[Physics] Why the weight force is not included in this free-body diagram

forcesstatics

Take a look at this question.

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The solution by the author is

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My question is why the weight force is ignored in the free-body diagram? Is it because the roller(A) and the pint(B) which cancel the effect of the gravitational force?

Best Answer

It must be that the weight of the member is assumed to be negligible in comparison to the vertical component of the 390 lb force. With no information to the contrary you have to assume this to draw the FBD.

Also, you assume correctly that a FBD must include all forces, including weight. You can't determine reactions at the pin and roller otherwise. The sum of the roller and pin vertical reactions must equal the sum of the weight and the vertical component of the 390 lb force.

Hope this helps.

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