't this mean that standing still on earth would be a acceleration of 9.8\m\s
With respect to what?

with respect to a free falling object. I understand now why my question gave little sense. Of course it's like that. But a object in free fall are not being puhsed or pulled in any way of newtonian force right?

Ok, we think of acceleration when we add force to a object. What i mean is that gravational pull cannnot be seen upon as the same thing right? because are not adding any force on the free falling object. If we add 1 N to a object in space it would be accelerating at 9.8\m\s^2, but if we add 1 N to the free falling object in the other direction it would only stand "still" compared to the object that makes the graviational pull...

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ShawnD
Gravity still adds force to your body as you fall, but air resistance adds equal force in the opposite direction.
It's sort of like being hugged. You don't move in any particular direction when somebody hugs you, but you do feel forces in all directions. Rawr

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Do you mean our weight from the 'pull' of earth's gravity can be seen as a gravitational acceleration? Like a 1g acceleration from a rocket is equal and indistinguishable from a 1g 'pull' you feel on the earth's surface.

The acceleration from free fall is quite different from the force of gravity while standing on the surface of the earth. This has me woundering if they are two different forces. Gravity is inversly proportional to its distance squared. Logically I see that in free fall you are weightless. How can this be? Could you weigh an object in free fall to test Newton's Inverse proportionality theory? And I don't mean measure its inertia either.

If you are satisfied with the standard model of gravity and motion then thats great, A satisfaction I fail to enjoy. The more time I spend reading and learning about physics, motion and in particular gravity the more I find questions, contradictions and confusion. And somehow I enjoy it all the more.

See someone who understands me :)

Anyway, doesn't this mean anything? Gravitational acceleration is not the same thing as inertial acceleration since you feel the G powers. So wouldn't it be most correct to compare 0 g in the universe with an acceleration of 9.8m\s^2 on earth? Since both is "free fall" and 0 g. Just that one of them is accelerating more than the other compared to eachother.

D H
Staff Emeritus
A body in free-fall is accelerating. It is not "standing still". Strictly speaking, a body is in free-fall when gravity is the only force acting on the body. Relaxing this strict definition a bit, one could say a body is in free-fall if the gravitational force dominates all other forces. A sky diver who jumps off a hovering helicopter reaches terminal velocity after falling a 1000 feet or so. The sky diver feels his full weight after reaching terminal velocity.

An spacecraft in low-Earth orbit is in near free-fall conditions. The spacecraft is not standing still; it is moving at more than 17,000 miles per hour. Because the exoatmosphere is so thin, atmospheric drag is several orders of magnitude small than the gravitational acceleration.

You do not feel the acceleration due to gravity. When you are standing on the surface of the Earth, the net force on your body is zero. So why do you feel anything? You don't feel gravity pulling every part of your body toward the center of the Earth. You do feel the upward mechanical force of the Earth on your feet that exactly the downward gravitational force. You don't feel gravity pulling your arms down. You feel the tensile forces on your bones, tendons, muscles, and skin that keep your arm from falling off of your body. As with the force on your feet, this tensile force is directed upward and exactly counterbalances the gravitational force.

ShawnD
If you are in free fall you would be standing still, with no acceleration right?
So in a way you are decelerating(or accelerating in the opposite direction) in 9.8m\s^2, when you are standing on the earth
I think I know where the confusion is coming from. The number of 9.8m/s^2 is actually a derived value.
http://en.wikipedia.org/wiki/Law_of_universal_gravitation

That formula is actually meant to solve for the force of gravity between 2 masses, which would be you and earth. Solving for it, you get your force of gravity, F. Another law of physics is that force is related to mass and acceleration,
F = ma
If you move the terms around you get:
a = F/m
If you fill in your body weight (using a scale) for F, and fill in your body mass (using a balance) for m, you get a = 9.8m/s^2. This is how fast you will accelerate in free fall, relative to the earth.

Now for where the confusion comes in. You're saying something to the effect of "if I'm standing still, how am I accelerating? this is total BS" and you are correct. When you are standing still, you are not accelerating. That acceleration is a derived value under the condition that you are falling. When you are not falling, it does not apply, and you would use the univeral gravitation formula to find your weight when standing still.

Well, you can't deny that gravitational acceleration and inertial acceleration is different? Inertial acceleration requires and external force pushing the object. And the accelerating object feels G power.

In gravitational acceleration an object does not feel G power. (i am not including air resistance here) it accelerates all right, but no force other that gravity "drags" the object.

My point actually was: It is more correct to say that an object falling due to gravity and accelerating is actually standing still, since it is only trying to find it's correct place in space.

So if an object experience 0 G, it won't move. If a star comes around and drags it towards it, the object still experience 0 G's even though it's moving. So why don't you just agree with me that gravity cannot be seen upon as a force like an object pushing another, attracting magnets and charges connecting. :)

ah,

"So what people standing on the surface of the Earth perceive as the 'force of gravity' is a result of their undergoing a continuous physical acceleration caused by the mechanical resistance of the surface on which they are standing."

