The Naked Scientists

The Naked Scientists Forum

Author Topic: How does a surface support a mass at rest?  (Read 2420 times)

Offline steviereal

  • First timers
  • *
  • Posts: 5
    • View Profile
How does a surface support a mass at rest?
« on: 28/04/2013 20:10:14 »
Hi guys!

I realized that I don't understand what's going on when an object rests on the table :-)
Gravitational force is applied to the table by the object (which I found out is stress force) and the table is exerting opposite force of the exact amount to keep the object lying there. But what is exactly going on inside the table? The molecules are kept together by binding energy, so the stress force is counteracted by these forces? Then, could we say that the binding energies inside the table counteract the object's force?
How does the table react to this force? Is it stored by deformity? When we lift the object off the table, the table is somehow "relieved". Is it translated to its deformity disappearing and the table regaining its shape?
So what I need to understand is the force-energy transition here. I know that if there is no movement there is no work done, still it seems that the table's molecular structure has a "hellofa job" keeping the object on the surface.

Thanks,

Stevie
« Last Edit: 01/05/2013 08:19:31 by chris »


 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: Stress force and energy
« Reply #1 on: 28/04/2013 20:47:56 »
Quote from: steviereal
Hi guys!

I realized that I don't understand what's going on when an object rests on the table :-)
Gravitational force is applied to the table by the object (which I found out is stress force) and the table is exerting opposite force of the exact amount to keep the object lying there.
First, let's be clear on what stress is. Stress is defined as force per unit area. It is completely described by a mathematical object known as a tensor. Forces can be normal or parallel to a surface or a combination of both.

Quote from: steviereal
But what is exactly going on inside the table? The molecules are kept together by binding energy, so the stress force is counteracted by these forces?
If only it were that simple. Molecules are kept together by electromagnetic forces and described by quantum mechanics. When two molecules are presse together there is a resultant force. First its very small attractive and when too close its a repulsive force which becomes greater the closer the molecules are pushed together. A solid is a complex structure composed of many molecules of different types and too difficult to explain using a simple picture of molecules. Its much easier tovisualize it with a potential energy diagram. Unfortunately I don't know where to find one on the internet that describes it. For simple things like steel you an described it easier.

See http://scienceworld.wolfram.com/physics/YoungsModulus.html

Quote from: steviereal
How does the table react to this force? Is it stored by deformity?
The table compresses.

Quote from: steviereal
When we lift the object off the table, the table is somehow "relieved". Is it translated to its deformity disappearing and the table regaining its shape?
Yes.
 

Offline steviereal

  • First timers
  • *
  • Posts: 5
    • View Profile
Re: Stress force and energy
« Reply #2 on: 28/04/2013 22:13:29 »
Thanks!
So could we say that that the gravitational force is counteracted by energy (that keeps the table together)?
Also, when we lift the object off and it regains shape, does it lose temperature?
Is the energy to be described by the W=F * delta L equation? (where delta L is the length to which the table is deformed.) Of course, here we suppose a very elastic table.
And yet, the more I write about it the more I get suspicious of the whole "force" concept. Aren't we talking about energy all the time? Force may only be energy that is inhibited by some "counterenergy" to do meaningful work.(?)
Maybe force is potential energy?
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: Stress force and energy
« Reply #3 on: 28/04/2013 23:19:42 »
Thanks!
So could we say that that the gravitational force is counteracted by energy (that keeps the table together)?
No. You're confusing force with energy.

Also, when we lift the object off and it regains shape, does it lose temperature?
No. This is work being done mechanically and not by a thermal process. If you had a weight on a piston that was being supported by gas pressure and took weight off then there it could be a temperature but change.

Is the energy to be described by the W=F * delta L equation?
Yes.

(where delta L is the length to which the table is deformed.) Of course, here we suppose a very elastic table.
All objects have some elasticity to them.

And yet, the more I write about it the more I get suspicious of the whole "force" concept. Aren't we talking about energy all the time?
Absolutely not. Force and energy are very different concepts.

Maybe force is potential energy?
No. At best a (conservative) force is the negative gradient of a potential energy function. i.e. F =- -grad U
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11978
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: Stress force and energy
« Reply #4 on: 29/04/2013 02:23:42 »
It is strange, isn't it :)

If I imagine a football, made of neutrons in deep space, and then place a cup of neutrons on it, I will find that it get stuck, by gravity. But if I treat it as a force then the cups attraction, as well as the balls to the cup, has to go somewhere, don't it? So where does it go? Equalized over the whole 'system', made out of the football and cup?
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11978
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: Stress force and energy
« Reply #5 on: 29/04/2013 02:41:47 »
The point is that gravity works both ways here. The ball attract the cup, and the cup attract the ball. So how does the force express itself? Normally one can think of it as when you sit down, you apply a force on the chair that the chair equalize by applying a equal force in the other direction. But that don't fit with me thinking of them both as attracting, does it? The world turned up side down somehow :)
 

Offline steviereal

  • First timers
  • *
  • Posts: 5
    • View Profile
Re: Stress force and energy
« Reply #6 on: 29/04/2013 20:40:33 »
No. You're confusing force with energy.
Okay, so if we concentrate on the force vs. energy problem here, could we say that force is a kind of factor that in practical circumstances always produces energy? I thought in the case of the table it is the difference of the gravitational potential energy of the supported object between its initial position (just touching the table) and its final position (lying, slightly compressing the table, it is somewhat closer to the center of Earth, so there is a difference in height, that is, a difference in grav. potential energy).
Also, if I'm pushing my palm against a wall for 5 seconds and I can measure all the energy released by the ATP molecules of my muscle cells, than the energy required to exert that force is equal to all that muscle energy exerted in 5 seconds. So can we say, there is always an energy associated with force?

