# The Naked Scientists Forum

### Author Topic: Does Gravity do any work?  (Read 69843 times)

#### Joe L. Ogan

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##### Does Gravity do any work?
« on: 12/12/2009 18:59:53 »
Does Gravity do any work?  I think there is a school of thought that believes that Gravity does no work.  It appears to me that the concept may be in error.  I welcome comments.  Thanks.  Joe L. Ogan

#### Mr. Scientist

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##### Does Gravity do any work?
« Reply #1 on: 12/12/2009 19:36:08 »
It is in error, because gravity is the same as matter, and since matter does work, the postulation speaks for itself :)

#### Joe L. Ogan

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##### Does Gravity do any work?
« Reply #2 on: 12/12/2009 21:15:02 »
Is that really true?  That is a rather clear definition of Gravity.  I wonder why Einstein didn't think of that.  Thanks.  Joe L. Ogan

#### Mr. Scientist

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##### Does Gravity do any work?
« Reply #3 on: 12/12/2009 21:36:34 »
he did

#### Farsight

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##### Does Gravity do any work?
« Reply #4 on: 12/12/2009 23:19:53 »
This is an interesting one, Joe. When we look at something as simple as a waterwheel, we say that gravity does do work. We get hydroelectric power from this, so it looks cut and dried.

But when we look deeper, we see there's something of a problem. Work is the transfer of energy, and if we examine two masses falling towards one another, we know from conservation of energy that no energy is added to the "system" that is those two masses. That's because energy causes gravity, and if energy was really being added, the gravity of the two combined masses would exceed the gravity of the two separate masses.

If the two masses spiralled around one another in a closing orbit and then fused gently and stayed cold, we wouldn't have extracted any work. If however we extracted work from the two masses falling together, the energy we extracted will be dissipated as heat. Then we know from E=mc² that the two masses will then weigh a little less. So the answer is: no, gravity doesn't do any work, but it does allow us to do work.

#### Joe L. Ogan

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##### Does Gravity do any work?
« Reply #5 on: 12/12/2009 23:31:20 »
Is Gravity the same as Matter?  Did Einstein so state?  Thanks.  Joe L. Ogan

#### Mr. Scientist

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##### Does Gravity do any work?
« Reply #6 on: 13/12/2009 00:18:07 »
Yes joe. He eqiuavalated gravity to mass and not only gravity, but acceleration and natural distortions. :)

#### Mr. Scientist

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##### Does Gravity do any work?
« Reply #7 on: 13/12/2009 00:20:51 »
This is an interesting one, Joe. When we look at something as simple as a waterwheel, we say that gravity does do work. We get hydroelectric power from this, so it looks cut and dried.

But when we look deeper, we see there's something of a problem. Work is the transfer of energy, and if we examine two masses falling towards one another, we know from conservation of energy that no energy is added to the "system" that is those two masses. That's because energy causes gravity, and if energy was really being added, the gravity of the two combined masses would exceed the gravity of the two separate masses.

If the two masses spiralled around one another in a closing orbit and then fused gently and stayed cold, we wouldn't have extracted any work. If however we extracted work from the two masses falling together, the energy we extracted will be dissipated as heat. Then we know from E=mc² that the two masses will then weigh a little less. So the answer is: no, gravity doesn't do any work, but it does allow us to do work.

Farsight.. enough. You have seen my reply - your archiac science has no place when considering these things. Obviously if gravity is a physical force, it must have a work associated to it.

#### Geezer

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##### Does Gravity do any work?
« Reply #8 on: 13/12/2009 00:57:40 »
Farsight.. enough. You have seen my reply - your archiac science has no place when considering these things. Obviously if gravity is a physical force, it must have a work associated to it.

I don't think so. Mr S, I presume you are sitting on a chair. Gravity is exerting a force on you and the chair is reacting to that force to support your mass. While the chair continues to support that mass, no work is done. If the chair collapses and you fall to the ground, work has been done. Just because there is a force it does not mean work is being done.

#### Karen W.

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##### Does Gravity do any work?
« Reply #9 on: 13/12/2009 01:56:22 »
By work do you mean there was no energy expended in sitting down on the chair Geezer?
« Last Edit: 13/12/2009 01:58:02 by Karen W. »

#### Joe L. Ogan

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##### Does Gravity do any work?
« Reply #10 on: 13/12/2009 02:29:38 »
Yes joe. He eqiuavalated gravity to mass and not only gravity, but acceleration and natural distortions. :)

Mr. Scientists.  As you know, I have great respect for your Scientific knowledge.  You have increased my vocabulary.  You have taught me a lot about Science.  But your statement that Mass and Gravity are the same thing begs the question with me.  Please let me explain.  If you give me some Mass, I can measure it, I can weigh it.  I can tell you if it stinks or if it smells good.  I can't do any of those things with Gravity.  It will be very interesting to see you tell me how I can do those.  Thanks for your comments.  Joe L. Ogan

#### Geezer

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##### Does Gravity do any work?
« Reply #11 on: 13/12/2009 02:34:07 »
By work do you mean there was no energy expended in sitting down on the chair Geezer?

