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    I apparently haven't been taught properly for mechanics and have no idea about a lot of what's happening, the fact that the book gives no relevant examples to the harder questions in each section doesn't help much either, was wondering if anybody could help me with this:

    "A cylindrical rock of mass 12kg is being pulled by a cable up a deep vertical shaft with rough sides, the tension in the cable is a constant 1520N for the first part of the operation which takes 2 minutes to complete, and lifts the rock 1440 metres"

    Calculate the resisting force, assumed constant, due to friction from the sides of the shaft"

    I though surely that as there is no acceleration, F-T = 0, and therefore the friction would be 1520N - (12 x 9.8). But the book answer is 4000 something.

    Any help solving it and explaining whats going on very much appreciated, thanks.
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    How do you know there is no acceleration?

    Also, if it is moving upwards then friction must be acting downwards. This can't be more than the 1520N of the tension in the cable, since then it would not move. Are you sure you checked the right question and answer?
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    Well it doesn't specifically say "there is no acceleration" but it says that the 1520N tension is constant.
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    Yes and F=ma, i.e. if there is a resultant force (which there may be) then it would be accelerating. So it is continually moving and doesn't start from rest? If this is one of those adapted questions, they are often rather rubbish.
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    Sorry i typed the answer wrong - it is 1400N in the book, which still makes no sense.

    The way i worked it out (ie f-t = ma) comes out with 1402.6 N

    Surely the distance it travelled and time taken have some relevance though, i just don't see how?
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    I would have thought (they wouldn't give you the distance and time taken otherwise) that it starts from rest and accelerates.

    Therefore, average v= 1440/120 = 12
    final v=2*12=24
    a=24/120=1.2ms^-2
    1520-12g-F=ma=12(1.2)
    1520-12(9.8)-12(1.2)=F=1400N

    That's my reckoning of the answer.
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    Thanks, that actually makes good sense, this book an absolute piece of crap when it comes to wording questions, literally every other question needs an analysis of what is actually going on :mad:

    Nice one mate.
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    Is it the OCR book? I only do ones that have OCR written in brackets.. I don't even do OCR adapted questions, since they've normally been adapted beyond comprehension.
 
 
 
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Updated: June 10, 2005
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