basic physics - opposing forces

Hi

I'm learning basic physics. I can't believe how little i knew. I've just found out the difference between weight and mass. My knowledge was that bad!

So i'm now reading about opposing forces. Apparently if you rest your elbow on a table there are both downwards and upwards forces. I imagine the downwards force is the weight of your arm due to the pull of gravity. However I have no idea what the upwards force might be. Any elabpration much appreciated.

Thanks
andiejeAsked:
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d-glitchCommented:
>>  the downwards force is the weight of your arm due to the pull of gravity.

Correct.  

But your arm is at rest.  
So there has to be an opposing force from the table on your arm.
Otherwise your arm would be accelerating.

If you put four scales under the legs of the table, you would be able to see the
the weight of the table and the force of your arm.
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David L. HansenCEOCommented:
For objects at rest (ie. arm on table) the force resisting is equal to the force imposed.  So the table exerts EXACTLY the same force upward as the arm does downward.  If you are doing karate on the table the table will only be able to exert an equal force (which will increase rapidly) against the arm or hand to a point...after that you'll have either a broken table or broken arm.
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andiejeAuthor Commented:
HI

I understand that the force from the table is equal to the force of the elbow because that's what the law says. I am finding it hard to conceptualise where exactly the force from the table comes from.

D-glitch, i understand if there wasn't a force on the elbow it would be accelerating, i just can't visualise where the force on the table is coming from. I'm sure its not too complicated, this is just new to me and that this time next week i'll be embarrassed i even asked something so trivial.

Is it the case that the force applied by the table actually results from the fact you are pressing on the table in the first place? I presume that's what the law means. For your action force there is a resulting reaction force. But i can't visualise how a table can be exerting a force. I read something just along the lines that your elbow on the table would be effecting the electrons in the table etc creating some sort of force back. If you could describe the force from the table in these sorts of terms it might help me visualise it

Many thanks as ever. I find your answers very helpful d-glitch
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David L. HansenCEOCommented:
Just remember gravity is still working even when something gets in the way.  If you were on a sheet of glass suspended say...a foot off the floor, and it was just barely think enough to hold your weight and your stand there for a few minutes, you would see no forces acting right?  True.  But now add a paper clip.  The glass breaks (sound effects go here) now, was it the force of the paper clip that broke the glass?  Of cource not, at least not the paper clip alone,  the forces of you standing there PLUS the force of the paper clip did the trick.  You see the force of you was already there....the paper clip just added to it and the resistive force of the glass could no longer meet the total downward force (even though it could with just you) so it breaks and all the forces continue to act but now the force is changed to you falling and glass shattering and falling etc.
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David L. HansenCEOCommented:
Think of holding somebody else on your shoulders, someone big.  You can barely hold them, but you manage.  If fact you can hold them up and not stagger around at all.  You may be red in the face but you hold perfectly still.  Now are you working?  You bet you are.  So is the glass or the table.  The atoms and molecules hold to each other and resist the downward force until there is too much force.
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andiejeAuthor Commented:
HI sl8rz, I didn't really get your comment ID: 30136074
 Your second comment about holding someone on your shoulders was more meaningful to me

How do resistant forces work at the molecular level? I will go and have a look myself. I presume the term i need to search is resistant force or opposing force
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phoffricCommented:
>> i just can't visualize where the force on the table is coming from. I'm sure its not too complicated.
It's pretty complicated, I think, and I'll just try to hand-wave until someone else comes along more clearly.

>> the force applied by the table actually results from the fact you are pressing on the table in the first place?
You're not pressing with intent; but the weight (i.e., force due to gravity) of your arm is trying to move down on the table and trying (and succeeding to some extent) in displacing the molecules of the table to allow your arm to pass through it. So, I believe you are correct.

Disclaimer: Everything below is what I think I learned or at least thought about at some point. I'm not sure I have all the facts exactly right when I start talking about the atomic level.

Now I learned that there is mostly space in an atom. And that when two object touch, it isn't so much that there is a contact of matter, but rather an interaction of atomic forces between the two objects due to a proximity of localized atomic forces - sort of a force field around the atoms of a solid.

Think of springs and trampolines. First springs. Apply a compressing force to a spring, and it's physical makeup is to restore itself to its original state. Let go and it springs back. Don't let go, and you feel its nature to restore itself in an opposing equal and opposite force to your compressing force.

