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Thread: please help me out on this physics question

  1. #1 please help me out on this physics question 
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    Hi gentlemen and ladies. This question is partially hooke's law and partially relative density, the question is a 'hard nut to crack' for me. It goes like this;'A spiral spring loaded with a piece of metal extends by 10.5cm in air. When the metal is fully submerged in water, the spring extends by 6.8cm. Calculate the relative density of the metal (Assume hooke's law is obeyed).


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  3. #2  
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    Is this a homework problem?


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    If it walks like a homework problem, and smells like a homework problem, and quacks like a homework problem, the odds are it is a homework problem. So, Dr. Nominee, how far have you progressed with trying to complete this homework problem. Many members are able to help you with this, but only if you have demonstrated that you have tried to help yourself.
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  5. #4  
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    My problem is that neither mass nor volume of water is given in the question. So i don't know where to start from.
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  6. #5  
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    If you knew the ratio of the submerged weight to the weight when it is not submerged, could you find the relative density?
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  7. #6  
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    C'mon Harold! From the question, how can we get the weight of the metal?
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    Universal Mind John Galt's Avatar
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    Since the question is asking about relative density (presumably relative to water) I'm not clear why one would need to know anything about the mass or volume of water. Whatever the volume of the metal it will displace a comparable volume of water. Whatever the bouyancy effect it will be relatively the same per unit volume.
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  9. #8  
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    You know the relative density of water is one and that a submerged object displaces its own volume of water. all you need are the weight in and out of water to calculate the relative density.

    I could just post the equation or answer, but since it is a homework question...

    Edit: I should have read John Galt's answer before posting because I basically repeated what he just said.
    Last edited by dan hunter; February 25th, 2014 at 10:54 AM.
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    Quote Originally Posted by Dr Nominee View Post
    C'mon Harold! From the question, how can we get the weight of the metal?
    You can't determine the weight in kilograms, but you can define your own weight units based on Hooke's law. Call them "scale units" or something instead of kilograms. Try that. Just write down the formulas for buoyancy and Hooke's law, then fiddle around with it for a while. It will come to you.
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  11. #10  
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    two small hints
    Hooks law just says change in the length of the spring is proportional to the force on it.
    Relative density is a ratio that does not require any specified units. It is the same in metric as it is in imperial measurements.

    You really should be able to write this out directly just from the change in the spring lengths.

    To make it even easier

    relative density = weight in air/ (weight in air - weight in water)
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  12. #11  
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    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by Dr Nominee View Post
    C'mon Harold! From the question, how can we get the weight of the metal?
    You can't determine the weight in kilograms, but you can define your own weight units based on Hooke's law. Call them "scale units" or something instead of kilograms. Try that. Just write down the formulas for buoyancy and Hooke's law, then fiddle around with it for a while. It will come to you.
    I've actually done what u said Harold, but the "unkown" in the equation are just too much and i can't solve it. I will be grateful if you can just give me the procedure please!
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  13. #12  
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    Thanks Dan for your opinion
    dan hunter likes this.
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  14. #13  
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    Quote Originally Posted by Dr Nominee View Post
    Thanks Dan for your opinion
    I hope that means you got it.
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    Quote Originally Posted by Dr Nominee View Post
    I've actually done what u said Harold, but the "unkown" in the equation are just too much and i can't solve it. I will be grateful if you can just give me the procedure please!
    Sorry, but we don't want to become a place where people can just come and get their homework done. Show us what you did so far, and we'll try to give you a nudge in the right direction.
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  16. #15  
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    Yea Dan. It worked! Thanks
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