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Question:

13(a) A uniform rectangular lamina ABCD is of mass 2m, with AB = DC = 4

cm and BC = AD = 6 cm. Particles, each of mass m, are attached to

the lamina at B, C and D. Calculate the distance of the centre of

mass of the loaded lamina from the sides AB and BC respectively.

Solution:

http://img.villagephotos.com/p/2005-1/929440/PhysicsDiagram1.JPG

http://img.villagephotos.com/p/2005-1/929440/Physicssol(a).JPG

I guess this part involves centre of gravity more than anything but I still don't understand how the moments are taken.

(b) A uniform rod AB, of length 4a and weight W, is hinged smoothly to a

fixed point at A. The rod is held at 60 to the horizontal, with B

above A, by a horizontal force F acting at B. Calculate in terms of W:

http://img.villagephotos.com/p/2005-1/929440/PhysicsHelpDiagram2.JPG

(i) the magnitude of F;

http://img.villagephotos.com/p/2005-1/929440/PhysicsSol(i).JPG

Where do these moments come from exactly? I have a vague idea from the solution and my knowledge of trigonometry but I'm not entirely sure.

(ii) the magnitude of the reaction force at the hinge and its direction to

the horizontal.

http://img.villagephotos.com/p/2005-1/929440/PhysicsSol(ii).JPG

Not sure here how the forces are resolved, a diagram of where the horizontal and vertical forces are in relation to R would really help.

If anyone could show me where these answers are coming from I would be really gratefull since this just doesn't seem to click with me and I have no more classes left to speak with the tutor. Thanks.