Now trampolines.. Just put a bowling ball in the middle of one. It doesn't fall through, but it does distort the original near-like plane. The trampoline likewise has spring-like properties - it's makeup is to restore itself to its original state. That tendency and the fact that it didn't break under the bowling balls weight results in the upward force on the bowling ball so that an equilibrium is reached. The trampoline stretches, and the more it stretches (analogous to the more compression on a spring), the more it resists; and the more force it applies until the ball's constant weight, and the upward force of the trampoline finally are the same.

Now your table. It's a trampoline. But a very stiff trampoline. Extremely stiff. It's a solid whose molecules form some kind of structure with atomic internal force bindings (chemical, physical??) that keep the relationship amongst the molecules fairly stable. If you put a bowling ball on that table, it will, thanks to its weight, begin to push the molecules of that solid trampoline down, and just like a trampoline, its physical nature resists that due to its own internal chemical and atomic forces. The net result is that there is little movement of the molecules (but there is some tiny displacement), and it is this trampoline, spring-like effect that exerts the upward force.
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d-glitchCommented:
Imagine that the table has four legs made from pogo sticks.
When you rest your arm on the table, all the springs compress a little.
Now you can see where the force is coming from.

Even if the table has boring wooding legs, they still have some spring
to them.  Resting you arm on the table increases the tension in the legs.
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d-glitchCommented:
>> phoffric and his trampoline crossed the line before me and my pogo sticks
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David L. HansenCEOCommented:
All the atoms in your body "want" to stick together.  It's electromagnetism...the same reason magnets attract each other.  Your body doesn't just fall apart into millions of little atoms rolling around on the floor because they are attracted to each other.  That attractions keeps things from pulling them apart whether that thing is a t-shirt or someone's fist coming at you -- your atoms resist those forces and try to stick together.  The same is true for every object around you...the fridge, the tupperware, and of course the table.
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phoffricCommented:
We can add the pogo sticks to the trampoline and make it a death-defying Olympic sport.

>> You may be red in the face but you hold perfectly still.  Now are you working?  You bet you are.
On one hand I agree with you. After an hour of holding a heavy box, my arms are aching (this is a thought experiment; I have never actually done this, and don't plan on doing this). I feel exhausted and I certainly used by strength to exert an upward force to counter the gravitational force on the box (which as the author knows, is equal to the weight of the box). But when work is done, there is a transfer of energy. If I picked the box up, then the work performed is the weight times the height. But if you handed me the box, and I just held it, then there is no transfer of energy on my part. Neither the kinetic nor potential energy of the box changes as long as I hold it still.
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phoffricCommented:
Oh I just wondered about the box again. After sweating and being exhausted for an hour and feeling like I did a lot of work, seems like we're missing something.

Here's disclaimer again:
Everything below is what I think I learned or at least thought about at some point. I'm not sure I have all the facts exactly right when I start talking about entropy and human metabolism.

I am burning calories during this isometric exercise. Therefore the internal energy of the closed system (including just me) is being depleted. It is not going into the box. So where is the energy going. I believe that as a result of converting some of my body into muscular energy, I am heating up. The blood then carries away the heat to the surface of the skin and it goes into the air. I think entropy is increasing. Why am I doing this stupid thought experiment only to increase entropy! There is also bio-chemical reactions during this conversion process where the by-products of the reaction have a lesser energy state than in its previous form. Some of these by products are toxins and are excreted from the body via the kidneys and the lungs.
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David L. HansenCEOCommented:
Well, now we're getting into chemical reactions, potential and kenetic energy, and entropy.  All very useful topics of conversation but I'm not sure it will help the author to get into all that just yet.  Shall we begin a new thread in the Math/Science forum (it's in the "other" zone).  Unless Andieje wants us to continue here....?
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phoffricCommented:
... chemical reactions..
Actually, I was just responding to the supposed work issue when getting red in the face. The chemical reactions, etc. were off topic but related to many, many questions that the author has questioned in the past, so I wanted to tie these things together. You are right in that this should not be discussed further here.
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David L. HansenCEOCommented:
Good point.
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andiejeAuthor Commented:
thanks for your help, that made it very clear in simple terms
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andiejeAuthor Commented:
many thanks
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