This video screencast was created by Dr Terry Brown at the University of Technology, Sydney with Doceri on an iPad. Doceri is free in the iTunes app store. Learn more at www.doceri.com
hi @jonathansanchez2854, sorry for the delayed reply. How are you going with your understanding? Your question indicates that you may have some misunderstandings. If you still need me to reply, please let me know. cheers.
thanks for the very good questions. Sorry for the delayed reply. Any structural component that has length greater than about 5x the other two dimensions can be considered a 'beam'. Of particular interest is the moment because we use this to calculate the normal stress due to bending, i.e. normal stress = My/I. A typical example in a mech eng application is a power/torque transmission shaft.
Hi, thanks for the question. Taking moments about point A, the line of action of Rbx passes through point A, therefore the perpendicular distance is zero. And therefore Rbx has no moment about point A.
Hi Abam, sorry, I don't have any other videos of this problem. Also, I don't really understand what you mean by "moment of A from 4 metres". A is a just a location on the beam. When we talk about moments, we talk about the moment of a force (or sum of moments) about a particular point, e.g. A, B or any other convenient or relevant point.
I'm not sure if you're still around Mr Brown, and I looked through the questions to make sure this wasn't asked already, but i am curious to why the Rbx reaction is running left to right (+), as it is a pin and I see the push force from the other end (+600cos45) is also running in the same direction, wouldn't that make the reaction at the pin (Rbx) oppose that +600cos45 force, thus be negative? I saw in the comments someone say you didn't use it anyway because it was in line with a moment, thus this is sort of moot. but just curious to the theory as reactions sort of confuse me. Edit: Having finished the video, I guess for both horizontal forces to =0 (to be in equilibrium), one has to be the negative of the other. Instinctively feel wrong though, but adds up.
Hi, yes, I’m still around, but missed your comment. Sorry. Yes, Rbx will be negative and that is the answer that I get at about the 13min mark. Your last sentence indicates you may have some misunderstanding about moments though.
@@TerryBrownMechEng I guess I I may be conflating horizontal equilibrium (LHS forces=RHS forces), where a moment is force x distance about a point, so I assume the reaction force will be the same but in opposition? I'm studying mechanics 1, we're looking at forces through a dissected beam currently. I did have an "A-ha" moment last week realising that the bend above and below the horizontal axis of the beam (tension and compression) indicates which direction the moment goes in (as often the reaction forces feel instinctively opposite to what I feel, I guess a reaction is always going to be opposite to the applied force) But thank you for replying, you must truly have passion for this subject.
Sometimes our instinct/intuition is wrong. Good and correct free body diagrams and use of the equilibrium equations will tell us the ‘truth’. Keep thinking more deeply about what you’re doing, and asking questions, and you’ll get it.
Eoin - not too late, I'm stil here :) . If you take moments about point B, the moment effect of the applied 200 N force about point B is zero (since the perpendicular distance from the line of action of the force to the point is zero). Try it and see that you get the same answers. This is a good way to check your equilibrium equations and calculations.
Ali Nasser Yes. Assuming you do the equations and algebra correctly. Give it a go and find out. This is a good thing to do anyway to check your calculations.
Thanks for the feedback. I will do so. I have a few new ones that are unlisted at the moment that I will make public soon. Do you mind telling me whether you are one of my UTS students or not?
thanks for letting me know. And thanks again for the feedback. I sometimes wonder how worthwhile it is doing these videos as there are now many others doing similar and better e.g. ru-vid.com and ru-vid.com. Wishing you all the best for your engineering studies.
do you mean the 200 N.m applied couple moment? I'm not sure why you think it is clockwise, the curved arrow is indicating anti-clockwise. If you mean the 200 N force applied at end B, then yes, it will cause a clockwise moment about point A and I have shown that moment as negative in my solution i.e. -200 x 7
