How Do You Calculate Forces in a Supported Bar with Uneven Loads?

[Questions999]

Homework Statement

We have a bar with the weight of 500 N which stands in A and B.In this bar an object which weights 1500 N has been hung.The length of the bar AB is 2 meters and AD is 0.8 meters.Find FA and FB just like in the picture.

http://s1171.photobucket.com/albums/r549/Elaia06/?action=view&current=sdsd.jpg

The Attempt at a Solution

The resultant momentum is zero so

FA*0+1900*0.8+500*1+2*FB=0
But FB in my book is 850 N.This answer does not fit my answer.

 

[Doc Al]

FA*0+1900*0.8+500*1+2*FB=0

(1) Why 1900?
(2) Distinguish clockwise and counterclockwise moments: they have different signs.

 

[Questions999]

it is 1900 sorry , I have written it the wrong way in the data.
FA*0-1900*0.8-500*1+2*FB=0
Still FB is not 860..

 

[azizlwl]

Your drawing is not clear enough.
Maybe you make the drawing, take a snapshot and insert image.

 

[Doc Al]

it is 1900 sorry , I have written it the wrong way in the data.
FA*0-1900*0.8-500*1+2*FB=0
Still FB is not 860..

Are you sure? When I solve for F_B using your original data (weight of object = 1500N) I get F_B = 850 N. Which matches the answer in your book (at least according to your first post).

 

[Questions999]

i have written it wrong ,i was sleepy last night :P
It should be 860 N, the Fb.
Yes I get 850 N too.The drawing is just like in my book.

 

[Questions999]

Not related to this exercise :Ah and by the way, If we have a bar with two forces acting in the beginning and the end of it, what is the relationship of the weight of the bar with the forces? I mean, is the distance from a force to the point of weight equal to the distance of the other force with the weight or not?

 

[Doc Al]

Not related to this exercise :Ah and by the way, If we have a bar with two forces acting in the beginning and the end of it, what is the relationship of the weight of the bar with the forces? I mean, is the distance from a force to the point of weight equal to the distance of the other force with the weight or not?

If the bar is uniform, the weight acts at its center. The support forces at each end will (of course) be equidistant from the center.

 

[Questions999]

what is a uniform bar? Sorry but english is not my native language :P

 

[Doc Al]

what is a uniform bar?

A bar whose weight is evenly distributed along its length.

 

[Questions999]

Yes this is an uniform bar, thanks :)