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GCSE Physics: Hooke's Law

I'm doing a past paper and I'm not sure how to answer this question. Help?

A student estimates that if she hangs a 45N load on a spring, the extension will be 920mm. Explain why this estimate may be unrealistic. (1 mark)

Thanks!
(edited 7 years ago)
Original post by ellecsmith
I'm doing a past paper and I'm not sure how to answer this question. Help?

A student estimates that if she hangs a 45N load on a spring, the extension will be 920mm. Explain why this estimate may be unrealistic.

Thanks!


Does that not seem like a very large extension to you?
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ellecsmith
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Original post by Smack
Does that not seem like a very large extension to you?

I don't know, I don't really understand it
Original post by ellecsmith
I don't know, I don't really understand it


What is it that you don't understand?

Consider an extension of 920 mm. That's almost 1 metre.
(edited 7 years ago)
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ellecsmith
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Original post by Smack
What is it that you don't understand?


How the weight is proportional to the extension, ie,. how do you know when the extension is too much for the weight?
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ellecsmith
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Original post by Smack
What is it that you don't understand?

Consider an extension of 920 mm. That's almost 1 metre.


In comparison to 45N, is that a lot?
Original post by ellecsmith
How the weight is proportional to the extension, ie,. how do you know when the extension is too much for the weight?


F = k*x

You know F and x.

In this question you do not have all of the information required to actually calculate whether the extension is too much for the weight (or whether the spring would fail mechanically). You need to make certain guesses/assumptions.

You can calculate k and see if it is relatively small for what you would expect of a spring. Or perhaps the extension is a lot more than you would typically expect of a spring that would typically be used for such an experiment.
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ellecsmith
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Original post by Smack
F = k*x

You know F and x.

In this question you do not have all of the information required to actually calculate whether the extension is too much for the weight (or whether the spring would fail mechanically). You need to make certain guesses/assumptions.

You can calculate k and see if it is relatively small for what you would expect of a spring. Or perhaps the extension is a lot more than you would typically expect of a spring that would typically be used for such an experiment.

I probably sound really stupid, but I really don't understand
Original post by Smack
F = k*x

You know F and x.

In this question you do not have all of the information required to actually calculate whether the extension is too much for the weight (or whether the spring would fail mechanically). You need to make certain guesses/assumptions.

You can calculate k and see if it is relatively small for what you would expect of a spring. Or perhaps the extension is a lot more than you would typically expect of a spring that would typically be used for such an experiment.

Would have answered this by saying that the spring has gone part it's limit of proportionality and therefore it's extension is no longer directly proportional to the force applied?
Original post by ellecsmith
I probably sound really stupid, but I really don't understand


Original post by SunnyBoys
Would have answered this by saying that the spring has gone part it's limit of proportionality and therefore it's extension is no longer directly proportional to the force applied?


Yes, that's another possible answer (I don't know the marking scheme so can't say for sure). But (this is more for your information), the question is not asking whether Hooke's law is valid. It's a fair guess that at an extension of close to 1 m that the spring is extended past it's proportionality limit. But even then, the spring can still continue to extend in the plastic region (the region after the elastic region where Hooke's law and the Young's Modulus apply), it's just that the strain is not proportional to the stress, or the extension is not proportional to the force, as you say. Hopefully that's not confusing.

The key to this question, I think, is being able to estimate and compare against the theory, as not enough information is provided in the question to perform a calculation and compare against a known value. It's only a 1 marker, so I don't think they're expecting an essay.
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ellecsmith
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Original post by SunnyBoys
Would have answered this by saying that the spring has gone part it's limit of proportionality and therefore it's extension is no longer directly proportional to the force applied?


Ohhh, that makes sense. Thanks!

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