Of course work was expended in moving your mass to sit in the chair. But once you sit in the chair, no work is being done. However, the gravitational force persists.

If mass is moved along the direction of the force of gravity, work is done by, or against, gravity. While you are sitting in your chair, gravity is doing no work.

"Ah ha!" you might say, "So why am I still orbiting the centere of the planet? Surely the force of gravity is supplying the energy to make this happen."

Simply, it cannot do that! The force of gravity acts towards the centre (of mass) of the earth. At any moment while you are sitting in your chair, your direction of travel is perpendicular to that force. Unless Einstein repealled the basic laws of mechanics, it is not possible for gravity to propel your movement around the center (of mass) of the earth. Your motion is a function of the friction between you and the earth. The friction is a function of the gravitational force between you and the earth, but unless you move along that gravitational line of force, no work is done by, or against, the force of gravity.
« Last Edit: 13/12/2009 02:35:40 by Geezer »

#### Karen W.

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##### Does Gravity do any work?
« Reply #12 on: 13/12/2009 02:46:48 »
Is the balancing you do to stay in an upright position constitute force still happening or does it become different from side to side why I ask this is that when I have passed out in my chair my body has slumped down and out of the chair to the ground where I find myself when I have come too, so is there not a constant work happening in just trying to sit upright in a chair to begin with?

I am not trying to be difficult but to understand as I am horrible with gravity and it is a difficult subject even with basic understandings...

#### Geezer

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##### Does Gravity do any work?
« Reply #13 on: 13/12/2009 02:59:57 »
Is the balancing you do to stay in an upright position constitute force still happening or does it become different from side to side why I ask this is that when I have passed out in my chair my body has slumped down and out of the chair to the ground where I find myself when I have come too, so is there not a constant work happening in just trying to sit upright in a chair to begin with?

I am not trying to be difficult but to understand as I am horrible with gravity and it is a difficult subject even with basic understandings...

No problem. It's quite simple really. If your "center of mass" moved closer to the earth's center of mass when you slumped forward, (which it probably did) then the force of gravity did work. When you realized that you were about to start inhaling your soup, you had to do a corresponding amount of work to straighten yourself up again, and you did that work against the force of gravity.

If you are sitting on an office type chair with wheels, and you scoot across the floor on it, you did some work to do the scooting, but gravity didn't do any work because the distance between your center of mass and the earth's center of mass didn't change (unless your floor has a nasty slope, in which case you should sell your house soon.)

BTW, we are all acutely aware of the force of gravity, particularly as we get older. We tend to take it for granted. We do have a pretty good idea about what it does, certainly in close proximity to our star (the sun), but although we can predict quite accurately what it does, we don't really have a very good handle on how the heck it does it! There are many schools of thought on that one.
« Last Edit: 13/12/2009 03:28:46 by Geezer »

#### Karen W.

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##### Does Gravity do any work?
« Reply #14 on: 13/12/2009 03:22:01 »
Thank you Geezer.. I need to contemplate that a while! maybe look up some references..or diagrams..you have any... diagrams that is? I am a visual girl!

#### Geezer

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##### Does Gravity do any work?
« Reply #15 on: 13/12/2009 03:36:31 »
Le sigh!

#### Geezer

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##### Does Gravity do any work?
« Reply #16 on: 13/12/2009 05:20:44 »
As Cyril Fletcher put it, "The effects of gravity can be grave."

Seriously, gravity is a major embarrassment to modern science. We are all continuously aware of the effects of gravity. Scientists can describe very accurately how gravity influences all objects with mass, but (as far as I am aware) there is no consensus in the scientific community that describes how gravity actually manages to exert its invisible force.

We can talk all we want about the other forces in nature, but if we can't even explain gravity, why would we believe any of them?

(I'm heading for my underground bunker right now.)

#### Mr. Scientist

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##### Does Gravity do any work?
« Reply #17 on: 13/12/2009 07:01:21 »
I give up. I am telling you all, there is work assocoiated to the gravity of something. If it didn't, gravitational objects could not move. It's very simple.

#### Mr. Scientist

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##### Does Gravity do any work?
« Reply #18 on: 13/12/2009 07:04:21 »
Is the balancing you do to stay in an upright position constitute force still happening or does it become different from side to side why I ask this is that when I have passed out in my chair my body has slumped down and out of the chair to the ground where I find myself when I have come too, so is there not a constant work happening in just trying to sit upright in a chair to begin with?

