Homework help - Models.

[studiot]

Did you not say the drawer was stuck?

 

So what is holding it back?

[Relative]

Did you not say the drawer was stuck?

 

So what is holding it back?

I do see the concept you are trying to get me to understand, but an offset drawer, often made of wood, or sometimes metal, behave in accordance to heat, cold, humidity,

 

swelling etc, I can not see been a force.

 

I understand you are saying a force holds it jammed shut, but I see that has geometry mechanics, thermodynamics, elements.

 

Not force, unless you are suggesting there is friction, but there is no friction until movement of the drawer.

I suppose I could consider the weight of the set of drawers been a force , because of gravity.

Edited September 3, 2014 by Relative

[studiot]

 

I do see the concept you are trying to get me to understand, but an offset drawer, often made of wood, or sometimes metal, behave in accordance to heat, cold, humidity,

 

swelling etc, I can not see been a force.

 

I understand you are saying a force holds it jammed shut, but I see that has geometry mechanics, thermodynamics, elements.

 

Not force, unless you are suggesting there is friction, but there is no friction until movement of the drawer.

 

I suppose I could consider the weight of the set of drawers been a force , because of gravity.

 

 

 

Yes of course and perhaps the drawer is locked or painted shut or something inside is sticking up and catching or.......................

 

But none of these matter since they all apply the same force - just enough to resist your pull no more and no less.

 

 

The whole point of a model is to extract only the important facts and discard all the rest, even though they are facts.

 

The important facts are

The drawer is a box

You are applying a force by pulling

Something is applying a foce by resisting.

 

One drawer, two forces, no other facts matter.

[Relative]

 

 

Yes of course and perhaps the drawer is locked or painted shut or something inside is sticking up and catching or.......................

 

But none of these matter since they all apply the same force - just enough to resist your pull no more and no less.

 

 

The whole point of a model is to extract only the important facts and discard all the rest, even though they are facts.

 

The important facts are

The drawer is a box

You are applying a force by pulling

Something is applying a foce by resisting.

 

One drawer, two forces, no other facts matter.

Hmmm, ok, in the drawer instant, if a person applied force, there would be a resistance force, but if a person did not apply the force, there would be no resistance force, the resistance only occurring when we apply force.

 

So the only force involved is surely the force of just us applying force, creating resistance force?

You do not drown in water because you can not swim, you drown in water because you can not stop yourself falling.

[studiot]

 

Hmmm, ok, in the drawer instant, if a person applied force, there would be a resistance force, but if a person did not apply the force, there would be no resistance force, the resistance only occurring when we apply force.

 

 

 

Exactly.

 

There is an applied force (often called 'the action')

 

This stimulates a second opposing force, often called the reaction.

 

As I noted bfore they are exactly equal in magnitude, but opposing.

 

Recognise Somebody's Law?

 

Note also that this conclusion you have drawn, does not follow.

 

That is it contradicts your first statement above and of course itself since it talks of creating a second force.

 

 

So the only force involved is surely the force of just us applying force, creating resistance force?

 

Edited September 3, 2014 by studiot

[Relative]

 

Exactly.

 

There is an applied force (often called 'the action')

 

This stimulates a second opposing force, often called the reaction.

 

As I noted bfore they are exactly equal in magnitude, but opposing.

 

Recognise Somebody's Law?

 

Note also that this conclusion you have drawn, does not follow.

 

That is it contradicts your first statement above and of course itself since it talks of creating a second force.

Hmmmm, your logic is of ''Data'' status.

 

Yes every action has an opposite reaction. I do see that is what you are saying, but I miss the part where every action has an opposite reaction, having anything to do with every force has an opposite force,

 

I paused after the last sentence above...........and think that is actually correct and all force has an opposite force?

[studiot]

 

When two material bodies A and B interact so that body A applies a force on body B,

body always B applies a counterforce on body A.

 

Note also the specification when two material bodies interact.

 

Then yes there is always a second opposing force.

 

Note that Newton also included words to this effect.

 

This extra specification is necessary because he needed to exclude the situation such as gravity in the space around objects.

The force of gravity only comes into operation if a second object appears, but we cannot say there is no gravity around an object just because there is no second object there.

[Relative]

 

Note also the specification when two material bodies interact.

 

Then yes there is always a second opposing force.

 

Note that Newton also included words to this effect.

 

This extra specification is necessary because he needed to exclude the situation such as gravity in the space around objects.

The force of gravity only comes into operation if a second object appears, but we cannot say there is no gravity around an object just because there is no second object there.

OK, I understand physical interactions of forces, but i still can not see why a static object would have any more than one force with no applied force.

 

 

Where does the acceleration come from in this diagram?

 

Unless you consider the Earth's rotation has movement, and then the force direction tag would depend on global positioning.