- Wikipedia, General Relativity

Another statement that proves my point that we are decelerating (accelerating in the opposite direction of the earth):

"Similarly, two balls initially at rest with respect to and above the surface of the Earth (which are parallel paths by virtue of being at rest with respect to each other) come to have a converging component of relative velocity as both accelerate towards the center of the Earth due to their subsequent free-fall".

- Wikipedia, General Relativity

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Inertial Mass is the resistence to change of motion, I see it as a time delay. 2 masses one is half the size of the other and with the same force applied it would take the larger mass twice as long to react, accelerate and travel the same discance as the smaller one. This does not hold true for free fall and Jarle brings up a good point that free falling bodies don't feel a G force with gravitational acceleration. I had not thought of that.
When you are standing on the surface of the Earth, the net force on your body is zero.
I have a problem with this statment and its not a problem with the posters reply but that this is what is taught in physics today. The net force on your body is not zero. I don't mean to argue but 'come on' there is work being done even if the work equation is showing zero work due to zero distance. This equation was desgined to measure the amount of work being done, and this is where it fails. I believe the reason why we are taught that 'net force is zero while standing on the earth' is that it might make gravity seem like perpetual energy. So dismissing it is way easier then opening up that can of worms.

You do not feel the acceleration due to gravity.
D H, think of this, how do you know when your leaning to far to the left and starting to fall over while your standing up? It is your ability to detect gravity, or more precisely detect accelertation, in you inner ear. A sensory organ that can detect acceleration (I think of it as our sixth sence).

I also have trouble with imagining the upward force that holds objects from falling through the earth. Again general physics here, a stack of books sitting on a table are pushing down with 'x' force and the table is pushing up with equal opposite 'x' force. so If we can magically take the books away fast enough does the table fly up due to this upward force? I see it as weight displacement, no opposing force here unless you see displacement as equal and opposite resistance.

Jarle, I look at it as all objects are in motion and the term 'at rest' is realative but not absolute. I remember reading a quote from einstein and it caused me to think of gravity as an acceleration....perhaps an acceleration into time. I am really stretching this though here but it helps me understand time dilation due to mass.

D H
Staff Emeritus
You do not feel the acceleration due to gravity.
D H, think of this, how do you know when your leaning to far to the left and starting to fall over while your standing up? It is your ability to detect gravity, or more precisely detect accelertation, in you inner ear. A sensory organ that can detect acceleration (I think of it as our sixth sence).
The otoliths in our ears and the accelerometers in a spacecraft or fighter jets sense linear acceleration due to all forces but gravitation. No device that directly senses the acceleration due to gravity can exist. True free-fall is indistinguishable from inertial motion per the equivalence principle. General relativity went one step further: Per general relativity, free-fall is inertial motion.

Think of how this relates to what you said at the start of your post:
Jarle brings up a good point that free falling bodies don't feel a G force with gravitational acceleration.
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I also have trouble with imagining the upward force that holds objects from falling through the earth. Again general physics here, a stack of books sitting on a table are pushing down with 'x' force and the table is pushing up with equal opposite 'x' force. so If we can magically take the books away fast enough does the table fly up due to this upward force? I see it as weight displacement, no opposing force here unless you see displacement as equal and opposite resistance.
The mechanical resistance is a true force. What would happen if you replaced the table with springs? The springs will fly up you take the books away fast enough. The table is nothing more than an extremely stiff spring.

I just read the article about general relativity, or at least half of it on wikipedia. It got me thinking. If we are accelerating at the speed of 9.8m\s^2 as this quote means "people standing on the surface of the Earth perceive as the 'force of gravity' is a result of their undergoing a continuous physical acceleration caused by the mechanical resistance of the surface on which they are standing.", don't we get a higher velocity? let's say we accelerate at 9.8m\s for a long time, let's say thirty years, as most people live at this earth, wouldn't we get a velocity higher than speed of light? I know we wouldn't but we would be very near the limit since further adding of N force would decrease amount of acceleration. But that would mean that we had a very very high weight, since an object moving very close to speed of light, experience a extreme weight.

I know that gives little sense, but i didnt think logically, I just thought the effect of an acceleration of 9.8m\s^2 over thirty years, and that would give that effect. and the GR points out that we ARE accelerating 9.8m\s^2 AWAY from the earth at all times when we are standing on it. I know everything is relative. but wouldn't a perosn feel that he is moving at 9.9999999% at the speed of light, in the way of measuring the force needed to be added to reach a higher acceleration? (it's humonguos!)

And the table are not pushing the books upwards. the atoms are only sharing the weight of the books, and the earth's atoms is sharing the weight of the table. When the books are removed, the atoms have freedom to organaize as they were when the books werent there, this may cause a slight "jump", but of course a string's atosm would organize in a way more dramatically way than the table as D H says.

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D H
Staff Emeritus
The table most certainly is pushing the books upward. There is no "sharing of atoms". At the interface between the book and table, the electrons orbiting the nuclei at the face of the book and the electrons orbiting the nuclei at the top of the table repel each other. The upward force is electromagnetic.

