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    A packing case in the shape of a cuboid is on a rough plane inclined at an angle A to the horizontal.
    The packing case is being pushed by a horizontal force of P N applied perpendicular to and in the centre of an edge of the case, as shown in Figure 1 below.
    Figure 2 below is a side elevation showing the dimensions of the packing case and the position of G, the centre of mass of the packing case and its contents.

    The weight of the packing case and contents is 840 N, sin A = 7/25, cos A= 24/25 and the coefficient of friction between the packing case and the plane is u.
    (i) Initially P = 0 and the packing case is in equilibrium. Show that u>=7/24 . [4]

    (ii) Subsequently P > 0. Write down the components of P parallel to and perpendicular to the plane. Show that the moment of the pushing force about the edge AB is 27/25 P Nm clockwise. [4]

    I was able to do part 1 by using F = uR and resolving the weight, but Im stuck on part 2.
    The components I got were 24/25P and 7/25P but I don't know which lengths to use for the moment. What is Figure 2 actually showing?

    Thank you
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Updated: January 21, 2017
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