Why does a spring exercises force when stretched?

[Ifat Glassman]

Why does a spring exercises force when stretched or contracted?

[Maarten]

Off the top of my head I would say that the spring has a certain equilibrium condition to which it always tries to return. This is mainly determined by the spring constant (which I think is a property of the material the spring is made of, or at least it's influenced by this). If you pull the spring apart the force you exert on the spring will be measured against the other force that tries to keep the spring in its original shape, whichever force is stronger determines whether the spring actually stretches or whether it stays the same length.

A similar situation occurs when you press the spring, the force you exert is counteracted by the normal force of the spring. Why the spring wants to return to its equilibrium condition is probably due to this being the most energetically advantageous position for it to be on; why this is so I cannot tell you

I hope someone with more knowledge of physics can explain this in more detail; mine is a little rusty so I may have gotten some things wrong. It's mostly a case of using my common sense on this problem, but I don't know if I am correct or not.

[Ifat Glassman]

Why the spring wants to return to its equilibrium condition is probably due to this being the most energetically advantageous position for it to be on; why this is so I cannot tell you

This is exactly what I want to know.

There is a certain game here between plastic and elastic deformation, that I don't understand.

Plastic deformation is when layers of atoms in a metal slide over one another and cause the macro-dimensions of the object to change (like taking a cord of metal and putting it in a spiral shape). The change is final, and when the force is removed the metal will not go back to it's original form.

Elastic deformation is when the atoms change the distance from one another temporarily, but layers of atoms don't slide over one another, and when the force is released the metal will go back to it's original form.

A spring seems to be behaving like elastic deformation between atoms, only the strange thing is that it acts this way due to the macro-structure of the metal (instead of the micro structure in elastic deformation), and will not react the same under the same force if it's structure was different.

[Maarten]

One thing you have to keep in mind is that there is a limit to the elastic capacity of the material. If you pull a spring too hard it will probably not revert to its initial form anymore. I say this mostly to indicate that it's not two completely seperate mechanisms that occur here; rather which one occurs probably depends upon a combination of the type of material and the amount of force used on it.

I would suggest that the bonding strength between the individual atoms is the main component here; if the strength of the bond is very high it will be much harder to deform the material permanently. However, I am not sure if the strength and elasticity of a material can be explained by the same theory or if they are completely seperate attributes.

[Jay P]

This is exactly what I want to know.

There is a certain game here between plastic and elastic deformation, that I don't understand....

Elastic deformation of a material is non-permanent deformation in which the bonds between the atoms get temporarily deformed. It is governed by Hooke's law, which states that strain (the amount of stretch per unit length) is proportional to stress (the amount of force per unit cross-sectional area). So for a spring, the stiffness is a function of the material and the geometry, but it will always be the case, because of Hooke's law, that the amount of stretch is proportional to the force applied, for any particular spring. If it takes 1 pound of force to stretch it 1 inch, then two pounds will stretch it two inches. (That is assuming you don't stretch it so much that it deforms plastically.)

Every material has its own inherent stiffness, called the "modulus of elasticity", from which you can tell how much a bar of it will stretch when loaded.

Why is Hooke's law valid? To really understand that, you'd have to study elasticity theory.

Plastic deformation is permanent deformation that is caused by the motion of line defects ("dislocations") in the material. (Slipping whole planes of atoms over each other would take much more force.)

Stretch a piece of metal, and it first deforms elastically. Eventually, when the stress reaches the "yield point", it begins to deform plastically. Then, eventually, when there is too much plastic deformation, it will break, at its "tensile strength".

The yield point and tensile strength, which deal with plastic deformation, are affected by alloying and thermal and mechanical history, because these things can affect how many dislocations there are and how easily they move. Modulus of elasticity, on the other hand, which affects elastic deformation, isn't strongly affected by these variables.

A first-year physical metallurgy text would be a good place to begin a study of this.

[Ifat Glassman]

So basically, you're referring me to books about metallurgy and elasticity...

thanks for all the explanations you provided, but it still doesn't answer my question. And while the answer to this question does interest me, it doesn't interest me that much that I would start a research about it and all...

Just a little comment about plastic deformation: I was taught that it is sliding of layers of atoms over one another, especially layers that have many dislocations in them, so the sliding become easier (less force is needed since there are fewer connections).

My question is basically, what is so special about this specific structure (coil) of the metal (or another material) that causes the deformation to be elastic (or in simpler words, that allow the spring to go back to it's original shape)?

what atoms in that structure actually get drifted from one another to allow the stretch to occur? is it all the atoms along the coil? just the external atoms in the coil? How come if we arrange the metal in a different structure we will not get this elasticity?

Just figured out something: Say we take a thin stripe of metal: If we apply force parallel to it, then the equation for movement will be strain=stress/(young modulus), but if we apply the force perpendicular to the strip of metal, other things happen. They are too, Elastic deformation, but a different type: In perpendicular force, when the metal stripe is bent (to an arch), the atoms in the upper layer get too close to one another, while the atoms in the bottom layer get too far from one another. That's why, when we release the metal stripe it will try to go back to it's original structure, and start oscillating. Something similar must be happening with a spring as well...