If the ground was a spring, then yes, there would be a force extra.

It is theoretically impossible for the ground to push back.

 

Ok i will try another model example.

 

 

 

[Fuzzwood]

It is theoretically impossible for the ground to push back. <-- and yet it does.

 

In your elevator-example, the elevator will just keep falling with an acceleration of 9.81 m/s². Put a floor under it, strong enough to support the elevator, and the elevator will of course stop. Yet the force pulling at the elevator that can potentially give it an acceleration of 9.81 m/s² remains. So with no compensating force, the elevator will keep falling.

Edited September 4, 2014 by Fuzzwood

[Relative]

It is theoretically impossible for the ground to push back. <-- and yet it does.

 

In your elevator-example, the elevator will just keep falling with an acceleration of 9.81 m/s². Put a floor under it, strong enough to support the elevator, and the elevator will of course stop. Yet the force pulling at the elevator that can potentially give it an acceleration of 9.81 m/s² remains. So with no compensating force, the elevator will keep falling.

Yes that is what I am visualising, The force of a static object on the ground been equal to the speed it would fall at.

 

I associate force with energy, something that has to be applied , exerted, I do not and can not see how the ground can exert a force, when it is surely just a matter of ''viscosities'', densities, and how much compacting there is of the matter underneath the object.

 

Y is equal to Y

 

X is equal to X

 

But Y is not equal to X.

 

 

Y been vertical axis, X been horizontal axis, and in context , compression.

 

Meaning Y becomes more compact, where X expands because density is lost to surface motion/expansion,

 

Force of A is equal to force of B?

 

FA=FB?

 

would that be a simple model?

FA+FB=G?

what is the science symbol for <and> FA and FB is equal to G

Edited September 4, 2014 by Relative

[Fuzzwood]

I associate force with energy <-- and that is where you go wrong. Force has a unit of kg*m/s². Energy is measured in

kg * m²/s² or a force multiplied by a distance. Even with a distance of 0 and thus no applied energy, there can still be a force.

 

Also, stop overcomplicating things if you do not understand the basics of something. It seems you fail at the progress of abstracting a physical phenomenon.

 

To help you, you are indeed correct in saying that something doesn't fall through a table as long as molecular cohesion (forces like atom bonds, van der Waals forces, dipole interactions, etc.) inside that table is maintained. The moment that particular force is overcome by something with a very high mass, the table simply breaks. Until that point, however, the table will exert a force equal but opposite to the gravity force.

Edited September 4, 2014 by Fuzzwood

[Relative]

You are saying that the ground is directed one way by gravity, centripetal, and also saying that the ground exerts a force the opposite direction to gravity, outwards, centrifugal,

 

I would except this explanation if you admitted that the ground is been attracted to a center point,

But also at the same time the ground of Earth, is been attracted to the Sun at the same time, and the core been repelled at the same time by the sun.

 

Ok I take away the block, just the ground, you are saying this.

 

 

Does not make sense unless above.

Edited September 4, 2014 by Relative

[Fuzzwood]

That is not what I am saying.

[Relative]

That is not what I am saying.

I quote you

 

''It is theoretically impossible for the ground to push back. <-- and yet it does.''

 

so if the ground pushes back , it must have a repelling force to ''push'' has in my diagram.

''Until that point, however, the table will exert a force equal but opposite to the gravity force.''

like my diagram

[Fuzzwood]

The ground will only push if there is something to push against. In your case, the normal force is 0. Like the gravitational force working on nothing is also 0.

Edited September 5, 2014 by Fuzzwood

[Endy0816]

I think the easiest way to understand this particular force pair would be to simply pick up something heavy and hold it over your head.

 

You are going to feel the effects pretty quickly. No repelling going on.

[Relative]

The ground will only push if there is something to push against. In your case, the normal force is 0. Like the gravitational force working on nothing is also 0.

But the force of an object on a horizontal plane is not zero.

 

The F is equal to falling speed, even though static, the amount of F is the amount of weight an object is, hence something's are heavy where others are light.

 

 

You are trying to state that a static object has an equilibrium of force to the ground.

 

0 and 0.

 

But no, the ground is 0, and the object continues its force always, hence subsidence.

 

I am sorry for making a simple model into a complicated issue for me, I am still doing models, but I can not except this simple model if the logic does not work.

I think the easiest way to understand this particular force pair would be to simply pick up something heavy and hold it over your head.

 

You are going to feel the effects pretty quickly. No repelling going on.

Well actually, your arms and muscles would do the repelling, the opposite force, but the force is not strong enough to hold an heavy object.

The ground pushes the same direction has the object.

 

The object tries to push you through the ground.

 

Both you, the object and the ground all are falling at the same force comparative to weight , comparative to gravity.

 

All falling, isotropically ,centripetally.

