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Static Eqmunsure about forces to be drawn 
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#1
Mar2709, 08:16 PM

P: 128

1. The problem statement, all variables and given/known data
A uniform beam of mass mb and length supports blocks with masses m1 and m2 at two positions, as in Fig. P12.3. The beam rests on two knife edges. For what value of x will the beam be balanced at P such that the normal force at O is zero? 2. Relevant equations sum of F = 0 sum of moments = 0 3. The attempt at a solution I'm quite unsure regarding the forces to be drawn in my free body diagram. Example, the two normal forces by beam on the each masses. Do i draw them out? And at points P and O, is there a horizontal reaction force? How do i determine the forces at point P? Like when i looking at point P, do i think as the way below? (treat the knife edge at O as not existing) the beam can rotate at P, so no moments. but the beam cannot translate horiz and vert at P, so there are horiz and vert reaction forces. or I cannot treat the knife edge at O as not existing? If so, means the beam cannot rotate at P and there will be a moments? Do you all know what i mean? =x Can explain to me? Thanks!! 


#2
Mar2709, 11:52 PM

HW Helper
P: 5,343

The best treatment is to consider that point O doesn't exist.
Then all you care about are the sum of the moments about P, which you can write out by inspection. As to the forces at P, that's just the Σ m*g if it is in balance. 


#3
Mar2809, 03:33 AM

P: 128

Then for the two normal forces by beam on the each masses, are they considered as internal forces? 


#4
Mar2809, 07:53 AM

Mentor
P: 41,323

Static Eqmunsure about forces to be drawn



#5
Mar2909, 08:48 AM

P: 128

There are horizontal forces cos the beam cant translate horizontally at these points? (like the support reactions..) hmm.. is it? 


#6
Mar2909, 09:56 AM

Mentor
P: 41,323

If you place a book on a table, what's the horizontal reaction force? Why is that case any different than this one?



#7
Mar2909, 10:31 AM

P: 128




#9
Mar2909, 10:45 AM

P: 128

No force?



#10
Mar2909, 10:48 AM

Mentor
P: 41,323




#11
Mar2909, 10:53 AM

P: 128

Means it is not like those support reactions? Like if we cannot rotate abt that point, the there is a moment; if we cannot translate vertically or horizontally, then there is a reaction vertically or horizontally?



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