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Physics
Classical Physics
Mechanics
Calculating Free Body Diagram Forces
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[QUOTE="Merlin3189, post: 6046366, member: 542077"] The calculation follows to determine Fy etc. But if you mean, how did they decide what direction to draw the Fy arrows on the FBD, then I would say it is an arbitrary decision. You know that w acts downwards (at A and D) and that Rx acts inwards (at B and C, in the first setup), from your understanding of the operation of the device. But you may not have a clear idea of which way Fy and Fx are acting at E. So you just have a guess and mark them in. If you are right, then they work out to be positive. If you are wrong, they work out negative, meaning they actually point in the opposite direction. That's why you have to be clear in your calculations to add and subtract forces strictly according to the direction you've marked on the diagram. You'll notice that in the calculations, they mark the direction in which they are summing forces. So, for eg., w initially appears as -w, because they are summing forces upwards and w points downward. [/QUOTE]
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Classical Physics
Mechanics
Calculating Free Body Diagram Forces
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