I also thought about the gas piston analogy as an obvious example when force translates to energy and I think it doesn't matter if it's gas or solid, compression causes a rise in temperature.

Maybe what is causing the problem here is the mystery of fields, could it be that the infinity problem is at play here? (Gravitational and other fields have infinite range and lasts forever in time if the object causing it lasts forever.) Without fields, if we look at machines or creatures exerting the force, the energy required for it is possible to calculate. Fields, on the other hand are just plain weird though, aren't they?
« Last Edit: 29/04/2013 20:43:45 by steviereal »
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4096
  • Thanked: 244 times
    • View Profile
Re: Stress force and energy
« Reply #7 on: 29/04/2013 22:16:18 »
I prefer to say that "A Force is capable of doing Work, if it is applied to something which is capable of movement". Work has the same units as Energy (Joules), and is calculated as (Force applied) x (Distance moved).

Physics is full of mythical objects which don't move and so force does not do any work on them. In real life, pushing a palm against a brick wall or placing a book on a normal table is like applying a force to an immovable object so that it does no work (or a close enough approximation for most practical purposes).

Where you have a weak table (maybe plastic outdoor furniture) and a great force (like the weight of a bag of cement), there will be a significant deformation of the table, so the force is doing some work. This work goes into stretching the forces between some plastic molecules in the table, and compressing the forces between other molecules, effectively storing energy.
  • If the weight is not too great, the table will "bounce back", doing work to help you lift the bag of cement. In this case, the energy is temporarily stored as mechanical potential energy; only a small fraction of the work will result in permanent deformation of the plastic and friction between molecules, most of which will appear as heat.
  • If the table is too weak, the intermolecular forces will be be stretched to breaking point and the table will collapse. Because the movement is larger, more work will be done in this case. Some of the work will go into breaking intermolecular bonds, permanently deforming the plastic, throwing pieces of plastic into the air, increasing the temperature of the plastic and the cement - and generally making a loud noise.

The biological processes of muscle producing a force do consume energy from glucose and ATP. However, this energy is only efficiently transferred to an object which applies a resisting force in the "right" range - for example, a bicycle in the "right" gear.
  • If you try to start a bicycle with a too-high gear ratio, you will not be able to turn the pedals, so you do no effective work (but you still spend ATP energy). The Force is high, but there is no movement.
  • Conversely, if you try to start  a bicycle in a too-low gear ratio, you will be able to pedal furiously, but the pedals will provide no opposing force on your foot; the bike will go nowhere, so you do no effective work (but you still spend ATP energy). There is considerable movement, but no force.
  • Pushing a palm against a brick wall is effectively pushing an immovable object, so it does no useful work.
  • In these cases, the energy from ATP goes into a muscle force which stretches your tendons, and is released when you release the pressure. In the end it does no useful work, and it ends up as heat.
« Last Edit: 29/04/2013 22:42:16 by evan_au »
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11978
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: Stress force and energy
« Reply #8 on: 30/04/2013 00:23:13 »
pressing your palm against a wall means creating a pressure, that pressure must be a force too, even if the wall doesn't move. But that is what you mean Evan, right? Saying 'if it could move'. So you're defining it as a force, but not as 'work done'? 'Work done' is really a tricky one.. Me carrying a tray, straight above me head, is that 'work done' :) for example?
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4096
  • Thanked: 244 times
    • View Profile
Re: Stress force and energy
« Reply #9 on: 30/04/2013 11:16:56 »
Quote
carrying a tray, straight above me head, is that 'work done'?
The force from the weight of the tray is in an "up & down" direction. There is no horizontal force required.

If you managed to carry the tray above your head perfectly smoothly - on a skateboard, for example:
  • Force vertically = (say) 1kg weight. Distance the tray moves vertically = 0. Vertical work done on the tray = FxD=0
  • While rolling, force horizontally = 0 Newtons. Distance the tray moves horizontally = (say) 10m. Horizontal work done on the tray = FxD=0
  • Of course, there is work that you do to get your body & the skateboard moving horizontally - plus a small amount to get the tray moving horizontally. In principle, you get this work back when you stop the skateboard, but our bodies can't turn unwanted movement back into ATP.
  • If you were carrying the tray above your head, you would do work to lift the tray as you took a step, and the tray would do work on you as the step ended. Unfortunately, our bodies spend energy to lift the tray, and spend even more energy to absorb the force of the descending tray at the end of the step.
So it's fairly easy to talk about force & energy in objects like books, tables, walls & trays.
But biological systems spend energy just hold the same position, whether pushing on a wall or holding up a tray.
 

Offline steviereal

  • First timers
  • *
  • Posts: 5
    • View Profile
Re: How does a surface support a mass at rest?
« Reply #10 on: 20/05/2013 14:32:01 »
I've been thinking about this some more and think that fields are what causing this mystery. In this case, it is the gravitational field vs. electric field (holding the particles of the surface together). All fields have energy and the bodies that the field acts on, have work done on them. In the case of gravity there is a formula that establishes the relationship between two bodies based on their distance and mass. The shorter the distance, the greater the potential energy is. Maybe this is the energy that is acted on the body when it is on the surface. On the other hand, the energy of the surface (that holds it together, the sum of binding energies) acts against this energy, so the two cancel each other out. They have opposite directions.
So I really think that the field is constantly working on the body it acts upon, only another energy is keeping it from doing anything useful. Quantum mechanically this has gotta be the case, let's just think about all those mediating particles (photons, gravitons) being exchanged constantly. Don't they carry energy just like any other photon/graviton?
 

The Naked Scientists Forum

Re: How does a surface support a mass at rest?
« Reply #10 on: 20/05/2013 14:32:01 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length