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moments question

My textbook solves this question by resolving around P, but I just wanted to know if it was possible to solve by resolving around the centre of mass of the rod. I tried it and haven't got the correct answer.

Is it possible and why / why not?


thanks for any help in advance I appreciate it
tsr help 98.1.jpg84.8KB
Original post by pondering-soul
My textbook solves this question by resolving around P, but I just wanted to know if it was possible to solve by resolving around the centre of mass of the rod. I tried it and haven't got the correct answer.

Is it possible and why / why not?


thanks for any help in advance I appreciate it


When you say resolving around the centre of mass, I presume you mean taking moments about the centre of mass, however I can't follow your working.


If you do take moments about the centre of mass, you'll need to take into account the reaction at P.
Original post by ghostwalker
When you say resolving around the centre of mass, I presume you mean taking moments about the centre of mass, however I can't follow your working.


If you do take moments about the centre of mass, you'll need to take into account the reaction at P.

Yeah I meant taking moments around the centre of mass. How would I go about doing that?
Original post by ghostwalker
When you say resolving around the centre of mass, I presume you mean taking moments about the centre of mass, however I can't follow your working.


If you do take moments about the centre of mass, you'll need to take into account the reaction at P.

oh wait, does that mean its impossible since we don't know the reaction at P?
Reply 4
Avatar for mqb2766
mqb2766
21
Original post by pondering-soul
oh wait, does that mean its impossible since we don't know the reaction at P?


You'd have to resolve both horizontally and vertically and take moments about the .com to get both P (magnitude and angle) and F.
Much easier to take moments about P. One simple equation gives you F directly.
Original post by mqb2766
You'd have to resolve both horizontally and vertically and take moments about the .com to get both P (magnitude and angle) and F.
Much easier to take moments about P. One simple equation gives you F directly.

Yeah, sorry for the extra hassle. I just want to know the other way of doing it to understand the topic better.


Where have I gone wrong in my working for the reaction at P?


Edit: I've just realised I can't just change the position of 8g, can I, since that force isn't acting on P since P isn't centre of mass. So, if we don't know what that force is (the one that goes straight down), can we answer it like this?
(edited 3 years ago)
tsr help 99.1.jpg37.1KB
Reply 6
Avatar for mqb2766
mqb2766
21
You have not resolved horizontally and vertically. There is not 8g acting downwards at P as F is acting partially upwards. Are you assuming the direction of the reaction at P?

Write the equations down clearly if you want, but there must be better ways of practicing?
(edited 3 years ago)
Original post by mqb2766
You have not resolved horizontally and vertically. There is not 8g acting downwards at P as F is acting partially upwards. Are you assuming the direction of the reaction at P?

Write the equations down clearly if you want, but there must be better ways of practicing?

yeah sorry about that thanks for the help anyways
Original post by ghostwalker
When you say resolving around the centre of mass, I presume you mean taking moments about the centre of mass, however I can't follow your working.


If you do take moments about the centre of mass, you'll need to take into account the reaction at P.

thank you

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