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I'm trying to make an application for Hooke's law using a spring, but the law doesn't give any correct result with my spring, because when I hang a $100\,\mathrm{g}$ object on the spring it's elongates about $0.3\,\mathrm{cm}$ and when I hang a $200\,\mathrm{g}$ object the spring elongates about $1\,\mathrm{cm}$ while it should elongates only $0.6\,\mathrm{cm}$ .

I'm sure that the problem is with the spring design.

The spring I'm using is just like in picture below:

image of a tight-coiled spring

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    $\begingroup$ do you have any other data points or just the two? $\endgroup$ – Jim Jul 25 '13 at 15:50
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Let me guess: you take the spring as it is and hang your objects, right? Then measure the displacement.

Try to do the following: hang any arbitrary object so that the string will stretch a bit from its initial state. Then add you 100g and 200g objects to the initial mass and measure the difference in spring's length. I will be surprised if you won't get good results.

Explanation: there are other forces involved when the spring is in its initial condition (as in the picture). When you initially stretch it a bit, you neutralize these forces and the only force left is Hooke's one.

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    $\begingroup$ As Vasiliy says, it looks as if the spring has been made so it's under tension i.e. there is a force holding the coils together. You'll need to add a certain weight to conteract this tension before the spring starts extending, but once you've added that weight you should find the weight:extension is linear. $\endgroup$ – John Rennie Jul 25 '13 at 16:06

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