Edited September 5, 2014 by Relative

[Endy0816]

But the force of an object on a horizontal plane is not zero.

 

The F is equal to falling speed, even though static, the amount of F is the amount of weight an object is, hence something's are heavy where others are light.

 

Because the object has mass. In what you posted last:

 

Ok I take away the block, just the ground, you are saying this.

 

 

without the block there is no mass and thus no force produced.

 

Assuming it is adjusted correctly, your bathroom scale will read zero if 'nothing' is on it.

 

 

Well actually, your arms and muscles would do the repelling, the opposite force, but the force is not strong enough to hold an heavy object.

The ground pushes the same direction has the object.

 

The object tries to push you through the ground.

 

Both you, the object and the ground all are falling at the same force comparative to weight , comparative to gravity.

 

All falling, isotropically ,centripetally.

 

You can probably hold it stationary for awhile. You can do this in open space as well and two counterbalancing force pairs will still be present.

 

I strongly recommend thinking of it as attraction and not falling. You attract the Earth, the Earth attracts you.

[Relative]

 

Because the object has mass. In what you posted last:

 

 

 

 

Assuming it is adjusted correctly, your bathroom scale will read zero if 'nothing' is on it.

 

 

 

You can probably hold it stationary for awhile. You can do this in open space as well and two counterbalancing force pairs will still be present.

 

I strongly recommend thinking of it as attraction and not falling. You attract the Earth, the Earth attracts you.

You are attracted to the core and the core is attracted to you ,

 

 

''without the block there is no mass and thus no force produced.''

 

Incorrect, the ground has mass, the ground has force, equal to the force of the block even without the block, direction also equal to gravity direction.

 

The ground is made of lots of blocks.

[Endy0816]

It'll have a gravitational attraction on other objects with mass.

 

Though since there is no block, this particular force pair is gone or to put it another way zero force exerted downwards is balanced out by zero force exerted upwards.

 

Forces are simple, they always balance out.

[Relative]

It'll have a gravitational attraction on other objects with mass.

 

Though since there is no block, this particular force pair is gone or to put it another way zero force exerted downwards is balanced out by zero force exerted upwards.

 

Forces are simple, they always balance out.

You can not see the contradiction in your own statement?

 

Zero force exerted ''upwards''. zero has no force.

 

F=MA, you are insinuating that the ground accelerates ''upwards''.

[Endy0816]

It was just another way of saying that there won't be any force observed.

 

F=MA, you are insinuating that the ground accelerates ''upwards''.

 

Sure, typical example given is for a falling mass.

 

a_Earth = (Mass_Object x 9.8 m/s²) / Mass_Earth

[Relative]

It was just another way of saying that there won't be any force observed.

 

 

Sure, typical example given is for a falling mass.

 

a_Earth = (Mass_Object x 9.8 m/s²) / Mass_Earth

There is no force observed, because there is no force from the ground. Mass Earth is just the absorbing volume of a density compared to the V and weight of a falling object.

 

I thank you for your considerable patience and trying to explain, but I can not see any other force except gravity been involved, and have still no idea what force Fn is trying to represent.

 

Now if you explained Fn has a weight shift on a molecular level of the object that was ''sliding'' down a hill, then I think I could understand that.

[imatfaal]

Relative, you seem to forget that science is based on what actually happens and not the most pleasing idea. I, and most of the other posters in this thread who have been giving you advice, have actually done these experiments; we have rolled balls down slopes and timed them, pushed little carts into other carts, pulled blocks with spring balances...

 

Intuition is fine for cracking problems - sometimes. But the ultimate arbiter of a hypothesis' value is reality - and the very very basic physics based on newtons laws of motion is incredibly predictive, repeatable, and reliable. Unfortunately you need to start to read others' work with a little less confidence in your own ability to discern good ideas from bad. What you have been told in this thread works your ideas will not.

 

 

 

and for those who don't understand the reference

 

http://xkcd.com/54/

[physica]

The main reason why I sat back after a while is that you continue to go off on tangents. This is fine once you've mastered the material. In my experience very clever people have been accused of going off on tangents because the other person can't see the connection however, you have to stick with one concept and see it all the way through in order to master the material. I haven't seen you do this once. I'm going to say it again. You need to let the maths guide you on what's going on. This is why physics is such a great science. Human intuition is terrible. Once you've learn't vector addition and the dot product of vectors you will be able to apply them and the maths will tell you where a force needs to be in order for an object to move or stay still. Use the framework I've given you earlier on in this thread, it will prevent you going round in circles and losing focus. Another rule is not to add things like density to a problem if you can't solve the initial problem. This will just make it harder. It wil not clarify anything. Also have the humility to realise that the world is bigger than your brain. If these laws didn't work engineering as we know is wouldn't work.

Edited September 9, 2014 by physica