Textbook says reaction force at A is upward. In my mind I'm keep thinking it should go downward. Or is reaction force on y axis always upward?
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1$\begingroup$ Tip: SE policy is that you credit the authors of material referenced in your posts. Otherwise it's considered plagiarism. There's an edit link below your post. Welcome to E.SE. $\endgroup$– TransistorCommented Sep 29 at 20:50
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$\begingroup$ the direction depends on whether C is hanging from A, or C supports A $\endgroup$– jsotolaCommented Sep 29 at 21:14
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2$\begingroup$ This question is similar to: What's going on with internal forces and reaction forces?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. $\endgroup$– Solar MikeCommented Sep 30 at 9:59
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Upper part is in tension and lower part in compression. Other way of thinking about it is to split the beam at $C$ and also split the load $P$ into $P_A$ acting on the top part and $P_B$ acting on the bottom part, after which you end up with 2 sets of opposite forces; $P_A$ and $R_A$ on the top and $P_B$ and $R_B$ on the bottom. If deflection of both parts at $C$ is the same, this situation is equivalent to the original example.
