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

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1. 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)
2. You need to take into account the o.150 kg mass has remained throughout.
3. You already have 0.15 kg on the spring, so you need to work out what to add in excess to this
4. 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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Updated: January 27, 2009
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