Your interpretation of "http://en.wikipedia.org/wiki/General_relativity#Treatment_of_gravitation"" is too three-dimensional. It is spacetime, not just space, that gravity curves. The mechanical resistance of the Earth against the people standing on it, and of table against the books piled on it, are real forces that counteracts the spacetime curvature. Without those mechanical forces, the books would follow a geodesic in spacetime. In other words, they would fall through the table.

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Well, if you look at it that way, then yes. The table are "pushing" the books at the same force as the books are "pushing" the table. But the electrons are only resisting the force of the "pushing" books, isnt that right?

Well, the quotes are taken directly from wikipedia about the general relativity which indeed are working with space time, a four dimensional space... There is no inertial force that are pushing the books. The gravity are letting the books "fall" into the earth. Or at least that is what it want. The table is resisting this acceleration. And the mechanical forces is excactly what I am saying that is decelerating for example the books from falling through the table. I didnt deny the mechanical forces, they was what i was thinking of....

Anyway, the main point was that an object in inertial movement is moving along the coordinates, the geometrical 2 dimensial area we can look at spacetime as. In free fall, the object is standing still in this geometrical area, it is not moving along any coordinates. At least that was what I ended up with when i was finished reading that article.

Ok, i don't know if this is right, it just seems logical at the moment: Look at it like this: If we only hypothetical lived in a 2 dimensial world, moving upwards wont give any meaning. Moving upwards in a three dimensional world would give meaning There was no force that moved a hypothetical object upwards, but if you wanted to move sideways, it needs force. Falling due to gravity would not give any meaning in a three dimensional world, but we are not living in a three dimensional world, we are living in Spacetime a four dimensional world, where free fall give a meaning. we are changing our position in this 4d world and see this as accelerating towards a point. If we only lived in a three dimensional world we would be standing still (with no external force added)

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D H
Staff Emeritus
Jarle, you appear to have some misconceptions. What do you mean by "inertial force", "inertial acceleration", "inertial movement", and "standing still"?

An object in "free fall" is moving along a geodesic in curved spacetime. It is not "standing still".

:\ I am not so into the definitions of the terms...

I think I meant inertial acceleration.

An object in "free fall" is moving along a geodesic in curved spacetime. It is not "standing still".
Yes, but isn't that the system used to coordinate the four dimensional spacetime we live in? If we strictly think if a coordinatal field in a three dimensional world we would be standing still? I am pretty sure i read about it in the article. I think what I meant was the three dimensional Euclidean space, that we are standing still in this three dimensional system. Not in Spacetime, the four dimensional geodesic space. Am I right, or is this way out into the wilderness?

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Whats important for me is to try and understand the thoughts here. Because I said I had a problem with the upward force explination did not mean I thought it was incorrect, I just visualize it in a different way. I have been going over the replys here and I still need help with D H's explination of that force, upward force seems unrealistic. Maybe I am looking at it in the wrong manner.

The definition of terms used is important to help get our thoughts across as I am interested in understanding your ideas here and I hope you feel the same.

The table are "pushing" the books at the same force as the books are "pushing" the table. But the electrons are only resisting the force of the "pushing" books, isnt that right?
No, I don't think that is correct.

The books are not pushing the table and neither are the electrons resisting anything. It is the opposite the electrons are pusing the books up!

But of course this view is general relativity not Newtonian.

The key in understanding this leads to the core of general relativity, namely the equivalence principle.

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If we strictly think if a coordinatal field in a three dimensional world we would be standing still?
One of the conclusions we can draw from relativity is that it is useless to say if something is standing still or is in constant motion. It is the same, the laws of physics are invariant to inertial motion.
We are only moving relative to something else, and that something else must have mass. But whether we accelerate or not is absolute and something we can measure.

A comet approaching the earth is in free fall (if we exclude atmospheric resistance for a moment). Now is it standing still or moving? It does not matter, if we say it is moving we always have to provide a refererence as to what it is moving in relation to. Is it accelerating? Well in a general relativity sense no, the movement is inertial. While movement is a relative concept in relativity acceleration is not. So when something stops the comet or slows it down, for instance the earth's atmosphere, then we can say that the electro-magnetic forces in the air accelerate this comet, accelerate away from the direction of the center of the earth.

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You are saying that acceleration and a constant velocity is not the same, and I assume it is because in acceleration you experience G force. The thing that doesn't say click in my head then, is gravity.

An object could be in parallell acceleration in outspace with an object on earth, with the object in space moving by inertial force, and the object on earth moving because of the gravity. The difference is that the object on earth DOES NOT feel G powers, it is like it is just standing still(standing still=constant velocity, I may have explained myself a bit wrong here), even though it is accelerating.

If find that pretty impossible, that's why i need to (in my head) differ the two kind of movements. One is inertial motion\acceleration, and one is gravitational motion\acceleration.

I thought since the object on earth accelerationg is not feeling any G's they can't be "moving" in a 3 dimensional space, but in a 4 dimensional space, yes. If a object moved upwards in a 2 dimensional space it would be standing still in the observers eyes. If it moved upwards in a three dimensional spave it WOULD move in the observers eyes too.

That's why I mean that there can be a coherence between this when we add another dimension, the fourth. That the object in free fall are "standing still" in the three dimensional space, but not in the fourth.

.......or what?