Recent content by mathandphysics

Excuse me, but here is where I am confused. If I want to be thorough as you said, then the fact that I would consider "normal force from the beam on the load" means that this force is no longer internal force as mentioned in previous replies. However given what I have understood from the above...

Then why didn't he just put Np instead of mg, and I was checking the new answers here and they are saying that in this case and given the chosen system, the drawn weight force is considered as an external force and needs to be included. I am not sure how both of these statement complete each other?

Thank you.. Given what @Orodruin said, that choosing and including forces depends on the system we are considering. How do you decide what system to take? For example in this problem and given the first reply, it seems that the author took is the (beam+box/man). How do I know which system to choose?

Thank you for your reply, it makes sense. While I was checking answers, I ran across this video: . If you have time to check it, in particular from minute 2:23 till 2:51. I failed to understand what he meant by saying that it is NOT the weight of the person, but he is going to put it there...

So please find both screenshots below taken from these youtube videos: Why in the first problem, the tension in the string, from which a mass hangs, is not included when the teacher is placing all torques in the equation (see image)? Why in the second problem, the normal force from the rod...