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    really need help with this question - i just cant get to the right answer

    Find the least value of P that prevents the sledge (weight - 175kg) from sliding down the plane


    The answer should be 580N but i cant get that
    please help my exam is tomorrow
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    I think it is wrong because i get P = 990N.
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    I've attached my version of the diagram. Because of the right angles marked in red, P is at an angle of 30 degrees to the slope. If it's not sliding, there will be no resultant force on the sledge. Parallel to the plane
    Pcos30 = 175g*sin30
    P = 175g*tan30 = 175*9.8/sqrt3 = 990.15...
    Do you mean weight 175 N? Then it would be P = 175/sqrt3 = 101.03..
    Neither is right answer so maybe it's wrong. Or I'm missing something - is there meant to be friction included too?
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    (Original post by MiniBarSmarties)
    really need help with this question - i just cant get to the right answer

    Find the least value of P that prevents the sledge (weight - 175kg) from sliding down the plane


    The answer should be 580N but i cant get that
    please help my exam is tomorrow
    If there is a "least" value, then there must be friction - otherwise there would only be one value for P that would keep the block in equilibrium. So your questions is impossible without giving us the coefficient of friction.
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    (Original post by mik1a)
    If there is a "least" value, then there must be friction - otherwise there would only be one value for P that would keep the block in equilibrium. So your questions is impossible without giving us the coefficient of friction.
    working backwards, µ comes out at exactly 0.2. So looks like maybe that's what's missing.
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    sorry yes there is a co-efficient to frcition
    its 0.2
    very sorry i missed that off!
    ive just spent last hr tryin to work it out and still cant
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    ive worked it out now i was being incredibly dumb and had my angles wrong! i had 70 instead of 60 because im stupid
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    (Original post by MiniBarSmarties)
    sorry yes there is a co-efficient to frcition
    its 0.2
    very sorry i missed that off!
    ive just spent last hr tryin to work it out and still cant
    Forces down plane
    =============
    F1 = 175*9.8*sin 30

    Forces up plane
    ===========
    F2 = Pcos30
    F3 = F (friction)

    Forces normal to the plane
    ==================
    F4 = 175*9.8*cos30
    F5 = Psin30

    Therefore normal reaction is

    N = F3 + F4
    N = 1485.2 + P/2

    Balancing forces up and down pl,ane,

    F1 = F2 + F3
    857.5 = P√3/2 +F
    857.5 = P√3/2 +µN
    857.5 = P√3/2 +0.2(1485.2 + P/2)
    857.5 = P√3/2 + 297.04 + P/10
    560.46 = P(√3/2 + 0.1)
    560.46 = 0.966*P
    P = 580.17
    P = 580N
    ======

    Edit: I had Psin30° at the wrong angle in the sketch - it's right now.
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    (Original post by MiniBarSmarties)
    ive worked it out now i was being incredibly dumb and had my angles wrong! i had 70 instead of 60 because im stupid
    Ah, sorry, you already gottit!

    My excuse,
    The server is too busy at the moment. Please try again later
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    Not that it means much now, but i got the same answer as Fermat using the origional angle.
 
 
 
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