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Hooke's Law - a problem..... HELP PLEASE! watch

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    ok so i was doing a few practice questions on Hooke's law this afternoon to make sure i know what i'm doing... and i came across a question with a strange answer....

    An object of mass 0.150 kg is attached to the lower end of a vertical spring of unstretched length 300mm, which is fixed at its upper end. Witht he object at rest, the length of the spring becomes 420mm as a result.

    Calculate the weight that needs to be added to extend the spring to 600mm


    I have calculated the spring constant as 12.25 N/m

    then i would have thought that F = kx
    F = 12.25 x ((600-300)x 10^-3)
    F = 12.25N/m x 0.3m
    F = 3.675N

    however.... the answer in the back of the book is given as 2.2N????

    Where have I made the mistake? the answer i calculated for the spring constant is the same as the book gives... so it is from there onwards where i am not sure where i have made the mistake...

    Thanks a lot for your help!

    This is the AQA AS Physics A nelson thornes textbook btw (p166, Q4.c)
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    You need to take into account the o.150 kg mass has remained throughout.
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    You already have 0.15 kg on the spring, so you need to work out what to add in excess to this
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    ahhhhh thanks you two!
    yeah missed that completely.... i am suprised i didn't notice that from my answer. Ah well!

    Thank you
 
 
 
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