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M2 Question

Discussion in 'Mathematics' started by Maths_Mike, Aug 4, 2011.

  1. Maths_Mike

    Maths_Mike New commenter

    I cant get the answer in the back of the book and would appreciate a second opinion!
    A loft door OA of weight 100N is propped open at 50 degrees to the horizontal by a strut AB. the door is hinged at O; OA=OB=1metre. Assuming the mass of the strut can be neglected and that the weight of the door acts through the midpoint of OA, find:
    a) the force in the strut
    b) the reaction at the hinge.
    Book says 35.5 and 69.5 - I got 32.6 and 77.9 !
     
  2. Maths_Mike

    Maths_Mike New commenter

    I cant get the answer in the back of the book and would appreciate a second opinion!
    A loft door OA of weight 100N is propped open at 50 degrees to the horizontal by a strut AB. the door is hinged at O; OA=OB=1metre. Assuming the mass of the strut can be neglected and that the weight of the door acts through the midpoint of OA, find:
    a) the force in the strut
    b) the reaction at the hinge.
    Book says 35.5 and 69.5 - I got 32.6 and 77.9 !
     
  3. DM

    DM New commenter

    The book looks right to me.
    Have you drawn the diagram properly? The strut AB is at an angle forming the base of an isosceles triangle.
    For the first part I took moments about O:
    100 x 0.5 x cos 50 = F x 1 x cos 25
    where F is the force in the strut.

     
  4. Maths_Mike

    Maths_Mike New commenter

    yep - i had the strut as being one of the equal sides length 1 - thanks.
    I checked this about 3 times and just couldnt see what I had done wrong[​IMG]
     
  5. Maths_Mike

    Maths_Mike New commenter

    Fancy another one DM!
    Thti time i get double the answer in the book but just cant find my mistake.
    mass 30 grams on a spring natural length 3cm and modulus of elasticity = 4.5N.
    Find compression in spring when in equilibrium.
    I have assumed that EPE gained will be equal to GPE lost. (no friction or anything like that involved)
     
  6. DM

    DM New commenter

    Just use Hooke's Law:
    The weight of the 30 g mass will be 0.294 N so compression (in metres) = 0.294 x 0.03 / 4.5.

     
  7. Maths_Mike

    Maths_Mike New commenter

    thanks again - I was doing ex3B and hookes law was ex3A so I had obviously forgotten it!!
    (Feels very embarrassed at such a stupid mistake!)[​IMG]
     

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