Hi, I have my first year university exam coming up soon and this type of question seems to feature quite prominently in the past paper questions we are given where I have to take moments about a point and also resolve forces horizontally and vertically. I suppose the best way to show you would be with a question and the solution (which we were given by the tutor).(adsbygoogle = window.adsbygoogle || []).push({});

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:

Diagram(a)

Solution(a)

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:

Diagram(b)

(i) the magnitude of F;

Solution(i)

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.

Solution(ii)

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.

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# Homework Help: Taking Moments About a Point and Resolving Forces

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