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HOMEWORK 25.B - SP 24
11 thoughts on “HOMEWORK 25.B - SP 24”
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If the structure here were not made up of 3 conjoined segments but were just one continuous object, would there not be the need for a moment here? In other words, is the need for another moment due only to the need to account for internally balanced forces between members?
I believe so. The only external forces are the loads at E and the reaction forces at A and B. So if it were one rigid body then there would be either internal stress or the reaction forces would change in the absence of the applied moment.
For all practical purposes if we treat it like a rigid body, that's exactly the same as treating it as segments - the individual members disappear and things become internal forces.
Is there anything that can be found using the whole free body diagram here or do we have to break it up immediately? It seems like without breaking it up we cannot find certain moments due to the length of DC being unknown.
I'm stuck on the same issue. I tried setting up similar triangles to find the distance of CD for a moment arm but that didn't seem to help much.
Without breaking the frame up, the only values that can be found are external, ie the reaction forces at A and B. While these can be found, they're completely unnecessary as neither should show up in the equation for M.
Try splitting the figure in a way to determine the two-body force from CD, and use that to find M with a moment equation about the proper joint and proper member.
There are too many unknowns if you solve the whole body. Breaking up the body allows you to focus on one set of reactions at a time.
So in other words, splitting the frame up allows us to include internal forces in the moment equation and solve for M? If so, I think that makes sense.
Yep, this is right, this accounts for the forces of the members on each other, and makes sure the indivual members themselves are stable and in equilibrium
The wording on this question confused me a little so I just wanted to clarify. My understanding is M is not supposed to be equal to the moment at B. M is a moment force that in addition to the other forces creates an equilibrium moment at B that equals 0. Is that how you interpreted it?
This is how I interpreted it. Because M acts on the member BC, it doesn't matter where it acts--it will have the same effect on the sum of moments at any point along the member. So, M in addition to any other forces/moments applied on the member achieves static equilibrium, so the sum of moments at any point along the member should be zero (but this also means that at point B, M will be the same as the sum of all other moments at point B).