I am not trying to be difficult but to understand as I am horrible with gravity and it is a difficult subject even with basic understandings...

Hi Karen

It's technically true to say there was a gravitational work being done - such things are possible for objects and its called a gravitational potential. However, most of the atoms in your body are able to sit down because of electrostatic repulsion, where all the electrons that are in your body which meet the chair all repel each other like little magnets, so in most cases scientists prefer to think of it being far more ovewhelming than lets say a gravitational influence.

#### Mr. Scientist

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##### Does Gravity do any work?
« Reply #19 on: 13/12/2009 07:07:34 »
Farsight.. enough. You have seen my reply - your archiac science has no place when considering these things. Obviously if gravity is a physical force, it must have a work associated to it.

I don't think so. Mr S, I presume you are sitting on a chair. Gravity is exerting a force on you and the chair is reacting to that force to support your mass. While the chair continues to support that mass, no work is done. If the chair collapses and you fall to the ground, work has been done. Just because there is a force it does not mean work is being done.

Now you're talking rubbish.
To have any physical system do work, there must be a force applied to it. This was simple Newtonian dynamics. The chair collapsing does not because of the overwhelming electrostatic force, not the gravity which almost surely cancels out.

#### Geezer

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##### Does Gravity do any work?
« Reply #20 on: 13/12/2009 07:29:55 »
Farsight.. enough. You have seen my reply - your archiac science has no place when considering these things. Obviously if gravity is a physical force, it must have a work associated to it.

I don't think so. Mr S, I presume you are sitting on a chair. Gravity is exerting a force on you and the chair is reacting to that force to support your mass. While the chair continues to support that mass, no work is done. If the chair collapses and you fall to the ground, work has been done. Just because there is a force it does not mean work is being done.

Now you're talking rubbish.
To have any physical system do work, there must be a force applied to it. This was simple Newtonian dynamics. The chair collapsing does not because of the overwhelming electrostatic force, not the gravity which almost surely cancels out.

Mr S,
It is unfortunate that you have to descend to terms like "talking rubbish". Kindly review my explanation and provide me with some data that points out a flaw in my reasoning.
G

#### Mr. Scientist

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##### Does Gravity do any work?
« Reply #21 on: 13/12/2009 07:32:02 »
I apologize. I've only just woken up. I have had many things on my mind and it does not excuse my use of language. I will though, point out your mistakes, properly.

#### Mr. Scientist

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##### Does Gravity do any work?
« Reply #22 on: 13/12/2009 07:32:17 »
How are you with math?

#### Geezer

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##### Does Gravity do any work?
« Reply #23 on: 13/12/2009 07:39:03 »
My math is limited. My understanding of mechanics and dynamics is not so limited. Work is a very simple concept. Force applied without movement (displacement) does NOT constitute work.

Get your facts straight before you start rubbishing other posters.

#### Mr. Scientist

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##### Does Gravity do any work?
« Reply #24 on: 13/12/2009 07:51:03 »
Well, this won't be too hard. :)

Take the constant g - this is gravitational acceleration. This is the kind of acceleration attributed to moving objects. In fact, we can pretty much tell the motion of how thing move to earth because of this constant, which has a value of about 9.8N.

x=x_0+v_0(t)+1/2(at)^2

where t is the function of time, x_0 is the initial position and v_0 is the initial velocity. Now, you can plug in pretty much any values you want for the time, position, but you can specifically replace (a) which is the acceleration symbol with [g] - ''that'' gravitational acceleration so as you might surmise to understand, force is actually attributed to the gravitational acceleration and we often give it as F=Mg - meaning that force is inversely-related to the acceleration of something, a point you where completely ignoring when i told you.

Suppose we did change the formula. It would now look like:

y=v_{0v}(t) - 1/2 (gt)^2

This means we can work out the height of an object - an the essential word here is being ''work'' - so how is work associated to everything conjectured so far?

well, this can be easier to understand. The energy of any mass is given in terms of a possible work it may do. This is the gravitational potential, and if there is a potential for any mass to move, then it SURELY CAN be said there is an associated possible work.

GPE=Wh

is the Gravitational potential energy (GPE) equation, and W is the weight and h is the height. And so:

GPE=Mgh

Where M is the mass. So i have proven here that the mass is certaily related to the work it can do. Just to prove it with a final set of equations, h/2 will be our units above the ground, so to have something move, we now yield the formula:

W_k=Fh

where W_k is the work due to kinetic energy and this work of any material body i inversely related to the force and hieght of the system from some h/2 from the earth's surface.

Done.

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##### Does Gravity do any work?
« Reply #24 on: 13/12/2009 07:51:03 »