Which bits of the spring metal stretch and in what directions?

[Cheman]

When we stretch a spring like the one on this link, http://auto.howstuffworks.com/car-suspension3.htm
it tends to obey hookes law (up to a point) - does this mean, like when wires are stretched (which also obey hookes law) the metal of the spring is actually streching and not just being moved as in would appear from looking at a stretched spring? Which bits of the metal stretch and in what directions?

Thanks in advance.

 

[brewnog]

For the moment, forget about springs. Just consider a straight, metal bar (or wire!)being stretched along its length. If you apply a load, it will begin to stretch, and will become longer. Initially, the metal will obey Hooke's law, and the load applied will be proportional to the amount by which it deflects. The metal itself actually deforms.

Now, consider the spring. The same thing is happening here, except that the spring is a metal bar which has been formed into a different shape. This shape will vary depending on the type of spring being used. In a spring, the metal is deforming just like the simple case stated above, except that the deformation will happen in different directions depending on the geometry of the spring.

So yes, the metal is actually being stretched and compressed. With car suspension, the spring is usually under compression (rather than tension), but the idea is just the same.

 

[Cheman]

So which bits of the spring are stretching when you pull it? The whole of the spring wire or just certain areas? eg - at bend in the helix are atoms tryin to move up one side and down the other causing strecthin between the two? Its harder to visualize what the atoms are doing in a spring compared to a straight wire - but that is what i would like to know about! lol.

Thanks in advane.

 

[brewnog]

Well, obviously it depends what kind of spring, but I'll toy with coil springs here (because that's what you're talking about, anyway!).

If you consider a coil spring under tension, then (apart from the ends) there aren't any 'bends' as such, it's just a continuous curve. It all behaves the same way. If you consider just a small section of spring, then one end of it is being pulled up, and the other end down. If you're asking whether the deformation of a spring is the same as of a straight wire (only all coiled up) then the answer is no.

I'm sure you can understand that it's hard to explain (especially just in text) exactly what's going on, but a good plan might be to make a model (perhaps out of rubber, Fimo or Blu-Tak or something) of a section of spring, and draw lines on it with a felt tip. Then deform your model, and see how the lines look afterwards.

 

[Cheman]

But with so many coils, how can one bit be moving up and one bit down? Are you saying that eg the centre stretches first, then the curves before and behind it, etc?

 

[brewnog]

I'm not quite sure what you're asking.

If you consider a small segment of a vertical, uniform, coil spring under tension below its yield, one end of your segment will deform upwards in relation to its centre, and the other end will deform downwards.

The coils are not separate objects, they're just parts of the same bit of metal. The whole of the spring behaves the same, - no parts of the spring stretch more, or before, any other parts. All the 'coils' are identical.

 

[russ_watters]

When you stretch a metal bar, its cross-sectional area decreases - its exactly the same as the shrinking you notice when stretching a rubber band. Typically, the same thing happens for springs.